B.

Ed Science Education

BIOLOGY-IV
Course Code: 6454 Units: 1–9

SCIENCE EDUCATION DEPARTMENT

FACULTY OF EDUCATION
ALLAMA IQBAL OPEN UNIVERSITY, ISLAMABAD
 (All rights reserved with the publisher)

First Edition ......................................

Quantity ............................................ 1000

Price .................................................. Rs.

Printer ............................................... AIOU-Printing Press, Sector H-8, Islamabad.

Publisher ........................................... Allama Iqbal Open University, H-8, Islamabad.

ii
 COURSE TEAM

Dean: Prof. Dr. Nasir Mehmood

Chairman: Prof. Dr. Nasir Mahmood

Course Development
Coordinator: Arshad Mehmood Qamar

Writers: 1. Dr. Muhammad Waseem, Assistant Professor

Department of Biology, AIOU
2. Dr. Sobia Mushtaq, Assistant Professor
Adjunct Faculty, PMAAU, Rawalpindi
3. Dr. Tauseef Anwar, Assistant Professor (Biology)
IMCB, I-8/3, Islamabad
4. Arshad Mehmood Qamar, Lecturer
Science Education Department, AIOU

Reviewers: 1. Dr. Muhammad Waseem, Assistant Professor

Department of Biology, AIOU
2. Dr. Sobia Mushtaq, Assistant Professor
Adjunct Faculty, PMAAU, Rawalpindi
3. Dr. Aftab Ahmed, Lecturer, Science Education
Department, AIOU
4. Arshad Mehmood Qamar, Lecturer
Science Education Department, AIOU

Editor: Fazal Karim

Composer: Rehan Yaqoob

Producer: Shahzad Afridi

Typeset by: Asrar ul Haque Malik

iii
 FOREWORD

This course has been designed for fulfilling the content expertise of prospectus teachers
who will be enrolled in B.Ed 4.0Year or B.Ed 2.5 Year in Allama Iqbal Open University.
This book is very useful for making up the need of the advance content for students and
teachers. This book will be helpful to reduce the controversy that what type of
knowledge, skills and values Science teachers need. Some teachers need more content
knowledge, whereas some science teachers want to enhance the knowledge. Further
Scientific knowledge is expanding at very high speed. This is the era of scientific
innovation and creations. Innovations and creations need skillful technologies. Allama
Iqbal Open University and Science Education Department has promised to maintain the
quality and acceptability. This book is one of those series of books which will enable the
teachers to cope with changing needs of the society and students.

The focus of this book is to provide with best knowledge, skills and content in the subject
of biological sciences. With the help of this book science students can explore the natural
world, maintain their health by avoiding the diseases and discover new dimensions in the
field of Bio-sciences. Keeping in view the qualitative aspect of education and an
increasing demand of science teachers, stress is laid upon science content as well as
strengthening their professional skills and knowledge. The elements of motivation and
love are also considered.

We welcome suggestions and comments for improvements from the readers, teachers and
public at large for the improvement of this course.

Prof. Dr. Nasir Mahmood

Chairman
Science Education Department

iv
 PREFACE

Though there is lot of books available in market, but there is no book which fulfills the
requirements of University’s approved outlines. Some cover one area of content while
other covers another area. In this way there would be a lot of financial burden and
dispersed focus.

Further AIOU has its own requirement either to provide compiled material or text book.

This book is one of those series of Biology Books for coverage of content area
requirement for B.Ed 4 Year and B.Ed 2.5 Year in the field of Biology.

This book is written as per prescribed procedure of book development. After approval of
content from all statuary bodies, approval for starting development of this book was
sought. Then selection of writers and reviewers was completed. Time and again
reminders to Unit writers and telephonic conversation were done with the writers to
expedite the process of writing and review. In spite of very tedious work of writers and
reviewers, coordinators had to look into everything i.e. format, self assessment exercises,
alignment of the content and addition of some essential things and removal of irreverent
things.

Great stress has been laid in making the course to facilitate prospectus, in service and pre-
service teachers for content knowledge regarding Biology. The course is equipped with
illustrations for better understanding of the reader. Each unit is equipped with necessary
illustrations, activities and self assessment exercises.

It is hoped that this book will prove best for the content knowledge regarding Biology.

Arshad Mehmood Qamar

Course Development Coordinator

v
 ACKNOWLEDGEMENTS

Allama Iqbal Open University and the Course development coordinator along with
course development team are grateful to the writers and publishers of Biology books for
adopting their books and materials, internet for providing useful information regarding
Biology, and reference materials for the development of the course of Biology-IV for
B.Ed Science Education.

All are specially acknowledged whose information and material has been quoted in the
course that Allama Iqbal Open University is a non- commercial educational University in
Pakistan which is providing educational facilities to under-privileged remote rural areas
through distance and non-formal mode.

It is a matter of pleasure for department of science education and AIOU to acknowledge

all those whose efforts and hard work make it possible to frame Contents of this book.
Committee of courses tried her best to make necessary changes and then approved the
contents of this course.

Highly acknowledged members of CoC Prof. Dr. Rizwan Akram Rana, Dr. Muhammad
Idrees, Dr. Hafiz Athar Khan, Dr. Fazal Ur Rahman, Dr. Muhammad Samiullah, Dr.
Farkhunda Rashid Ch. and Arshad Mehmood Qamar.

My special thanks to Mr. Arshad Mehmood Qamar Course Development Coordinator

who got the approval of Course Team including Dr. Muhammad Waseem, Dr. Sobia
Mushtaq and Dr. Tauseef Anwar I acknowledge the writers for writing the units
efficiently.

I also acknowledge the team of CP, Editor, lay out designer and producer for giving their
input to make this book more beautiful. Our PPU team is very cooperative and helpful for
publishing the book. Finally I acknowledged all those who in one way or the other put
their efforts for completion of this task.

Prof. Dr. Nasir Mehmood

Chairman/ Dean
Faculty of Education

vi
 OBJECTIVES OF THE COURSE

After completing this course, you will be able to:

1. explain evolution, taxanomy, nutrition in protzoans and symbiotic relationship of
protozoans
2. describe the multicellular and tissue levels of organization and symbiotic
relationship among them.
3. explain general characteristics, origin of diploblastic organization, radial
symmetry, polymorphism, formation of coral reefs in coelenterates.
4. elaborate the genarl characteristics, adaptations for parasitic mode of life,
importance of infestation and disinfestations(tapeworm) in platehelmonthes.
5. explain the general characteristic and salient features , importance and parasitic
adaptation of nematodes(Aschelmenthes)
6. explain the general feature, segmentation and its advantage, coelom and its
advantages in annelids, classify the phylum annelid upto classes; describe the
importance of phylum annelid.
7. describe the common characteristics , structure and function of different organs
of arthropods and related groups.
8. describe important features, classification, feeding modes, organs, evolutionary
perspectives of Invertebrates.
9. highlight characteristics of chordates and classify phylum chordate into classes
and sub-classes.

vii
 INTRODUCTION

Due to globalizations and frequent expanding knowledge the challenges for teaching and
learning are emerging at faster rate as compared to past. Content Knowledge and how
content knowledge is delivered to the learner are both equally important for researchers
and society. Demands of society regarding education of their children are very high.
These demands can be fulfilled by equipping the teachers with high level content
knowledge along with modern and tested teaching methods and technologies.

With the development of world, paradigm has been shifted from traditional to most
modern teaching and learning methodologies. The focus of this course is to reinforce and
enhance the content knowledge of the perspective teachers in the subject of Biology so
that they may teach the science students with best of their capabilities to the science
students.

Zoology is the branch of biology that deals with the study of animals that live around us.
They are of many types, some are beneficial to us and others are harmful. Some animals
are very large and other are very small. Some resembles to each other and other differs.
This course focus on characteristics, structures, functions of different organs, their
feeding modes and their evolutionary perspectives. Different phla and classes of
invertebrates and vertebrates are discussed in this course.

This course has been approved by different statuary bodies and is equally important for
B.Ed 4.0 Year and B.Ed 2.5 Year in Science Education.

This course is quite comprehensive and is not only useful resource for B.Ed graduates,
but also useful for the students of BS Biology, teachers, coordinators and working
science teachers.

Arshad Mehmood Qamar

Lecturer
Course Development Coordinator

viii
 CONTENTS

Unit No. Page #

Unit 1: Animal-like Protists: The Protozoa ............................................................ 1

Unit 2: Multicellular and Tissue Level of Oranization ........................................... 31

Unit 3: Triploblastic and Acoelomate Body Plan of Animals ................................ 59

Unit 4: Pseudocoelmate Body Plan: Aschelminthes ............................................... 91

Unit 5: Molluscan Sucess ....................................................................................... 115

Unit 6: Annelida: The Metameric Body Form ........................................................ 145

Unit 7: Arthropods .................................................................................................. 171

Unit 8: Echinoderms ............................................................................................... 213

Unit 9: Chordates .................................................................................................... 245

ix
x
UNIT-1

ANIMAL LIKE PROTISTS:

THE PROTOZOA

Written by: Dr. Sobia Mushtaq

Reviewed by: Arshad Mehmood Qamar

1
 CONTENTS
Introduction ....................................................................................................... 3

Objectives ......................................................................................................... 3

1.1 Evolutionary Perspectives ..................................................................... 4

1.2 Life within a Single Plasma Membrane ................................................ 5

1.3 Symbiotic Life-Styles ........................................................................... 7

1.4 Protozoa Taxanomy: (Up to Phyla, Subphyla and Super Classes,

Wherever Applicable) ........................................................................... 8

1.5 Pseudopodia and Amoeboid Locomotion; Cilia and other Pellicular

Structures .............................................................................................. 23

1.6 Nutrition: Genetic Control and Reproduction....................................... 23

1.7 Symbiotic Ciliates: Further Phylogenetic Considerations .................... 26

2
Introduction

Animal-like protists which are also named as protozoa. They are single-celled
eukaryotes that resembles animals because they can move and are heterotrophs. They
depend on other organisms for their food. Animal-like protists are very small only about
0.01-0.5mm including flagellates, ciliates and sporozoans. Flagellates have long flagella
which rotate in a propeller-like fashion, pushing the protist through its environment. An
example of flagellate is Trypanosoma responsible for causing African sleeping sickness.

In some protists cell surface extends out to form feet-like structures known as
pseudopodia that propel the cell forward. For example, amoeba. Some ciliate protozoans
contains small hair like projections throughout the body called as cilia which helps in
movement. For instance, Paramecium has cilia that propel it. The sporozoans are protists
that produce spores, such as the toxoplasma. These protists do not move at all. The spores
develop into new protists (Figure 1.1).

Figure 1.1: Different protists movements. (a) Cilia, small hair like projections
throughout the body help in movement such as paramecium (b) Pseudopodia, finger like
projections such as amoeba (c) Flagella, hair like structure such as euglena.

Objectives
After completion of this unit, you will be able to:
 describe evolutionary perspective of protozoa.
 explore life within a single plasma membrane.
 elaborate symbolic life styles.
 classify protozoan up to classes or super classes level
 explain movement of different types in protozoan.
 describe nutrition in protozoan.
 discuss genetic control and reproduction in Protozoa.

3
1.1 Evolutionary Perspective
The fossil record shows that all Protists and animal phyla were present during the
Cambrian period. Cambrian period is about 550 million years ago. But only a few fossil
evidences are available. Ancient Archaea were the first living organisms on earth which
gave rise kingdom Protista about 1.5 billion years ago. The endo-symbiont hypothesis
explains the mechanism of this evolution of protista. Most dentists agree that the protists
arose from more than one ancestral group. The zoologists recognize between 7 and 45
phyla of protists having many evolutionary lineages Therefore, these protists are
polyphyletic. Groups of organisms having separate origin are called polyphyletic (Figure
1.2).

Kingdom protista is sub-divided into two groups:

a) Plant like protists (algae): These are primarily autotrophic.
b) Animal like protists (Protozoa): These are primarily heterotrophic

Figure 1.2: Evolutionary prespective of protists. Generalized evolutionary tree

depicting possible lines of descent for the protozoa

Activity 1: Observe and illustrate animal-like protists also keeping in view

evolutionary prospective.

4
1.2 Life within a Single Plasma Membrane
Protozoans are single-celled animals found in marine, freshwater habitats as well as in
moist soil. In addition to many free-living species they live in association with other
animals as well as plants such as commensals or as parasites. They vary in size,
morphology, mode of nutrition, reproduction and mechanism of locomotion. They belong
to polyphyletic group.

a) Plasma Membrane
The entire organisms are bounded by the plasmalemma (cell membrane). It is often
differentiated into a clear, outer gelatinous region (gel or semisolid) the ectoplasm and an
inner more fluid region (fluid or sol state), the endoplasm.

b) Locomotory Organelles
Cilia are shorter and more in number while long flagella less in number. They both have
similar in structure but functionally different. Their microtubules are arranged in ring of 9
microtubule doublets surrounding by central pair of microtubles (9+2 arrangement). They
are anchored to the cell by a basal body (Figure 1.3). Pseudopodia are temporary cell
extensions. The most familiar are lobopodia, broad cell processes containing ectoplasm
and endoplasm and are used for locomotion and engulfing food.

Activity 2: Develop model for cilia ultra structure depicting its important features.

Figure 1.3: Ultrastructure of cilia and flagella. Both perform motility and sensory
functions which are essential for cell survival in protozoans.

c) Mode of Nutrition and Excretion

Digestion is mostly intracellular known as Endocytosis where food particles becomes
surrounded by membrane forming food vacuole. This vacuole moves into the cytoplasm
and becomes absorbed by digestive enzymes. Indigestable material then starts moving
near to plasma membrane becoming a part of it and moving outside by Exocytosis.
Contractile vacuoles are organelles involved in expelling water from the cytoplasm. Fluid

5
is collected from the cytoplasm by membranous vesicles and tubules called spongiome
(Figure1.4).

Figure 1.4: Mode of nutrition in protozoans. Processes of Exocytosis and endocytosis.

In exocytosis, materials are exported out of the cell via secretory vesicles while
endocytosis is way by which materials move into the cell.

d) Reproduction
Asexual reproduction takes place by fission where mitotic replication of chromosomes
and splitting of the parent into two or more parts takes place. Binary fission occurs when
the protozoan splits into two individuals. In fission, nuclei divide equally among two
individuals when cytoplasm divides (cytokinesis). Cytoplasmic division could be
longitudinal or transverse. In multiple fission or schizogeny, firstly many nuclear
divisions followed by division of cytoplasm and as result many individuals formed. It
begins with multiple mitotic nuclear divisions in a mature individual. After many nuclei
produced, cytokinesis results in the separation of each nucleus into new cell. In budding,
part of parent breaks off differentiating into new individual. Mitosis is followed by the
incorporation of one nucleus into cytoplasmic mass that is much smaller than parent cell
(Figure 1.5).

Sexual reproduction includes gametes formation followed by their fusion to produce

zygote. Gametes are formed by mitosis followed by their union through meiosis.

6
Figure 1.5: Asexual reproduction in protozoans. (A) Binary fission in paramecium in
which parent cell divides into two daughter cells (B) Multiple fission in plasmodium
where single parents cells produces many daughter cells and released when get mature
(C) Budding in which small bud is produced on the body of parent and get separate after
getting into adult form.

1.3 Symbiotic Life Styles

Most protozoans follow symbiotic lifestyles. Symbiosis is a relationship between two or
more organisms that live closely together. It can be obligatory in which both organisms
depend on each other for survival or facultative where they live independently. Symbiosis
can vary between mutualism, commensalism, and parasitism. In mutualism, both
organisms benefit. In commensalism, one benefits and the other is unaffected; in
parasitism, one benefits and the other, known as host, is harmed. Being harmed but
survives, at least long enough for parasite to complete its life cycle.

Parasites are classified on the basis of their hosts and life cycles. An obligate parasite is
totally dependent on the host to complete its life cycle while facultative parasite do not.
A direct parasite has only one host while an indirect parasite has multiple hosts, definitive
and intermediate host. Parasites that live on the outside of the host are called
ectoparasites e.g. lice, and some mites) while endoparasites lives inside the host such as
all parasitic worms (Figure 1.6).

7
Figure 1.6: Human head lice. An obligate ectoparasite.

1.4 Protozoan Taxonomy: up to Phyla, Subphyla and

Superclasses, wherever Appliable
Proto-zoologists are zoologists who study about protozoans. The protists base diversity of
ultrastructure, life cycle, mitochondria, DNA sequence data, life styles and evolutionary
lineages. Therefore, they cannot be put in a single kingdom. Thus classification scheme
of protozoan have been changed. New evidences have been collected from electron
microscopy, genetics. biochemistry and molecular biology–. These evidences shows that
phylum protozoa has itself may phyla. Therefore, protozoa have been given the status of
kingdom. Number of species of protozoan are 64,000. Most of these are fossils. Most
protozoologists regard protozoa as sub-kingdom, having several phyla (singular=
Phylum) within kingdom protista (Table 1.1). These are:

1. Phylum Sarcomastigophora, having flagellates and amoebae.

2. Phylum Labyrinthomorpha
3. Phylum Apicomplexa
4. Phylum Microspora
5. Phylum Acetospora
6. Phylum Myxozoa
7. Phylum Ciliophora, having ciliated protozoa with two nuclei types.

Table 1.1: Protozoan Taxonomy.

Kingdom Protista
Sub-Kingdom Protozoa
1. Phylum Sarcomastigophora
a. Sub-phylum Mastigophora
i. Class Phytomastigophora
ii. Class Zoomastigophora

8
 b. Sub-phylum Sarcodina
i. Super-class Rhizopoda
ii. Super-class Actinopoda
c. Sub-phylum Opalinata
2. Phylum Labyrinthomorpha
3. Phylum Apicomplexa
4. Phylum Microspora
5. Phylum Acetospora
6. Phylum Myxozoa
7. Phylum Ciliophora

Activity 3: Build an understanding of the diversity of species that makeup the

Kingdom Protista.

1. Phylum Sarcomastigophora:
The phylum sarcomastigophora is one of the largest phylum belongs to the Protista or
protoctista kingdom. Its main features are:
i. Unicellular or colonial having single nucleus.
ii. Locomotion by flagella, pseudopodia or both.
iii. Autotrophic (produce own food), saprozoic (depend on dead particles) and
heterotrophic (obtain energy from other organisms).
iv. Single type of nucleus
v. Sexual or asexual mode of reproduction.
It is divided into three subphyla: the Mastigophora, the Sarcodina and the Opalinata

a. Sub-Phylum Mastigophora: Flagellar locomotion

Members of the sub-phylum mastigophora move by the help of whip-like organelles
called flagella (Figure 1.7). These were grouped in Flagellata (Mastigophora), it is
divided in to Phytoflagellata (autotrophic) and Zooflagellata ( heterotrophic). So it is
further divided into two classes: Phytomastigophorea and Zoomastigophorea.

Figure 1.7: Sub-Phylum Mastigophora: Flagellated protozoa. Flagella is locomotory

organ and helps in locomotion.

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i. Class Phytomastigophorea
It is a class of flagellated, plant-like microscopic organisms and posses chloroplast and
are photosynthetic (Figure 1.8). These are mostly regarded as algae and consists of many
subgroups: Chloromonadida, Dinoflagellida, Euglenida, Heterochlorida, Silicoflagellida,
Volvocida. Chloromonadida flagellates are grass-green or colorless having two equal
flagella, one anterior and other trailing. They are free-swimming, although Reckertia and
Thaumatomastix form pseudopodia. They vary in size from 30 to 100 micrometers.
Chromatophores are small disks, lacking stigmas and fat is the storage product.

Dianoflagellates are mostly marine but also common in fresh water habitats. They
include about 2,294 species found in marine, freshwater and parasitic habitats. Some
dinoflagellates forms red tide which is bloom of visible coloration of the water but it
causes shellfish poisoning if humans consume contaminated shellfish. Some
dinoflagellates also exhibit bioluminescence primarily emitting blue-green light.

Euglena is freshwater phytomastigophorean. Chloroplast has pyrenoid which synthesizes

and stores polysaccharides. If cultured in dark, euglenoids feed by absorption and lose
their green colour. Some species such as peranema lack chloroplast and heterotrophic.
Asexual reproduction takes place by binary fission by mitosis followed by cytokinesis.
First, the basal bodies and flagella replicate, then the cytostome and microtubules (the
feeding apparatus), and finally the nucleus and remaining cytoskeleton. Once this occurs,
the organism begins to cleave at the basal bodies, and this cleavage line moves towards
the center of the organism until two separate euglenids are formed. It is also called
longitudinal cell division or longitudinal binary fission. Sexual reproduction is unknown.

Silicoflagellida is an order of marine flagellates having internal, siliceous, tubular

skeleton, numerous yellow chromatophores and a single flagellum. Volvocales, also
known as Chlamydomonadales, are flagellated or pseudociliated green algae. They form
planar or spherical colonies which range from Gonium (four to 32 cells) up to Volvox
(500 cells or more). Each cell has two flagella and similar in appearance to
Chlamydomonas, with the flagella throughout the colony moving in coordination.
Asexual reproduction occurs in the spring and summer when some cells withdraw to
water interior of parental colony forming daughter colonies.

Sexual reproduction occurs during autumn. Some are dioecious having separate sexes
while some are monoecious with both sexes in the same colony. Specialized cells
differentiate into macro-gametes (non-motile) and micro-gametes (Motile).
Microgametes as packet of flagellated cells leave parental colony travelling towards
macrogametes. Packet breaks apart and fertilization occurs between micro and
macrogametes. Zygote secretes resistant wall around itself releasing parental colony
when it dies. Zygote undergoes meiosis to reduce chromosome number, one product of
among them undergoes repeated mitotic divisions forming colony is released from
zygotic capsule in the spring while all others degenerate.

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Figure 1.8: Phytomastigophoreans. (A) Dianoflagellate, marine as well found in fresh-
water habitat (B) Euglena, large well-organized chloroplast and photoreceptors which
allows organism to move towards light (C) Silicoflagellida, marine chromists that are
both photosynthetic and heterotrophic (D) Volvox, Chlorophyte green algae, colonial
flagellate and form spherical colonies.

ii. Class Zoomastigophorea

They are single-celled organisms having spherical, elongated bodies with single central
nucleus. They are heterotrophic eukaryotes or may be symbionts such as Trichomonas.
They have one or more flagella and with or without plastids or cell walls. A few are
mutualistic such as those that live in the guts of termites and helping bacteria to break
down wood. Some are parasitic causing diseases such as African Sleeping Sickness
caused by Trypanosoma brucei and Chagas disease, caused by Trypanosoma cruzi. T.
brucei completes its life cycle between tsetse fly and mammalian hosts, including
humans, cattle and wild animals. Infection occurs when a vector tsetse fly bites a
mammalian host. The fly injects the metacyclic trypomastigotes into the skin tissue. The
trypomastigotes enter the lymphatic system and into the bloodstream. The initial
trypomastigotes are short and stumpy. Once inside the bloodstream, they grow into long
and slender forms. Then, they multiply by binary fission. The daughter cells then become
short and stumpy again. The long slender forms are able to penetrate the blood vessel
endothelium and invade extravascular tissues, including the central nervous system
(CNS).

11
Mental dullness, lack of coordination and sleepiness develops after entering of
trypanosomes into central nervous system. Pathology can also lead towards death. It can
be cured if detected only at early stage. Sometimes, wild animals can be infected by the
tsetse fly. In these animals, they do not produce the disease but the live parasite can be
transmitted back to the normal hosts (Figure 1.9).

Figure 1.9: Life cycle of trypanosome gambiense. An example of zooflagellate. When

tsetse fly feeds on vertebrate host, trypanosomes enters vertebrates ciruculatory system
where they multiply binary fission. When another tsetse fly bites this vertebrate host
again, trypanosomes move into the gut of the fly and undergoes binary fission. Then,
migrate into fly’s salivary glands where they are available to infect a new host.

Activity 4: Depict out major differences about phytomastigophoreans and

zoomastigophoreans.

b. Sub-Phylum Sarcodina: Pseudopodia and Amoeboid Locomotion

Sarcodina, the largest phylum consists about 11,500 living and 33,000 fossil species.
These organisms use temporary cytoplasmic extensions called pseudopodia for feeding
and locomotion. These include Amoeba and some pathogenic species, e.g., dysentery
causing Entamoeba histolytica. These protozoans cells may be spherical or irregular in
shape with thin envelope. The cytoplasm, composed of ectoplasm and endoplasm, may
contain more than one nucleus. Food, which attaches to the body surface or is engulfed
by pseudopodia and digested in food vacuoles.

12
Sexual reproduction is by fusion of two gametes known as syngamy and asexually by
division or budding. In multi-nucleate forms, cytoplasmic division with distribution of
the nuclei occurs. In some cases flagellated while in some flagellated generation meet
with un-flagellated. Some lives alone or form colonies while some are parasitic on plants
or animals. Most species are free-living, feeding on bacteria, algae or organic debris.
Pseudopodia are locomotory organs in amoeba. It reproduces asexually by binary fission.
It starts by pulling in the pseudopodia to attain a spherical shape. Mitotic cell division
takes place, which constricts the nucleus and cytoplasm to form two daughter cells. Thus,
the genetic information in the nucleus is copied in both the cells which make them
identical. The nucleus is the most important part of an amoeba and is the key to its
survival. The free amoebae are called trophozoites.

Binary fission in amoebae may lasts for about 30-60 minutes under ideal conditions.
Under abnormal conditions where environment lacks nutrients amoeba reproduces by
multiple fission. The process begins in the same way as binary fission. Amoeba retracts
its pseudopodia and attaining spherical shape. Sometimes it forms covering around itself
called as cyst to survive harsh and rough conditions. Mitosis takes place multiple times
producing many daughter cells inside the cyst, which are protected against unsuitable
environment. Once favorable conditions, the cyst ruptures releasing daughter amoebae.
These cysts are responsible for causing infection and contaminating food and water and
termed as microbial cyst. If the amoeba stays in this stage for long time, it may die due to
lack of food and will also be incapable of reproducing (Figure 1.10).

Figure 1.10: Reproduction in amoeba. It reproduces both by the processes of binary

fission and multiple fission. In both, reproduction starts when amoeba starts pulling its
pseudopodia. During binary fission, mitotic division separates nucleus and cytoplasm to
form two daughter cells and under multiple fission, a wall (cyst) developed around
amoeba and inside of which multiple mitosis takes place forming multiple daughter cells.
Under favourable conditions, cyst ruptures releasing daughter amoebae.

13
Amoeba has no specific head or tail but finger like projections called as pseudopodia are
present throughout the body which helps in locomotion. It moves in irregular manner
towards the side of these pseudopodia. When it moves along one side it pulls amoeba in
one direction but when pseudopodia moves in opposite irection the amoeba also starts
moving that side. Such movements are known as “amoeboid movements” (Figure 1.11).

Figure 1.11: Amoeboid locomotion. Amoeba moves in direction where pseudopodia

move.

i) Super-Class Rhizopoda: Class Lobosea

Three types of pseudopodia or cytoplasmic extensions used for locomotion and digestion
found in members of this superclass:
1. Long, thin reticulopodia, which fuse to form network.
2. Non-fusing filopodia( having ectolplasm only and stream-like movement that
delivers food in conveyor belt like fashion).
3. Blunt and fingerlike lobopodia (the cell processes with ectoplasm and endoplasm
used for locomotion and engulfing).

All free living amoebae are particle feeders, using their pseudopodia to capture food, a
few are pathogenic. For example, Entamoeba histolytica is a protozoan parasite causes
disease called amoebiasis. It causes internal inflammation of large intestine. It spreads
through contaminated food or water containing cyst stage of the parasite. These cysts are
ingested and enter the lower part of small intestine. These are ingested and enters in the
lower part of intestine. Cyst under the influence of enzymes disintegrates and transform
into quadrinucleate ameba. This ameba may escape through the wall and forms
trophozoites which move to the large intestine and establish the infection in the colon
(Common site is cecal colon). In the intestine, trophozoite forms the flask shape ulcer and
may form extra intestinal abscesses (Figure 1.12).

In some members, tests (protective shells) are common which are the protective
structures secreted by cytoplasm. They may be calcarious (made of calcium carbonate),
proteinaceous (made of protein), siliceous (made of silica) and chitinous (made of chitin-
a polysaccharide). For example, Difflugia is common freshwater shelled amoeba. Its test
is vase- shaped and composed of mineral particles embedded in secreted matrix.

14
Figure 1.12: Amoebiasis caused by Entamoeba histolytica. Cysts of E. histolytica are
ingested by water or uncooked food contaminated by feces. In the colon, vegetative
trophozoite forms emerge from cysts and may invade the mucous membrane of the large
bowel, producing lesions that are maximal in the caecum but found as far down as the
anal canal. These are flask-shaped ulcers and may form amoebic abscesses.

Activity 5: Students must know about various kinds of pseudopodal movements with
major differences among them.

ii) Super-Class Actinopoda: Foraminiferans, Heliozoans and Radiolarians

Foraminifera are amoeboid protists characterized by streaming granular ectoplasm for
feeding and external shell of diverse forms and materials. Tests of chitin is believed to be
most primitive type. Most foraminifera are marine, the majority of which live on or
within the sea-floor sediment (benthic), while smaller variety floats in the water column
at various depths (planktonic). They produce shell with one or more chambers and made
of calcium carbonate (CaCO3) or agglutinated sediment particles. Over 50,000 species
are recognized, both living (10,000) and fossils (40,000). They range in size from about 1
mm and up to 20 cm.

Heliozoan are microbial eukaryotes (protists) having stiff arms (axopodia) radiating from
their spherical bodies, which are responsible for their common name. The axopodia are
projections from the amoeboid cell body used for capturing food, movement and
attachment. They are similar to Radiolaria, but they are distinguished from them by
lacking central capsules and other complex skeletal elements, although some produce
simple scales and spines. They may be found inboth freshwater and marine environments.
The Radiolaria are protozoa of diameter 0.1–0.2 mm that produces intricate mineral
skeletons made up of silica, with central capsule dividing the cell into the inner and outer

15
portions of endoplasm and ectoplasm. They recognized as important diagnostic fossil
from the Cambrian period (Figure 1.13).

Figure 1.13: Examples of Actinopoda. (A) Radiolarian, produce silica skeleton with
central capsule dividing inner and outer portions of cell (B) Heliozoans, tough arms
radiating from their bodies (C) Foraminiferans, ectoplasm for catching food and other
uses

Activity 6: Compare various examples about apicomplexans

C) Sub-phylum Opalinata
They are highly unusual protists with large cells, multiple flagella and two to hundreds of
nuclei. The name Opalina is derived from the iridescent appearance when light reflects
on the delicately folded surface of the cells. Opalinids are found exclusively in the
intestines of frogs and some other hosts.

Figure 1.14: Opalina ranarum. An example of sub-phylum opalinata.

16
2. Phylum Labirynthomorpha
A small phylum of free-living organisms that produce networks of slime in which
spindle-shaped cells about 10μm long, live and through which they move. They
reproduce both asexually and sexually. Most species are marine, forming colonies on the
surfaces of algae and sea-grasses or decomposers on dead plant material. Although they
are outside the cells, the filaments are surrounded by a membrane connecting to
cytoplasm by a unique organelle called a sagenogen or bothrosome. The cells are uni-
nucleated and move back and forth along the amorphous network at speeds varying from
5-150 μm per minute. Among the labyrinthulids, the cells are enclosed within the tubes
and among the thraustochytrids, they are attached to their sides (Figure 1.15).

Figure 1.15: Thraustochytrid, the coral symbiont. An example of phylum

Labyrinthomorpha.

3. Phylum Apicomplexa
i. Apical complex for penetrating host cells
ii. Single type of nucleus
iii. No cilia or flagella
iv. Both sexual and asexual phases

a) Class Sporozoea
Sporozoa (phylum Protozoa) is class of apicomplexa in which the life cycle includes a
spore-forming or cyst-forming stage. Asexual reproduction occurs by multiple fission.
All members are parasitic, parasitizing hosts throughout the animal kingdom. For
example, Plasmodium and coccidian causing variety of diseases in domestic animals and
humans.

The life cycle of Plasmodium involves several distinct stages in the insect and vertebrate
hosts. In infected mosquitoes, parasites in the salivary gland are called sporozoites.
When the mosquito bites a vertebrate host, sporozoites are injected into the host with the
saliva, enters the bloodstream and transported to liver, where they invade and replicate

17
within hepatocytes. The parasites that emerge from infected hepatocytes are called
merozoites and return to the blood to infect red blood cells.

Within the red blood cells, the merozoites grows and forms trophozoite which then
matures to schizonts, divide several times to produce new merozoites. The infected red
blood cell eventually bursts, allowing the new merozoites to travel within the
bloodstream to infect new red blood cells. Most merozoites continue this replicative
cycle, however some merozoites differentiate into male or female sexual forms called
gametocytes. These gametocytes circulate in the blood until they are taken up when a
mosquito feeds on the infected vertebrate host, taking up blood which includes the
gametocytes.

In the mosquito, the gametocytes move along with the blood meal to the mosquito’s
midgut and develops into male and female gametes which fertilize each other, forming a
zygote. Zygotes then develop into a motile form called an ookinete, which penetrates the
wall of the midgut. Upon traversing the midgut wall, the ookinete embeds into the gut’s
exterior membrane and develops into an oocyst. Oocysts divide many times to produce
large numbers of small elongated sporozoites. These sporozoites migrate to the salivary
glands of the mosquito where they can be injected into the blood of the next host the
mosquito bites, repeating the cycle (Figure 1.16).

Figure 1.16: Life cycle of Plasmodium vivax. It causes malaria and life cycle involve
two hosts. During a blood meal, infected female Anopheles mosquito inoculates
sporozoites into the human host. Sporozoites infect liver cells and mature into schizonts,
which ruptures and release merozoites. These move into the blood where they infect red
blood cells producing gametocytes which are taken up by insects which feed on the
vertebrate host. In the insect host, gametocytes merge to sexually reproduce. Parasites
grow into new sporozoites, which move to the insect’s salivary glands, from which they
can infect a vertebrate host bitten by the insect.

18
Coccidiosis is disease of poultry, sheep, cattle and rabbits. Infected animals spread spores
called oocysts in their stool. The oocysts mature, called sporulation. When another
animal passes over the location where the feces were deposited, they may pick up the
spores, which they then ingest when grooming themselves. The spores may also be
ingested by mice; when another animal eats the mouse it becomes infected inside the
host, the sporulated oocyst opens and eight sporozites are released. Each one finds a
home in an intestinal cell and starts the process of reproduction. These offspring are
called merozoites. When the cell is stuffed full of merozoites, it bursts open and each
merozoite finds its own intestinal cell to continue the cycle. As the infection continues,
thousands and thousands of intestinal cells may become infected. As they break open,
they produce a bloody, watery diarrhea. This can cause dehydration, and can lead to
death in young or small pets (Figure 1.17).

Figure 1.17: Life cycle of coccidian. Parasite has life cycle where definitive host ingests
oocysts containing sporozoites which are released and enter intestinal epithelium where
they undergo merogony. Then, gametogony occurs and macro/microgametes unites to
form oocytes which are passed in feces and matures into oocysts.Finally, sporozoites
formed in the oocyst through meiosis.

4. Phylum Microspora
Members of this species are parasites of vertebrates including humans in which they can
cause microsporidiosis. Some species are lethal and few are used in biological control of
insect pests. They produce highly resistant spores, capable of surviving outside their host
for up to several years. Spore morphology is useful in distinguishing between different

19
species. Spores of most species are oval but rod-shaped or spherical spores are not
unusual. (Figure 1.18).

Figure 1.18: Microsporidian spore. Unicellular parasite which forms spores.

In the gut of the host the spore germinates, it builds up osmotic pressure until its rigid
wall ruptures at its thinnest point at the apex. The posterior vacuole swells, forcing the
polar filament to rapidly eject the infectious content into the cytoplasm of the potential
host. Simultaneously the material of the filament is rearranged to form a tube which
functions as a hypodermic needle and penetrates the gut epithelium.Once inside the host
cell, a sporoplasm grows, dividing or forming a multinucleate plasmodium, before
producing new spores. The life cycle varies considerably. Some have a simple asexual
life cycle, while others have a complex life cycle involving multiple hosts and both
asexual and sexual reproduction.

The infective form of microsporidia is the resistant spore and it can survive for a long
time in the environment. The spore extrudes its polar tubule and infects the host cell. The
spore injects the infective sporoplasm into the eukaryotic host cell through the polar
tubule. Inside the cell, the sporoplasm undergoes extensive multiplication either by
merogony (binary fission) or schizogony (multiple fission). This development can occur
either in direct contact with the host cell cytoplasm (e.g., E. bieneusi) or inside a vacuole
(e.g., E. intestinalis). Either free in the cytoplasm or inside vacuole, microsporidia
develop by sporogony to mature spores. During sporogony, a thick wall is formed around
the spore, which provides resistance to adverse environmental conditions. When the
spores increase in number and completely fill the host cell cytoplasm, the cell membrane
is disrupted and releases the spores to the surroundings. These free mature spores can
infect new cells thus continuing the cycle (Figure 1.19).

20
Figure 1.19: Life cycle Microsporida. (A) Microsporidian spore (B) Merogonic and
sporogonic stages. Spore injects the infective sporoplasm and inside cell sporoplasm
undergoes multiplication by merogony (binary fission). In cytoplasm, microsporidia
develops by sporogony to mature spores.

5. Phylum Acetospora
They are parasites of animals especially marine invertebrates. There are two groups, the
haplosporids and paramyxids, which are not particularly similar morphologically but
consistently group together on molecular trees, which place them near the base of the
Cercozoa. Both produce spores without the complex structures found in similar groups
(such as polar filaments or tubules). Haplosporid spores have a single nucleus and an
opening at one end, covered with an internal diaphragm or a distinctive hinged lid. After
emerging, it develops within the cells of its host, usually a marine mollusc or annelid,
although some infect other groups or freshwater species. The trophic cell is generally
multinucleate. Paramyxids develop within the digestive system of marine invertebrates,
and undergo internal budding to produce multi-cellular spores.

21
6. Phylum Myxozoa
Myxozoa is a group of aquatic, obligately parasitic cnidarian animals having about 1300
species. They have two-host life cycle, involving fish and annelid worm. The average size
ranges from 10 μm to 2mm and can live in both freshwater and marine habitats. They have
been evolved from free swimming, self-sufficient jellyfish-like creature into current form of
obligate parasites. During this process, they lost many genes responsible for multi-cellular
development, coordination and cell-cell communication. The genome of some myxozoans are
among the smallest genomes of any known animal species (Figure 1.20).

Figure 1.20: Cnidaria. An example of Myxozoa.

7. Phylum Ciliophora
They have small hair like outgrowths calle as cilia throughout the boy. Cilia are similar in
structure to flagella but smaller in size an more in number. They are used in swimming,
crawling, attachment, feeding and sensation. All behavioral patterns are coordinated by
signaling processes (Figure 1.21).

Figure 1.21: Paramecium. Hair-like organelles, cilia throughout the body and help in
movement, swimming, feeding and sensation.

22
1.5 Pseudopodia and Amoeboid Locomotion: Cilia and Other
Pellicular Structures
Cilia are arranged in rows called kineties. In some forms there are also body poly
kinetids, for instance, among the spirotrichs where they generally form bristles called
cirri. More often body cilia are arranged in mono- and dikinetids, which respectively
include one and two kinetosomes (basal bodies), each of which may support a cilium.
Trichocysyts are pellicular structures used for protection. They are rod-like or oval
organelles oriented perpendicular to plasma membrane. The pellicle can discharge
trichocysts, which then remain connected to body by sticky thread.

1.6 Nutrition: Genetic Control and Reproduction

Food vacuoles are formed by phagocytosis where food particles are digested and broken
down by lysosomes so the substances the vacuole contains are then small enough to
diffuse through the membrane of the food vacuole into the cell. Anything left in the food
vacuole is discharged by exocytosis. Most ciliates also have one or more prominent
contractile vacuoles, which collect water and expel it from the cell to maintain osmotic
pressure or in some function to maintain ionic balance. In some genera, such as
Paramecium, these have a distinctive star shape, with each point being a collecting tube.
Some ciliates are mouthless and feed by absorption (osmotrophy), while others are
predatory and feed on other protozoa and in particular on other ciliates. Some ciliates
parasitize animals, although only one species, Balantidium coli, is known to cause
disease in humans

Ciliates have two different sorts of nuclei: tiny, diploid micronucleus (generative nucleus
which carries the germ-line of the cell) and large polyploid macronucleus (vegetative
nucleus which controls cell regulation, expressing the phenotype of the organism). The
micronucleus passes its genetic material to offspring, but does not express its genes. The
macronucleus provides the nuclear RNA for vegetative growth.

Division of the macronucleus occurs by amitosis followed by chromosomes segregation.

Macronuclei regenerate from the micronuclei during conjugation. Two cells line up, the
micronuclei undergo meiosis, some of the haploid daughters are exchanged and then fuse
to form new micronuclei and macronuclei (Figure 1.22).

23
Figure 1.22: Reproduction in paramecium. Sexual reproduction through the the
process of conjugation.

Fission may occur spontaneously, as part of the vegetative cell cycle. Alternatively, it
may proceed as a result of self-fertilization (autogamy), or it may follow conjugation, a
sexual phenomenon in which ciliates of compatible mating types exchange genetic
material. While conjugation is sometimes described as a form of reproduction, it is not
directly connected with reproductive processes, and does not directly result in an increase
in the number of individual ciliates or their progeny (Figure 1.23).

Figure 1.23: Reproduction in ciliates. (A)Trnsverse fission (B) Budding (C) Palintomy,
multiple fission usually within a cyst (D) Strobiltion or catenulation (Form of transverse
fission).

24
Ciliate conjugation is a sexual phenomenon that results in genetic recombination and
nuclear reorganization within the cell. During conjugation, two ciliates of a compatible
mating type form a bridge between their cytoplasms. The micronuclei undergo meiosis,
the macronuclei disappear, and haploid micronuclei are exchanged over the bridge. In
some ciliates (peritrichs, chonotrichs and some suctorians), conjugating cells become
permanently fused, and one conjugant is absorbed by the other. In most ciliate groups,
however, the cells separate after conjugation, and both form new macronuclei from their
micronuclei. Conjugation and autogamy are always followed by fission (Figure 1.24).

Stages of Conjugation
In Paramecium caudatum, the stages of conjugation are as follows:
1. Compatible mating strains meet and partly fuse
2. The micronuclei undergo meiosis, producing four haploid micronuclei per cell.
3. Three of these micronuclei disintegrate. The fourth undergoes mitosis.
4. The two cells exchange a micronucleus.
5. The cells then separate.
6. The micronuclei in each cell fuse, forming a diploid micronucleus.
7. Mitosis occurs three times, giving rise to eight micronuclei.
8. Four of the new micronuclei transform into macronuclei and old macronucleus
disintegrates.
9. Binary fission occurs twice, yielding four identical daughter cells.

Figure 1.24: Stages of conjugation in paramecium caudatum. Each Paramecium has a

diploid (2n) micronucleus that undergoes meiosis creating four haploid (1n) micronuclei.
Three of the resulting nuceli disintegrate, the fourth undergoes mitosis. Daughter nuclei
fuse and the cells separate. The old macronucleus disintegrates and a new one is formed.
This process is usually followed by asexual reproduction.

25
1.7. Symbiotic Ciliates: Phylogenetic Considerations
Most are free-living but some are commensalistic or mutualistic with few parasitic
species. Balantidium coli is parasitic species causes the disease balantidiasis.
Balantidium coli has two developmental stages, trophozoite stage and cyst stage. In
trophozoites, the two nuclei are visible. The macronucleus is long and sausage-shaped,
and the spherical micronucleus is nested next to it, often hidden by the macronucleus.
The opening, known as the peristome at the pointed anterior end leads to the cytostome or
the mouth. Cysts are smaller than trophozoites and are round and have tough, heavy cyst
wall made of one or two layers. Usually only the macronucleus and sometimes cilia and
contractile vacuoles are visible in the cyst. Living trophozoites and cysts are yellowish or
greenish in color. Balantidium is the only ciliated protozoan known to infect humans.
(Figure 1.25).

Figure 1.25: Balantidium coli. (A) Trophozote (B) Cyst

Balantidium coli has two developmental stages: a trophozoite stage and a cyst stage. The
cyst is the infective stage of Balantium coli life cycle. Once the cyst is ingested via
feces-contaminated food or water, it passes through the host digestive system. The tough
cyst wall allows the cyst to resist degradation in the acidic environment of the stomach
and the basic environment of the small intestine until it reaches the large intestine. There,
excystation produces trophozoite from the cyst stage. The motile trophozoite then resides
in the lumen of the large intestine, feeding on intestinal bacterial flora and intestinal
nutrients. Trophozoites multiply by asexual binary fission or sexual conjugation. These
are released with the feces and encyst to form new cysts. Encystation takes place in the
rectum of the host as feces are dehydrated or soon after the feces have been excreted.
Cysts in the environment are then ready to infect another host.

26
Figure 1.26: Life cycle of Balantidium coli. Cyst ingested through food travels into
digestive system and after excystation in large intestine and produces trophozoite which
multiply by transverse binary fission and when released through fecesbecomes cyst again
which is the infective stage.

Further Phylogenetic Considerations

Protozoa probably originated about 1.5 billion years ago. Although known fossil species
exceed 30,000, they are of little use to find origin and evolution of various protozoan
groups. Only protozoa with hard parts (tests) have left much of fossil record.
Foraminiferans and radiolarians have well-established fossil records in Precambrian
rocks. Recent evidences are leund on the basis of study of base sequences in ribosonsal
RNA. Some evidences indicate that all the seven protozoan phyla have different origins.
Therefore, each group is different from other. Thus protozoan have been given the status
of kingdom. Additional modifications in this scheme of classification have proposed. It is
possible that this scheme of classification with be changed in future.

27
Key Points/Summary
1. Kingdom protista is polyphyletic group arose about 1.5 billion years ago from
Archea. Evolutionary pathways leading towards modern protozoa are uncertain.
2. Protozoa are both single cells and entire organism. Organelles specialized for
unicellular lifestyle carries many of their functions, while many protozoans lives in
symbiotic relationship with other organisms oftenly host-parasite relationship.
3. Sarcomastigophoreans possess pseudopodia and one to many flagella.
Phytomastigophoreans are photosynthetic including Euglena, while
Zoomastigophoreans are heterotrophic including trypanosome which causes
sleeping sickness. Sarcodina include freshwater genera such as Amoeba (uses
pseudopodia for feeding and locomotion) and parasitic genera such as Entamoeba.
Marine amoeba include foraminiferans and radiolarians.
4. Labyrinthomorphans have ectoplasmic network with spindle-shaped non-amoeboid
cells and mostly marine. Members of Apicomplexa are all parasites including
plasmodium which causes malaria. Some apicomplexans have three part life cycle
including schizogony, gametogony and sporogony.
5. Phylum microspora consists of small protozoa that are intracellular parasites and
transmitted from one host to the next as a spore, the form from which this group
obtains its name.
6. Phylum Acetospora contains protozoa that produce spores lacking polar capsules.
These are primarily parasitic in molluscs. Memebers of phylum Myxozoa consists
of parasitic species usually in fishes.
7. Phylum Ciliophora contains some of the most complex of all protozoa. Its members
possess cilia, a macronucleus and one or more micronuclei.
8. Evolutionary relationships are difficult to determine. Ribosomal RNA sequence
comparisons indicate that each of the seven protozoan phyla probably had separate
origins.

Self Assessment Questions

Q: Answer the following questions:
1. What do you meant by protozoa? In what ways they resemble animals so that
they are known as animal-like protists.
Ans: See introduction.
2. Explain the origin of protozoans from evolutionary point of view.
Ans: See 1.1
3. Explain the mode of nutrition, excretion and reproduction among protists.
Ans: See 1.2
4. What are pseudopodia? Write about various psudopoal movements as well
their mechanism in amoeboid locomotion.
Ans: See 1.4
5. Explain life cycle of plasmodium vivax.
Ans: See 1.4

28
Q: Fill in the blanks with appropriate answers.
1. Animal like protists are also named as ………………… (Protozoa)
2. Locomotory organelles among protozoans are …………………………
(Cilia, Flagella, Pseudopodia)
3. Intracellular mode of nutrition is……………………. While ………………..
is extracellular nutrition. (Endocytosis, Exocytosis)
4. ………………… is a relationship between two or more organisms that live
closely together. (Symbiosis)
5. An ……………….. is totally dependent on the host to complete its life cycle
while ……………..do not. (Obligate parasite , Facultative parasite)
6. Parasites that live on the outside of the host are called
……………………..while ………………………. lives inside the host.
(Ectoparasites, Endoparasites)
7. …………………. is freshwater phytomastigophorean. (Euglena)
8. African Sleeping Sickness caused by ……………. (Trypanosoma brucei)
9. …………………………. is the only ciliated protozoan known to infect
humans. (Balantidium coli )
10. ……………… and ………………….have well-established fossil records in
Precambrian rocks. (Foraminiferans, Radiolarians)

29
References
 Cartwright, Paulyn (2015). "Genomic insights into the Evolutionary Origin of
Myxozoa within Cnidaria" (PDF). Proc. Natl. Acad. Sci. U.S.A. 112: 14912–7.
 Cavalier-Smith, T (1995). "Zooflagellate Phylogeny and Classification.".
Tsitologiia. 37 (11): 1010–29.
 Giere, Olav (2009). Meiobenthology: the Microscopic Motile fauna of Aquatic
Sediments (2nd ed.). Berlin: Springer.
 Hopla, C.E.; Durden, L.A.; Keirans, J.E. "Ectoparasites and Classification"
(PDF). Rev. sci. tech. Off. int. Epiz. 13 (4): 985–1017.
 Thomas Cavalier-Smith & Ema E.-Y. Chao (2003). "Phylogeny of Choanozoa,
Apusozoa, and other Protozoa and Early Eukaryote Megaevolution". JOURNAL
OF MOLECULAR EVOLUTION. 56 (5): 540–563.

30
UNIT-2

MULTICELLULAR AND TISSUE

LEVEL OF ORGANIZATION

Written by: Arshad Mehmood Qamar

Reviewed by: Dr. Muhammad Waseem

31
 CONTENTS
Introduction ....................................................................................................... 33

Objectives ......................................................................................................... 33

2.1 Evolutionary Perspectives ..................................................................... 34

2.2 Phylum Porifera .................................................................................... 38

2.3 Phylum Cnidaria (Coelcenterate) .......................................................... 44

2.4 Phylum Ctenophora .............................................................................. 55

32
Introduction

The scientists and common people are much interested in the multicellular level of
organization. The swimmers of the tropical water have found Physalia physalis. It is
commonly known as Portuguese man war. Such organisms are composed of group of
cells. These groups of cells are specialized for various functions. These cells are
independent. The simple multicellular organization includes three phyla: Phylum
Porifera., Phylum Cnidaria and Phylum Ctenophora.

Multicellular life has formed approximately 550 million years ago. But at that time there
we are only 10% multicellular organisms. Multicellular life arises in Precambrian and
Cambrian boundary. The scientists take this period as evolutionary explosion. All the
animal phyla appeared during this evolutionary era. Now 15 to 20 groups of multicellular
animals have become extinct. The multicellular animals have becoming extinct since
their origin.

Objectives
After completion of this unit, you will be able to:
 describe how multicellularity have arisen in the animal kingdom
 use natural sponges are used for cleaning purposes.
 value branching canal system to a sponge
 discuss how soft-bodied cnidarians support themselves.
 identify the hazards related to blue, gas –filled floats washed up on beaches of
temperate and tropical water.
 observe precautionary measures to avoid swimming in coastal water.
 explain coastal organisms which form coral reefs.

33
2.1 Evolutionary Perspectives
Animals with multicellular and tissue level of organization have captured the interest of
scientists and laypersons alike. A bright blue float invited me to come closer,. I was not
threatened, rather I was fascinated by this beauty As I swam nearer I could see that
hidden from my previous view was an infrastructure of tentacles, some of which dangled
nearly nine meters below the water’s surface! The creature seemed to consist of many
individuals and I wondered whether or not each individual was the same kind of being
because, when I looked closely, I counted eight different body forms!

I was drawn closer and the true nature of this creature was painfully revealed. The beauty
of the gas filled float hid some of the most hideous weaponry imaginable. When I
brushed against those silky tentacles I experienced the most excruciating pain Swimmers
of tropical waters who have come into contact with physalia physalis, the Portuguese
man-of-war, know that this fictitious account rings true in organisms such as physalia
physalis, cells are grouped, specialized for various function, and interdependent. This
chapter covers three animals phyla whose multicellular organization varies from a loose
association of cells to cells organized into two distinct tissue layers. These phyla are the
Porifera, Cnidaria and Ctenophora.

2.1.1 Origins of Multicellularity

Multicellular life has been a part of the earth’s history for approximately 550 million
years. Although this seems a very long time, it represents only 10% of the earth’s
geological history. Multicellular life arose quickly in the 100 million years prior to the
Precambrian/Cambrian boundary, in what scientists view as an evolutionary explosion.
These evolutionary events resulted not only in the appearance of all of the animals phyla
recognized today, but also 15 to 20 animals groups that are now extinct. Since this initial
evolutionary explosion, most of the history of multicellular life has been one of
extinction.

The evolutionary events leading to multicellularity are shrouded in mystery. 1 Many

zoologists believe that multicellularity could have arisen as dividing cells remained
together, in the fashion of many colonial protists. Although variations of this hypothesis
exist, they are all treated here as the colonial hypothesis (Fig.2.1& 2.3).

A second proposed mechanism is called the syncytial hypothesis (Figure 2.1 & 2.3). A
syncytium is a large, multinucleate cell. The formation of plasma membranes in the
cytoplasm of syncytial Protist could have produced a small, multicellular organism. Both
the colonial and syncytial hypotheses are supported by the colonial and syncytial
organization that occurs in some protest phyla.

34
Figure 2.1: Physalia physalis, the Portuguese Man-of-War. The bluish float is about 12
cm long, and the nematocyst-laden tentacles can be up to 9 m long. Nematocysts are
lethal to small vertebrates and dangerous to humans. Note the fish that the tentacles have
captured. Digestion will eventually leave only the fish’s skeletal remains.

2.1.2 Animal Origins

A fundamental question concerning animals origins is whether animals are monophyletic
(derived from a single ancestor) diphyletic (derived from two ancestors), as polyphyletic
(derived from many ancestors). The view that animals are polyphyletic is attractive to a
growing number of zoologists. The nearly simultaneous appearance Animal-like Protists
and Animalia of all animal phyla in fossils from the Precambrian/Cambrian boundary is
difficult to explain if animals are monopyletic. If animals are polyphyletic, more than one
explanation of the origin of multicellularity could be possible, and more than one body
form could be ancestral. Conversely, the impressive similarities in animal cellular
organization support the view that all or most animals are derived from a single ancestor.
For example, asters (see Fig 2.2 ) form during mitosis in most animals, certain cell
junctions are similar in all animal cells, most animals produce flagellated sperm,
assuming polyphyletic origins. If you assume one or two ancestral lineages, then only one
or two hypotheses regarding the origin of muticellularity can be correct (box 2.1).

35
Figure 2.2: Evolutionary Relationships of the Poriferans and the Radiate Phyla.
Members of the phylum Porifera are derived from ancestral protozoan stocks
independently of other animal phyla. The radiate animals (shaded in orange) include
member of the phyla Cnidaria and Ctenophora. This figure shows a diphyletic origin of
the animal kingdom in which sponges arise from the protists separate from other animals.
Other interpretations of sponge origins are discussed in the text.

Figure 2.3: Two Hypotheses Regarding the Origin of Multicellularity. (a) The colonial
hypothesis. Multicellularity may have arisen when cell that a dividing protest produced
remained together. Cell imagination could have formed a second cell layer. This
hypothesis is supported by the colonial organization of some Sacromastigophora. (The
colonial protest and the two-layered redial ancestor are shown in sectional views.) (b)

36
The syncytial hypothesis. Multicellularity could have arisen when plasma membranes
formed within the cytoplasm of a large, multinucleate protest. Multinucleate bilateral
ciliates support this hypothesis.

BOX 2.1 ANIMAL ORIGINS- THE CAMBRIAN EXPLOSION

The geological timescale is marked by significant geological and biological events,

including the origin of the earth about 4.6 billion years ago, the origin of life about 3.5
billion years ago )see box 11.1), the origin of eukaryotic life-forms about 1.5 billion years
ago (see box 3.1), and the origin of animals about 0.6 billion year ago. The latter event
marks the beginning of the Cambrian perin. Animals originated relatively late in the
history of the earth-in only the last 10% of the earth’s history. During a geologically brief
100-million-year period, all modern animal phyla (along with other animals that are now
extinct) evolved. This rapid origin and diversification of animals is often referred to as
“the Cambrian explosion.”

Scientists have asked important questions about this explosion since Charles Darwin.
Why did it occur so late in the history of the compared of the earth? The origin of
multicellularity seems a relatively simple step compared to the origin of life itself. Why
do no fossil records document the series of evolutionary changes during the evolution of
the animal phyla? Why did animal life evolve so quickly? Paleontologists continue to
search the fossil record for answers to these questions.

One interpretation regarding the absence of fossils during this important 100-million-year
period is that early animals were soft bodied and simply did not fossilize. Fossilization of
soft-bodied animals is less likely than fossilization of hard-bodied animals, but it mals
include very rapid covering by sediments that creates an anoxic environment that
discourages decomposition. In fact, fossil beds containing soft-bodied animals have been
know for many year.

The Ediacara fossil formation, which contains the oldest known animal fossils, consists
exclusively of soft-bodied forms. Although named after a site in Australia, the Ediacara
formation is worldwide in distribution and dates to Precambrian times. This 700-million-
years-old formation gives few clues to the origin of modern animals. However, because
paleontologists believe it represents an evolutionary experiment that failed. It contains no
ancestors of modern animal phyla.

A slightly younger fossil formation containing animal remains is the Tommotian

formation-named after a locale in Russia. It dates to the very early Cambrian period, and
it also contains only soft-bodied form. At one time, the animals present in these fossil
beds were assigned to various modern phyla, including the Porifera and Cnidaria, but
most paleontologists now agree that all Tommotion fossils represent unique body forms
that arose early in the Cambrian period and disappeared before the end of the period,
leaving no descendants in the modern animal phyla.

37
A third fossil formation containing soft-bodied animals provides evidence of the results
of the Cambrian explosion. This fossil formation, called the Burgess Shale, is in Yoho
National Park in the Canadian Rocky Mountains of British Columbia. Shortly after the
Cambrian explosion, mud slides rapidly buried thousands of marine animals under
conditions that favored fossilization. These fossil beds provide evidence of virtually all of
the 32 phyla described in this text, plus about 20 other animal body forms that are so
different from any modern animals that they cannot be assigned to any one of the modern
phyla (figure 1). These unassignable animals include a large swimming predator called
Anomalocaris and a soft-bodied, detritus- or algae-eating animal called Wiwaxia. The
Burgess Shale formation also has fossils of many extinct representatives of modern
phyla. For example, a well-known Burgess Shale animal called Sidneyia is a
representative of a previously unknown group of arthropods (insects, spiders, mites,
crabs).

2.2 Phylum Porifera

The Porifera (po-rif’er-ah) (L. porus pore + fera, to bear), or sponges, are primarily
marine animals consisting of loosely organized cells (Figure 2.4; table 2.1). The
approximately nine thousand species of sponges vary in size from less than a centimeter
to a mass that would fill your arms.

Characteristics of the phylum Porifera include:

1. Asymmetrical or radially symmetrical
2. Three cell types:” pinacocytes, mesenchyme cells, and choanocytes
3. Central caviy, or a series of branching chambers, through which water circulates
during filter feeding
4. No tissues or organs

2.2.1 Cell Types, Body Wall, and Skeletons

In spite of their relative simplicity, sponges are more than colonies of independent cells.
As in all animals, sponge cells are specialized for particular functions. This organization
is often referred to as division of labor.

This, flat cells, called pinacocytes, line the outer surface of a sponge. Pinacocytes may be
mildly contractile, and their contraction may change the shape of some sponges. In a
number of sponges, some pinacocytes are specialized into tubelike, contractile porocytes,
which can regulate water circulation (Figure 2.5 a) openings through porocytes are
pathways for water moving through the body wall.

Just below the pinacocyte layer of a sponge is a jellylike layer calld the mesohyl (Gr.
Meso, middle + hyl, matter). Amoeboid cells called mesenchyme cells move about in the
mesohyl and are specialized for reproduction, secreting skeletal elements, transporting
and storing food, and forming contractile rings around openings in the sponge wall.

38
Below the mesohyl and lining an inner chamber (s) are choanocytes, or collar cells.
Choanocytes (Gr. Choane, funnel + cyte, cell) are flagellated cells that have a collarlike
ring of microvilli, forming a netlike mesh within the collar. The flagellum creaters water
currents through the sponge, and the colar filters microscopic food particles from the
water (figure 2..5b). The presence of choanocytes in sponges suggests and evolutionary
link between the sponges, and a group of protists called choanoflagellates. This link is
dicussed further at the end of this chapter.

Sponges are supported by a skeleton that may consist of microscopic needlelike spikes called
spicules. Spicules are formed by take on a variety of shapes (figure 2.6). 2 Alternatively, the
skeleton may be made of sponging (a fibrous protein made of collagen), which is dried,
beaten, and washed util all cells are removed to produce a commercial sponge. The nature of
the skeleton is an important characteristic in sponge taxonomy.

Figure 2.4: Phylum Porifera. Many sponges are brightly colored with hues of red,
orange, green, or yellow. (a) Verongia sp. (b) Axiomella sp

Figure 2.5: Morphology of a Simple Sponge. (a) In this example, pinacocytes form the
out body wall, and mesenchyme cells and spicules are in the mesohyl. Porocytes that

39
extend through the body wall form ostia. ( b) Choanocytes are cells with a flagellum
surrounded by a collar of microvilli that traps food particles. Food moves toward the base
of the cell, where it is incorporated into a food vacuole and passed to amoeboid
mesenchyme cells, where digestion takes place. Blue arrows show water flow patterns.
The brown arrow shows the direction of movement of trapped food particles.

2.2.2 Classificatin of the Porifera

Phylum Porifera (po-rif-ah)*

The animal phylum whose members are sessile and either asymmetrical; body organized
around a system of water canals and chambers; cells nnot organized into tissues or
organs. Approximately 9,000species.

Class Calcarea (la;-kar’e-ah)

Spicules composed of calcium carbonate, spicules are needle shaped or have three or four
rays; ascon, leucon, or sycon body forms; all marine.Calcareous sponges. Grantia
(Seypha). Leucosolenia.

Class Hexactinellida (hex-act’’in-el-ah)

Spicules composed of silica and usually six rayed; spicules often fused into as intricate
lattice; cup or vase shaped: sycon or leucon body form; found at 450 to 900 m depths in
tropical West Indies and eastrn Pacific, Glass sponges Euplectella (Venus flower-basker).

Class Demospongiae (de-mo-spun-je-e)

Brilliantly coloted sponges with needle-shaped or four-rayed siliceous spicules or
sponging or both; leucon body form; up to 1m in height and diamtere.Includes one family
of freshwater sponges, Sportigillidae, and the bath sponges. Cliana, Sportgilla

2.2.3 Water Currents and Body Forms

The life of a sponge depends on the water currents that coanocytes crate. Water currents
bring food and oxygen to a sponge and carry away metabolic and digestive wastes.
Methods of food filtration and circulation reflect the body forms in the phylum.
Zoologists have described three sponge body forms.

The simplest and least common sponge body form is the Ascon (Figure 2.7 a) Ascon
spoges are vaselike. Ostia are the outer openings of porocytes and lead directly to a
chamber called the spongocoel. Choanocytes line the spongocoel, and their flagellar
movements draw water into the sponogocoel through the ostia. Water exits he sponge
through the osculum, which is a single, large opening at the top of the sponge.

40
Figure 2.6: Sponge Spicules. Photomicrograp of a variety of sponge spicules

In the sycon body form, the sponge wall appears folded (Figure 2.7b) water enters a
sycon sponge through openings called dermal pores. Dermal pores are the openings of
invaginations of the body wall, called incurrent canals. Pores in the obdy wall connect
incurrent canals to radial canals, and the radial canals lead to the spongocoel.
Choanocytes line radial canals (rather than the spongocoel), and the beating of coanocyte
flagella moves water from the ostia, through incurrent and radial canals, to the
spongocoel, and out the osculum.

Leucon sponges have an extensively branched canal systern (figure 2.7c) water enters the
sponge through ostia and moves through branched incurrent canals, which lead to
choanocytelined chambers. Canals leading away from the chambers are called excurrent
canals. Proliferation of chambers and canals has resulted in the absence of a spongocoel,
and often, multiple exit points (oscula) for water leaving the sponge.

In comples sponges, an increased surface ara for choanocytes results in large volumes of
water being moved through the sponge and greater filtering capabilities. Although the
evolutionary pathways in the phylum are complex and incompletely described, most
pathways have resulted in the leucon body form.

2.2.4 Maintenance Functions

Sponges feed on particles that range in size from 0.1 to 50 pm. Their food consists of
bacteria, microscopic algae, protists, and other suspended organic matter. The prey are
slowly drawn into the sponge and consumed. Large populations of sponges play an
important role in reducing turbidity of coastal waters. A single lucon sponge, I cm in
diameter and 10 cm high, can filter in excess of 20 liters of water every day recent
investigations have discovered that a few sponges are carnivorous. These deep-water
sponges (Asbestophama) can capture small crustaceans using spicule-covered filaments.

41
Figure 2.7: Choanocytes filter small, suspended food particles. water p[asses through
their collar near the base of the cell and then moves into a sponge chamber at the open
end of the collar. Suspended food is trapped on the collar and moved along microvilli to
the base of the collar, where it is incorporated into a food vacuole (see figure 2.5b).
digestion begins in the food vacuole by lysosomal enzymes and ph changes. Partially
digested food is passed to amoeboid cells, which distribute it to other cells.

Filtration is not the only way that sponges feed. Pinacocytes lining incurrent canals may
phagocytize larger food particles (up to 50 p.m). Sponges also may absorb by active
transport nutrients dissolved in seawater.

Because of extensive canal system and the circulation of large volumes f water through
sponges, all sponge cells are in close contact with water. Thus, nitrogenous waste
(principally ammonia) removal and gas exchange occur by diffusion.

Sponges do not have nerve cells to coordinate body functions. Most reaction result from
individual cells responding to sponges is at a minimum at sunrise and at a maximum just
before sunset because light inhibites the constriction of porocytes and other cells
surrounding ostia, keeping incurrent canals open. Other reactions, however, suggest some
communication among cells. For example, the rate of water circulation through a sponge
can drop suddenly without any apparent external cause. This reaction can be due only to
choanocytes ceasing activities more or less simultaneously, and implies some form of
internal communication. The nature of this communication is unknown. Amoeboid cells
transmitting chemical messages and ion movement over cell surfaces are possible control
mechanisms.

2.2.5 Reproduction
Most sponges are monoecious (both sexes occur in the same individual) but do not
usually self-fertilize because individual sponges produce eggs and sperm at different
times. Certain choanocytes lose their collars and flagella and undergo meiosis to form

42
flagellated sperm. Other coenocytes (and amoeboid cells in some sponges) probably
undergo meiosis to form eggs. Eggs are retained in the mesophyl of the parent. Sperm
cells exit one sponge through the osculum and enter another sponge with the incurrent
water Sperm are trapped by conanocytes and incorporated into a vacuole. The
choanocytes lose their collar and flagellum, become amoeboid, and transport sperm to the
eggs.

In most sponges, early development occurs in the mesohyl. Cleavage of a zygote results
in the formation of a flagellated larval stage. (A larva is an immature stage that may
undrgo a dramatic change in structure before attaining the adult body form.) The larva
breaks free, and water currents carry the larva out of the parent spnge. After no more than
2 days of a free-swimming existence, the larva settles to the substrate and begins to
develop into the adult body form (Figure 2.8a,b).

Asexual reproduction of freshwater and some marine sponges involved the formation of
resistant capsules, called gemmules, containing masses of amoeboid cells. When the
parent sponge dies in the winter, it releases gemmules, which can survive both freezing
and drying (figure 2.8c,d). When favourable conditions return in the spring, amoeboid
cells stream out of a tiny opening, called the micropyle, and organize into a sponge.

Some sponges possess remarkable powers of regeneration. Portions of a sponge that is

cut or broken from one individual regenerate new individuals.

Figure 2.8: Development of Sponge Larval Stages.

43
Self Assessment Questions
1. How do the colonial and syncytial hypotheses account for the origin of
multicellularity?
2. What evidence supports the idea that the kingdom animalia is polyphyletic?
3. What are three kinds of cells found in the Porifera, and what are their function?
4. What is the path of water circulating through (a) an ascon sponge, (b) a sycon
sponge, and (c) a leucon sponge?

2.3 Phylum Cnidaria (Coelentrata)

Members of the phylum Cnidaria (ni-dar’e-ah) (Gr. Knide, nettle) possess radial or
biradial symmetry. Biradial symmetry is a modification of radial symmetry in which a
singe plane, passing through a central axis, divides the animal into mirror images. It
results from the presence of a single or paired structure in a basically radial animal and
differs from bilateral symmetry in that dorsal and ventral surfaces are not differentiated.
Radially symmetrical animals have no anterior or posterior ends. Thus, terms of direction
are based on the position of the mouth opening. The end of the animal that contains the
mouth is the oral end, and the opposite end is the aboral end. Radial symmetry is
advantageous for sedentary animals because sensory receptors are evenly distributed
around the body. These organisms can respond to stimuli from all directions.

The Cnidaria include over nine thousand species, are mostly marine, and are important in
coral reef ecosystems.

Characteristics of the phylum Cnidaria include:

1. Radial or biradial symmetry
2. Diploblastic, tissue-level organization
3. Gelatinous mesoglea between the epidermal and gastro dermal tissue layers
4. Gastro vascular cavity
5. Nervous system in the form of a nerve net
6. Specialized cells, called cnidocytes, used in defense, feeding, and attachment

2.3.1 The Body Wall and Nematocysts

Cnidarians possess diploblastic, tissue-level organization (See Figure 2.10). Cells
organize into tissues that carry out specific function, and all cells are derived from two
embryological layers. The ectoderm of the embryo gives rise to an outer layer of the body
wall, called the epidermis, and the inner layer of the body wall, called the gastrodermis, is
derived from endoderm (figure 2.9). cells of the epidermis and gastrodermis differentiate
into a number of cell types for protection, food gathering, coordination, movement,
digestion, and absorption. Between the epidermis and gastrodermis is a jellylike layer
called mesoglea. Cells are present in the middle layer of some cnldarians, but they have
their origin in either the epidermis or the gastrodermis.

44
One kind of cell is characteristic of the phylum. Epidermal and/or gastrodermal cells
called cnidocytes produce structures called nematocytsts, which are used for attachment,
defense, and feeding. A nematocyst is a fluid-filled, intracellular capsule enclosing a
coiled, hollow tub (figure 2.10). A lidlike operculum caps the capsule at one end. The
cnidocyte has a modified cilium, called a cnidocil. Stimulation of the cnidocil forces ope
the operculum, discharging the coiled tube-as you would evert a sweater sleeve that had
been turned inside out.

Zoologists have described nearly 30 kind of nematocysts. Nematocysts used in food

gathering and defense may discharge a long tube armed with spines that penetrates the
prey. The spines have hollow tips that deliver paralyzing toxins. Other nematocysts
contain unarmed tubes that wrap around prey or a substrate. Still other nematocysts have
sticky secretions that help the animal anchor itself. Six or more kinds of nematocysts may
be present in one individual.

TABLE 2.2: Classification of the Cnidaria

Phylum Cnidaria (ni-dar’e-ah)
Radial or biradial symmetry, diploblastic organization,a gastrovascular cavity and
cnidocytes. Over 9,000 species.
Class Hydrozoa (hl’’fro-zo’ah)
Cnidocytes present in te epidermis; gametes produced epidermally and always release
to the outside of the body: no wandering mesenchyme cells in mesoglea; medusa
usually with a velum; many polyps colonial; mostly marine with some freshwater
species. Hydra, Obelia, Ganianemus and physalia.
Class Scyohizia (si’’fo-zo’ah)
Colonial or solitary polyps; medusa absent ; cnidocytes present in the gastrodermis;
gametes gastrodermal in origin; gastrovascular cavity divided by mesenteries that bear
nematocysts; internal bi or bilaterial symmetry present; mesoglea with wandering
mesenchyme cells; marine. Anemones and corals. Metridium.

Figure 2.9: Body Wall of a Cnidarian (Class Anthozoa). Cnidarians are diploblastic (two
tissue layers). The epidermis is derived embroyologically from ectoderm, and the

45
gastrodermis is derived embryologically from endoderm. Between these layers is
mesoglea. Source: after Bullock and horridge.

2.3.2 Alternation of Generation

Most cnidarians possess two body forms in their life histories (Figure 2.11) the polyp is
usually asexual and sessile. It attaches to a substrate at the aboral end, and has a
cylindrical body, called the column, and a mouth surrounded by food-gathering tentacles.
The medusa (pl.,medusa) is dioecious and free swimming. It is shaped like an inverted
howl, and tentacles dangle from its margins. The mouth opening is centrally located
facing downward, and the medusa swims by gentle pulsation of the body wall. The
mesoglea is more abundant in a medusa than in a polyp, giving the former a jellylike
consistency.

Figure 2.10: Cnidocyte Structure and Nematocyst Discharge. (a) A nematocyst develops
in a capsule in the cnidocyte. The capsule is capp0ed at its outer margin by an operculum
(lid) that is displaced upon discharge of the nematocyst. The trigger like cnidocil is
responsible for nematocyst discharge. (b) A discharged nematocyst. When the cnidocil is
stimulated, a rapid (osmotic) indlux of water causes the nematocyst to Evert, first near its
base, and then progressively along the tube from base to tip. The tube revolves at
enormous speeds as the nematocyst is discharged. In nematocysts armed with harbs, the
advancing rip of the tube is aided in its penetration of the prey as barbs spring forward
from the interior of the tube and then flick backward along the outside of the tube.

46
Figure 2.11: Generalized Cnidarian Life cycle. This figure shows alternation between
medusa and polyp body forms. Dioecious medusa produce gametes that may be shed into
the water for fertilization.

2.3.3 Maintenance Functions

The gastrodermis of all cnidarians lines a blind ending gastrovascular cavity. This vavity
functions in digestion, the exchange of repiratory gases and metabolic wastes, and the
discharge of gametes. Food, digestive wastes, and reproductive stage enter and leave the
gastrovascular cavity through the mouth.

The food of most cnidarians consists of very small crustaceans, although some cnidarians
feed on small fish (Figure 2.11) nematocysts entangle and paralyze prey, and contractile
cells in the tentacles cause the tentacles to shorten, which draws food toward the mouth.
As food enters the gastrovascular cavity, gastrodermal gland cells secrete lubricating
mucus and enzymes, which reduce food to a soupy broth. Certain gastrodermal cells,
called nutritive muscular cells, phagocytize partially digested foor and incorporate it into
food vacuoles, where digestion is completed. Nutritive muscular cells also have circularly
oriented contractile fibers that help move material into or out of the gastrovascular cavity
by peristaltic contractions. During peristalsis, ringlike contractions move along the body
wall, pushing contents of the gastrovascular cavity ahead of them, expelling undigested
material through the mouth.

Cnidarians derive most of their support from the buoyancy of water around them. In
addition, a hydrostatic skeleton aids in support and movement. A hydrostatic skeleton is
water or body fluids confined in a cavity of the body and against wiich contractile
elements of the body wall act. In the Cnidaria, the water-filled gastrovascular cavity acts
as a hydrostatic skeleton. Certain cells of the body wall, called epitheliomusclar cells, are

47
contractile and aid in movement. When a polyp closes its mouth (to prevent water from
escaping) and contracts longitudinal epitheliomuscular cells on one side of the body, the
polyp bends toward that side. If these cells contract while the mouth is open, water
escapes from the gastrovascular cavity, and the polyp collapses. Contraction of circular
epitheliomuscular cells causes constriction of a part of the body and, if the mouth is csed,
water in the gastrovascular cavity is compressed, and the polyp elongates.

Polyps use a variety of form of locomotion. They may move be somersaulting from base
o tentacles and from tentacles to base again, or move in an inchworm fashion, using their
base and entacles as points of attachment. Polyps may also glide very slowly along a
substrate whl attached at their base or walk on their entacles.

Medusa move by swimming and floating. Water currents movements are the result of
swimming. Contractions of circular and radial epitheliomuscular cells cause rhythmic
pulsations of the bell and drive water from beneathte bell, propelling the medusa through
the water.

Cnidarians nerve cells have been of interest to zoologists for many years because they
may be the most primitive nervous elements in the animal kingdom. By studing these
cells, zoologists may gain insight into the evolution of animal nervous systems. Nerve
cells are located below the epidermis, near the mesoglea, and interconnect to form a two-
dimensional nerve net. This net conducts nerve impulses around the body in reponse to a
localized stimulus. He extent to which a nerve impulse spreads over the body depends on
stimulus strength. For example, a weak stimulus applied dto a polyp’s tentacle may cause
the tentacle to be retracted. A strong stimulus at the same point may cause the entire
polyp to withdraw.

Sensory structures of cnidarians is distributed throughout the body and include receptors
for perceiving touch and certain chemicals. More specialized receptors are located at
specific sites on a polyp or medulla.

Because cnidarians have large surface area to volume rations, all cells are a short distance
from the body surface, and oxygen, carbon dioxide, and nitrogenous wastes exchange by
diffusion.

2.3.4 Reproduction
Most cnidrians are dioecious. Sperms and eggs may be released into the gastrovascular
cavity or to the outside of the body. In some cases eggs retain inside the body of parents
till after fertilization.

A blastula forms early in development and migration of surface cells to the interior fills
the embryo with cells that will form the gastrodermis. A swimming larva, called Planula
is form by elongation of embryo. This panula is get attached with some stone or
substrate. During this time interior cells form into a gastrovascular cavity which develops

48
into a young polyp. A number of other polyps are formed by process of budding. Some
polyps remain attached with the parent body.

Self Assessment Questions

Q.1 Describe three layers of the cnidarians body wall?
Q.2 What is nematocyst? Write its functions.
Q.3 Draw generalized life cycle of Cnidarian.
Q.4 How would you characterize the nervous organization of cnidarians?
Q.5 What is hydrostatic skeleton? How does this function in cnidarians?

2.3.5 Classification up to Class

A) Class Hydrozoa
Hydrozoans are small, relatively common cnidarians. Majority is marine, but the
members of this class found in freshwater. Most of them show alternation of generation,
however in some medusa is lost, while in others the polyp stage is very small.

Three features distinguish hydrozoans from cnidarians:

a) nematocysts are only in the epidermis.
b) gametes are epidermal and released to the outside of the body rather than into the
gastrovascular cavity, and
c) the mesoglea never contains amoeboid mesenchyma cells.

Most hrdrozoans have colonial polyps in which individuals may be specialized for
feeding, producing medusa by budding or defending the colony. In Obelia, a common
marine cnidarians, the planula develops into a feeding polyp, called gastrocoid. A
gastrocoid has tentacles, feeds on microscopic organism in the water, and secrets a
skeleton of protein and chitin, called the perisarc around itself.

Growth of an Obelia colony results from budding of the original gastrocoid. Roots grow
which anchor the colony and give rise to branch colonies. The entire colony has a
continuous gastrovascular cavity ,body wall, and perisarc and is a few centimeters high.
A gonozoid is a reproductive polyp that produce medusae by budding. When medusa
mature, they break free of the stalk and swim out an opening at the end of the gonozoid.
Medusae reproduce sexually to give rise to more colonies of polyps.

49
Figure 2.12: Structure and life cycle of Obelia

Hydra is a common freshwater hydrozoan that hangs from the underside of floating
plants in clean streams and ponds. Hydra lacks medusa stage and reproduces both
sexually by budding from the side of the polyp and sexually.Hydras are somewhat
unusual hydroazoans because sexual reproduction occurs in polyp stage. Testes are the
conical elevations of the body surface that form from mitosis of certain epidermal cells,
called interstitial cells. Sperm form by meosis in the testes.Mature sperm exit the testes
temporary openings. Ovaries also form from interstitial cells. One large egg forms per
ovary. Durng egg formation, yolk is incorporated into the egg cell from gastrodermal
cells. As ovarian cells disintegrate, a thin stalk of tissues attaches the egg to the body
wall. After fertilization, resistant chitinous shell is layed down by epithelial cells. The
embryo detaches from the parent in winters, hatches in spring and develop into an adult.

Figure 2.13: A Structure and life cycle of Hrdra

50
Physalia physalis ,commonly called Portugese man-of war. It is large colonial
siphnophore. It lacks swimming capabilities and move at the mercy of wind and waves.
Its cnidocytes –laden dactylozoids are lethal to small vertebrate and dangerous for human
beings.

Other features of Physalia physalis are given in the fig.2.14.

Figure 2.14: structure and features of Physalia Physalis

B) Class Scyphozoa
All members of this class are marine and are true jelly fish . The dominant stage is
medusa in jelly fishes.Medusae of scyphozoans lack a velum, the mesoglia contains
amoeboid mesenchyma cells, cnidocytes occur in the gastrodermis as well as epidermis,
and gametes are gastrodermal in origin.

Many scyphozoans are harmless to human beings. Some have unpleasant and even
dangerous stings, for example Mastigias quinquecirrha is called stinging nettle, is a
common Atlantic scyphozoan. Their population increase in late summar and become
hazardous to swimmers. A rule of thumb for swimmers is to avoid helmet-shaped jelly
fish with long tantacles and fleshy lobes hanging from the orla surface.

51
Figure 2.15: Representative Scyphozoans.

Kingdom of Cyanea capillata is Animalia, Phylum is Cnidaria, Class: Scyphozoa,

Order: Semaeostomaeida, Family: Cyaniidae, Scientific name: Cyanea capillata and
common name is Lions’s mane jelly fish, sea blubber or sea nettle.

The Lion's mane jellyfish is vivid yellow, orange or sometimes even red. The flattened
umbrella may reach a diameter of 2 meters. Swimmers should be aware. It is very
unpleasant to get tangled in the sometimes 30 meters long tentacles. As most other
jellyfishes, they reproduce in spring and die in the winter. Some survive the winter as
larvae.

Aurelia is a common scyphozoan in both pacific and Atlantic coastal water of North
America. The margins of its medusa has a fringe of short tantacles and is divided by
notches. The mouth of Aurelia leads to a stomach with four gastric pouches, which
contain cnidocytes-laden gastric filaments. Radial canals lead from gastric pouches to the
margin of the bell.

Aurelia is a plankton feeder. At rest it sinks slowly in the water and traps microscopic
animals in mucus on its epidermal surfaces. Cilia carry this food to the margins of
medusa. Four fleshy lobes, called oral lobes, hang from the manubrium and scraoe food
from the margin of the medusa. Cilia on the oral lobes carry food to the mouth.

In addition to sensory reception on the epidermis, Aurelia has eight specialized structures,
called rhopalia in the notches at the margin of medusa.

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Each rhopalium consists of sensory structures surrounded by rhopalial lappets. A
statocyst and a photoreceptor is associated with rhopalia.

Figure 2.16: Aurelia sp (an example of Scyphozoa)

Aurelia displays a distinct negative phototaxis, coming to the surface at twilight and
decending to greater depths during bright daylight. Scyphozoans are dioecious. Aurelia
eight gonads are in gastric pouches, two per pouch. Gametes are released into the gastric
pouches. Sperms swim through the month to the outside of the medusa. In some
scyphozoans, eggs are fertilized in the female gastric pouches, and early development
occurs there. In Aurelia eggs lodge in the oral lobes, where fertilization and development
take place. Planula develops into a polyp called a scyphystoma. Scyphistoma lives a year
or more. Durng this time a mini bud produces a medusa,called a ephyrae. Repeated
budding of the scyphistoma results in ephyrae being stacked on the polyp. When ephyrae
detach they become adult. Structure and life cycle of Aurelia can be understood by
understanding the Figure 2.17 in the following.

Figure 2.17: Structure and Life cycle of Aureliasp.

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C) Class Cubozoa
The medusa in cuboidal and tantacles hang from each of its corners.Polyps are very small
and, in some species are not knowen. Cubozoans are active swimmers and feeders in
warm tropical waters. Some have dabgerous nematocytess. See Figure 2.18.

Figure 2.18: Cubozoa

D) Class Anthozoa
The animals in this class lack medusa. They include anemone and corals. Antozoans are
all marine and found at all depths of sea.

Anthozoans polyps differ from hydrozoan polyps in three ways.

i. The mouth of an antozoans leads to pharynx, which is an invagination of the body
wall that leads into the gastrovascular cavity.
ii. Mesenteries that bear cnidocytes and gonads on their free edge divide the
gastrovascular cavity into sections. &
iii. The mesoglea contains amoeboid mesenchyme cells.

Externally anthozoans appear to show perfect radial symmetry. Internally, the

mesenteries and other structures convey bilateral symmetry to members of this class.

Sea anemones are solitary, colourful and larger in size. Some attach to solid substrate,
and some burrow in soft substrates and some live in symbiotic relationship. The polyp
attaches to its substrate by a pedal disk.An oral disk contains the mouth and hollow oral
tentacles. At slit like mouth lies a siphonoglyph ,a ciliated tract and moves water into
gastrovascular cavity to maintain the hydrostatic skeleton.

Anemones have limited locomotion. They glide on their pedal discs, crawl on their side,
and walk on their tentacles. Some anemone float using a gas bubble held within folders of
the pedal disk.

Anemones feed on invertebrates and fishes. Tentacles capture prey and draw it towards
mouth. Radial muscle fibers in the mesenteries open the mouth to receive the food.

54
Anemones show both sexual and asexual reproduction. In a-sexual reproduction a piece
of pedal disk may break away from the polyp and grow into a new individual .this
process is termed as pedal laceration. Further longitudinal and transverse fission may take
place and divide the individual into two, and missing parts are regenerated.

Figure 2.19: Class Anthozoa. Structure of anemones, metridium sp.

2.4 Phylum Ctenophora

Animals in this phylum are called sea walnuts or comb jellies. Almost 90 species are
marine. Most of them have spherical form, although several groups are flattened or
elongated.

2.4.1 Characteristics
Characteristics of phylum Ctenophora are following:
1. Diploblastic , tissue level organization
2. Biradial symmetry
3. Gelatinous mesoglia between the epidermal and gastrodermal tissue layers.
4. Gastrovascular cavity
5. Nervous system in the form of a nerve net
6. Adhesive structure called colloblasts
7. Eight rows of ciliary bands, called comb rows, for locomotion.

Pleurobranchia has a spherical or ovoid, transparent body about 2cm in diameter. It

occurs in the colder waters of the Atlantic and Pacific Oceans. Pleurobranchia, like most
ctenophorans, has eight meridional bands of cilia, called comb rwo, between the oral and
aboral poles. Comb rows are locomotor structures that are coordinated through a statocyst

55
at the aboral pole. Pleurobranchia normally swims with its aboral pole oriented
downward. The statocyst detects tilting, and the comb rows adjust the animal orientation.
Two long, branched tentacles arise from pouches near the aboral pole. Tentacles possess
contractile fibres that retract the tentacles, and adhesive cells, called colloblasts, that
captures prey.

Injestion occurs as the tentacles wipe the prey across the mouth. The mouth leads o a
branched blind gastrovascular canal system. Some canals are blind; however two small
and canals open to the outside near the apical sense organ. Thus unlike the cnidarians,
ctenophores have an analopening. Some undigested wastes are eliminated through these
canals, and some are probably also eliminated through the mouth.

Pleurobranchia is monoecious, as are all ctenophores. Two ban dlike gonads are
associated with the astrodermis. One of these is an ovary, and the other is a testis.
Gametes are shed through the mouth, fertilization is external and a slightly flattened larva
develops.

Figure 2.20: Phylum Ptenophora (Pleurobranchia)

Phylum Ptenophora is classified into two classes:

i. Class Tentaculata; with tentacles tht may or may not be associated with sheaths,
into which the tentacles can be reracted.
ii. Class Nuda; without tentacles, flattened, highly branched with gastrovascular
cavity.

2.4.2 Further Phylogenetic Considerations

The evolutionay position of the phyla is a subject of debate. If the animal kingdom is
polyphyletic, then all phyla could have had separate origins, although the scientists who
believe in multiple origins agree that the number of independent origins is probably
small. Some zoologists believe the animal kingdom to be at least diphyletic, with the
porifera coannocytes and coanoflagelates protists suggest evolutionary ties between these
groups. Many other zoologists believe that the sponge have a common, although remote,

56
ancestor with other animals. The amoeboid and flagellated cells in sponges and higher
animals support this view.

One thing that nearly everyone agrees upon is that, the porifera are evolutionary “dead
ends”. They gave rise to no other animal phyla.

If two origins are assumed, the origin of the non-poriferan lineage is also debated. One
interpretation is that the ancestral animal was derived from a radially symmetrical
ancestor, which in turn, may have been derived from a colonial flagellate similar in form
to Volvox. If this is true, then the radiate phyla could be closely related to the ancestral
group. Other zoologists contend that bilateral symmetry is the ancestral body from and
that a bilateral ancestor gave rise to both the radiate phyla and bilateral phyla. In this
interpretation, the radiate phyla are further removed from the base of the evolutionary
tree.

Self Assessment Questions

Q.1 What characteristics do the Ctenophora and the cnidarians share?
Q.2 What are colloblasts?
Q.3 How do Ctenophores use statocysts and comb row to maintain up right position in
the water?
Q.4 Why are the sponges considered evolutionary “ dead ends” ?
Q.5 Why hydrozoans have well defined alternation of generation? What hydrozoan has
Reduced alternation of generation?
Q.6 What kind of sensory structures occur in cnidarians?
Q.7 How do the following fit into the life history of a scyphozoan; a) scyphistoma b).
ephyra c). Planula.
Q.8 What is protandry? How does it apply to the Anthozoa?

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Summary/Key Points
1. Although the origin of multicellularity in animals is unkown, the colonial
hypotheses and the syncytical hypothesis are explanation of how animals could
have arisen. Whether the animal kingdom had origin in one, two or many ancestors
is debated.
2. Animals in the phylum porifera are the sponges. Cells of sponges are specialized to
create water currents, filter food, produce gametes, form skeletal elements and line
the songe body wall.
3. Sponge circulate water through their bodies to bring in food and oxygen and to
carry away wastes and reproductive products. Evolution has resulted in more
sponges having complex canal system and large water circulatory capabilities.
4. Hydrozoans differ from other cnidarians in having ectodermalgametes, mesoglea
without mesentchyme cells, and nematocytes only in their epidermis.
5. Most of the hydrozoans have well –developed polyp and medusa stages.
6. The class Scyphozoa contains the jelly fish. The polyp stage of ascyphozoans is
usually very small.
7. Members of the class Cubozoa live in warm, tropical waters. Some posses
dangerous nematocysts.
8. The Anthozoa lack the medusa stage. They include sea anemone and corals.
9. Members of the phylum Ctenophora are bilaterally symmetrical and diploblastic.
10. Bands of cilia, called comb rows, characteristize the Ctenophores.
11. Zoologists debate whether or not the porifera had a common origin with other
animals.
12. The cnidarians and ctenophore are distantly related phyla. Within the cnidaria, the
ancient hydrozoans are believed to be the stock from which modern hydrozoans
and other cnidarians evolved.

References
 Porifera: fossil Record. The Fossil History, Life History, Ecology, Systematic, and
Morphology of Sponges are described at this Site.
https://www.ucmp.berkeley.edu/porifera/poriferafr.html
 Introduction to Porifera. This site describes the evolutionary history, life
history,and ecology of the sponges.
https://www.ucmp.berkeley.edu/porifera/porifera.html
 Introductionto Cnidaria. This site describes the recent discovery of carnivorous,
crustacean-eating sponges.
https://outcast.gene.com/ae/WN/SU/carnivorous-sponges.html

58
UNIT-3

TRISPLOBALSTIC
AND ACOELOMATE BODY
PLAN OF ANIMALS

Written by: Dr. Sobia Mushtaq

Reviewed by: Arshad Mehmood Qamar
59
 CONTENTS
Introduction ....................................................................................................... 61

Objectives ......................................................................................................... 61

3.1 Evolutionary Perspectives; Phylum Platyhelminthes ........................... 62

3.2 Classification up to Class ...................................................................... 64

3.3 Phylum Nemetea ................................................................................... 85

3.4 Phylum Gastrotrichea............................................................................ 87

3.5 Further Phylogenetic Considerations .................................................... 88

60
Introduction

An acoelomate is defined as an animal that does not possess a body cavity. Unlike
coelomates (eucoelomates), animals with a true body cavity, acoelomates usually lack
fluid-filled cavity between the body wall and digestive tract. Acoelomates have a
triplobastic body plan, meaning that their tissues and organs develop from three primary
embryonic cell (germ cell) layers. These tissue layers are the endoderm (endo-derm) or
innermost layer, mesoderm (meso-, -derm) or middle layer, and the ectoderm (ecto-, -
derm) or outer layer. Different tissues and organs develop in these three layers. In
humans, for example, the epithelial lining that covers internal organs and body cavities is
derived from the endoderm. Muscle tissue and connective tissues such as bone, blood,
blood vessels and lymphatic tissue are formed from mesoderm. Urinary and genital
organs including the kidneys and gonads are also formed from mesoderm. Epidermis,
nervous tissue and specialized sense organs (eyes, ears, etc.) develop from the ectoderm.

Objectives
After completion of this unit, you will be able to:
 describe acoelomates body cavity and represented by phylum Platyhelminthes,
Nemertia and Gastrotricha.
 explain evolutionary perspectives of phylum Platyhelminthes
 classify phylum platehelmonthes up to class level
 compare characteristics of different classes
 find common features among different members of classes.
 use the free –living flatworms in our daily life.
 tell the importance and disadvantages of flatworms for human beings.
 draw structures of members of different classes and label them

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3.1 Evolutionary Perspective: Phylum Platyhelminthes
Members of the phyla Platyhelminthes, Nemertea and Gastrotricha show following
advance characters:
1. They are first bilaterally symmetrical animals. Therefore, they are more Complex
than the cnidarians.
2. All these animals are triploblastic.
3. They are acoelomate (lacks coelem or body cavity).

1) Phylum Platyhelminthes: They are normally flatworms. They can be free living
e.g, turbellarians or parasitic e.g, flukes and tapeworms.
2) Phylum Nemertea: It includes a small group of elongated, unsegmented and soft
bodied worms. They exist in marine form or they can be free living.
3) Phylum Gastrotricha: These animals live in the space between bottom sediments.
The evolutionary relationship of Platyhelminthes to other phyla is controversial.
There are three views about this evolution:

a) First View: Evolution from Radial Diploblastic Animals

Triploblastic acoelomate is a group between the radial, diploblastic and the triploblastic
coelomate plan. Therefore, the flatworms have evolutionary side branch from
triploblastic acoelomate ancestor. Thus evolution of flat worms took place from radial
ancestors. It formed a larval stage. This larva became sexually mature by process known
as paedomorphosis.

b) Second View: Evolution from Bilateral Ancestor

Other zoologists believe that triploblastic acoelomate was formed from a bilateral
ancestor. Primitive acoelomates and triploblastic were formed earlier than the radiate
phyla. Therefore the radial, diploblastic plan was secondarily derived from it.

c) Third View: Acoelomate Derived from Coelomate

There is recent discovery of a small group of worms (Lobato cercebridae, Annelida)
showing both flatworm and annelid characteristics. It suggests that the acoelomate body
plan is secondary characteristic. Thus, the flatworms represents side branch. It is formed
as a result of loss of a body cavity (Figure 3.1).

62
Figure 3.1: Evolutionary perspective. Phylogenetic tree of triploblastic, acoelomate
animals depicting major events and possible lines of descent from acoelomates.

Activity 1: Discuss various views about evolution of triploblastic animals.

3.1.1 Phylum platyhelminthes

The name platyhelminthes was derived from the Greek "platys" flat and helminthes
worms. Gagenbaur 1859 coined the word platyhelminthes for the flat worms which are
considered as the most primitive of all helminthes including both free-living or parasitic
forms. Minot (1876) separated nemertines from flat-worms and group them as phylum
platyhelminthes. The branch of biology study of helminthes is known as Helminthology.

1. They are soft bodied, unsegmented worms showing bilateral symmetry.

2. Body is divided into three germinal layers i.e. ectoderm, mesoderm and endoderm.
They may be free living (Turbellaria), ectoparasitic or endoparasitic.
3. Soft epidermis is present which is ciliated in case of Turbellarians, while covered
by cuticle in Trematoda and Cestoda worms.
4. Exo or endo skeleton is completely absent. The parasitic form shows suckers or
hooks or both for attachment to the body host.
5. Muscular system is well developed consisting of circular, longitudinal and oblique
muscles beneath the epidermis.

63
6. A true body cavity or coelome is absent filling the space between body organs with
loose parenchyma. The alimentary canal is either absent or highly branched. Anus
is completely absent.
7. Circulatory and respiratory systems are absent, while excretory system consists of
flame bulbs or flame cells or proto-nephridia connected to the excretory ducts.
Nervous system and sense organs are poorly developed.
8. Asexual multiplication and alternation of generations are seen in some cases.
Usually they are hermaphrodite and fertilization is internal with development may
be direct or indirect.
9. Examples include planaria, liver fluke, and tapeworm (Figure 3.2).

Figure 3.2: Examples of phylum platyhelminthes. (A) Planaria (B) Liver Fluke (C)
Tape worm

3.2 Classification of Phylum Platyhelminthes Upto Class

Phylum Platyhelminthes is divided into four classes (Table 3.1):
(a) Turbellaria (b) Monogenea (c) Tremetoda (d) Cestoidea

Table 3.1: Classification of Phylum Platyhelminthes.

1) Phylum platyhelminthes
(3.2.1) Class Turbellaria
(3.2.2 ) Class Monogenea
(3.2.3 ) Class Trematoda
(i) Sub-class Aspidogastrea
(ii) Sub-class Digenea
(3.2.4) Class Cestoidea
(i) Sub-class Cestodaria
(ii) Sub-class Eucestoda

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3.2.1 Class Turbellaria: Free-living Flat Worms
The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes
(flatworms) and incorporate all the sub-branches that are not parasitic. There are about
4,500 species, which extend from 1 mm (0.039 in) to large freshwater forms more than
500 mm (20 in) long or terrestrial species like Bipalium kewense which can reach 600
mm (24 in) in length. All the larger forms are flat with ribbon-like or leaf-like shapes,
since absence of respiratory and circulatory systems means that they need to depend on
diffusion of inner transport of metabolites.

1. They exist in mostly free-living forms both in fresh as well sea-water or on land.
2. Their body is unsegmented and dorso-ventrally flattened with cellular epidermis.
4. They can be without intestine or simple and sac like (Rhabdocoela) or can be branched.

Order (1) Acoela: They are mostly marine and small. Their mouth and pharynx could be
simple or absent while oviducts with two yolk glands are completely absent. Example:
Convoluta.

Order (2) Rhabdocoela: They are small having digestive system and sac-like intestine.
Most members are free-swimming with reproductive organs. Examples: Microstomum.
Order (3) Alloeocoela: They are mostly marine or small size worms having simple or
branched intestine. Example: Otoplana.
Order (4) Tricladida : They are mostly flat bodied having intestine with two lateral
limbs and one median limb. Genital aperture is single. Examples: Planaria.

 Body Wall
There are following layers in the body wall:
1. Epidermis
Its outer body layer is epidermis. Epidermis is ectodermal in origin and is in
contact with the environment. Some cells have micro-villi while some are ciliated.
2. Basement Membrane
A basement membrane of connective tissues is present below the epidermis which
separates it from tissues originate from mesoderm.
3. Muscular Layers
Below the basement membrane an outer layer of circular muscle and inner layer of
longitudinal muscles are present. Some muscles are dorso-ventrally and diagonally
placed. These muscles are present between the dorsal and ventral side.
4. Parenchyma
These are loosely organized cells. They are present between the longitudinal
muscles and the gastrodermis.
5. Gastrodermis
It is the innermost tissue layer. It is derived from the endodermis having single
layer of cells. Gastrodermis lines the digestive cavity and secretes enzymes. These
enzymes help in the process of digestion.

65
6. Glands
Several types of glandular cells are present on the ventral surface of the body wall.
These are epidermal origin. These are:
i) Rhabdites
These are rod like cells which swells forming protective mucous sheath
around the body. So, they protect the body from dehydration.
ii) Adhesive Glands
They are present in the epithelial surface producing sticky material which
helps in the attachment of turbellarians to substrate.
iii) Releaser Glands
They secrete a chemical that dissolves the attachment as need (Figure 3.3).

Figure 3.3: Body wall of Turbellaria. Cross section through body wall of planaria
elaborating relationship among various body structures.

 Locomotion
They have bilateral symmetry. Bilaterally symmetrical animals have active lifestyle.
They are mostly bottom dwellers which glides over the substrate and move with the help
of cilia and muscular undulations. They spread a sheet of mucus during their movement
which helps in adhesion. They have ciliated ventral surface flat body. These adaptations
help in locomotion.

 Digestion and Nutrition

i) Digestive cavity
Digestive cavity is absent in some marine turbellarians. But most of them have blind
digestive cavity. This cavity may be simple and unbranched chamber. Or it may be highly
branched system of digestive tubes. The digestive tracts of some other turbellarians are
lobed. Highly branched digestive system is an advanced structure. Gastrodermis has close
contact to the sites of digestion and absorption. Therefore, it reduces the distance for the
diffusion of nutrients. It partially compensates the absence of circulatory system.

66
ii) Food and its Ingestion
Most turbellarians are carnivores and feed on small invertebrates. Some are scavengers
which depend upon dead animals for their food. Some are herbivores and feed on algae
scrapping them from rocks. Presence of sensory cells on heads helps in the detection of
food from large distance. The turbellarians pharynx functions as an ingestive organ. It
may be a simple and ciliated tube or it may be a complex organ due to the folding of
muscle layers. In this case the free end of the tube lies in a pharyngeal sheath and projects
out of the mouth during feeding.

iii) Digestion
Extracellular digestion with pharyngeal glands secreting enzymes and breaking down the
food into small pieces. These smaller pieces are then taken into pharynx. Phagocytic cells
helps in the engulfment of these pieces from digestive cavity. Food is broken intracellular
vesicles (food vacuoles) and digestion is completed (Figure 3.4).

Activity 2: Find out and dissect tape worm to study its various structures.

Figure 3.4: Digestive system of planaria. An example of platyhelminthes. The digestive

system consists of mouth (located at the central underside of body), pharynx and
gastrovascular cavity.

 Exchanges with the Environment

i. Respiration
They do not have specialized respiratory organs. Exchange of respiratory
gases and excretion of metabolic wastes such as ammonia are removed by
the process of diffusion through body wall.
ii. Osmoregulation
Marine invertebrates are often isotonic to their environment. But they are
hypertonic to their aquatic environment in freshwater. Therefore, they must
regulate the osmotic concentration of their body tissues. This osmoregulation
enables them to live in freshwater.
iii. Excretion
Their excretory organs are protonephridia which are network of fine
tubules. These tubules run along the length of the body. The branches of
protonephridium end in flame cells having cilia which projects into the

67
 lumen of the tubule. A slit like openings is present in the wall of flame cell.
The beating of cilia creates a negative pressure in the tubule of flame cell.
Therefore, the fluid from the surrounding tissue is sucked by the tubule
through flame cell which joins and opens to the outside of the body through
nephridiopore (Figure 3.5).

Figure 3.5: Excretory system of platyhelminthes. A flame cell is excretory cell found
in the simplest freshwater invertebrates, including flatworms. It functions like kidney
removing waste materials.

 Nervous System and Sense Organs

Two types of nervous systems are found in turbellarians.

i. Diffused Nervous System
It is the most primitive type of flatworm nervous system. It is found in order
Acoela. It composed of a sub epidermal nerve plexus. This plexus resembles the
nerve net of cnidarians. A statocyst is present at anterior end which functions as
mechanoreceptor. It detects the position of body with respect to gravity.
ii. Central Nervous System
Some turbellarians have a more centralized nerve system. It is composed of with
cerebral ganglia. Several pairs of long nerve cords and sub-epidermal nerve net. It
is found in members such as planarian (Dugesia). Lateral branches arise from the
nerve cords. These lateral branches are called commissures connecting the nerve
cords and giving ladder like appearance.
iii. Neurons and Sense Organs
The neurons may be sensory or motor. It is an important evolutionary
advancement. The neurons concentrate in the anterior end to form a pair of cerebral
ganglia. It may be called a primitive brain. Turbellarians respond to many stimuli
in their external environment. They have following sensory cells:

68
i) Many tactile and sensory cells distributed over the body which helps to detect
touch with water currents and chemicals.
ii) Auricles (sensory lobes) projecting from the side of the head.
iii) Chemoreceptors: They help in food search and are especially dense in auricles.
iv) Eye spot (Ocelli): Most turbellarians have two simple eyespots called ocelli (single
ocellus) which helps the animal to detect light. Most of them are negatively
phototactic and move away from light. Each ocellus consists of a cuplike
depression. It is lined with black pigment. Photoreceptor nerve endings are present
in the cup. These are part of the neurons. They leave the eye and connect with
cerebral ganglia (Figure 3.6).

Figure 3.6: Nervous system. (A) Acoela (B) Polycladida (C) Tricladida

 Reproduction and Development

i. Asexual Reproduction
They reproduce asexually by transverse fission which begins as constriction behind the
pharynx. The two (or more) animals forming as result of fission are called Zooids. Zooids
separate from each other fission and regenerate the missing parts. Sometimes, the Zooids
remain attached till completion of development. Then they detach independent
Individuals (Figure 3.7).

69
Figure 3.7: Asexual reproduction in Planaria. Planarian detaches its tail end and each
half re-grows by regeneration thus resulting in two worms.

ii. Sexual Reproduction

Turbellarians are monoecious. Their reproductive organs are formed from the
mesodermal tissues in the parenchyma.

1. Male Reproductive Organs

Paired testes are present at sides of the worm. Sperm ducts (vas deferens) open into
seminal vesicle which acts as sperm storage organ. A protrusible penis is present.
The penis projects into a genital chamber.
2. Female Reproductive Organs
The female system has one to many pairs of ovaries. Oviducts start from the
ovaries and open into the genital chamber. Genital pores open outside through
genital pore.
3. Fertilization
Turbellarians are monoecious. But sperms are exchanged between two animals.
This cross-fertilization produces greater genetic vesicle diversity. The penis of each
individual is inserted into the copulatory sac of the partner. After copulation,
sperm move from the copulatory sac to the genital chamber. They reach ovaries
through oviducts. Fertilization occurs in ovary. Yolk is directly incorporated into
the egg during egg formation. Sometimes, yolk cells are attached around the zygote
as it passes through yolk glands.
4. Development
Eggs are laid with or without a gel-like mass. A cocoon encloses many turbellarian
eggs. Cocoon is a hard capsule attaches to the substrate by stalk. Cocoon contains
several embryos. Two kinds of capsules are laid:

70
 a) Summer capsules hatching in two to three weeks producing immature
animal.
b) Autumn capsules having thick walls which resists freezing and drying. They
hatch after over wintering.

Some turbellarians develop directly with gradual changes take place in embryo and
change it into adults with few of them producing free-swimming Muller’s larva having
ciliated extensions for feeding and locomotion. The larva develops into a young
turbellarian when it settles on the substrate (Figure 3.8).

Figure 3.8: Reproductive system in Planaria. Both male and female reproductive
organs are present within Single individual.

3.2.2 Class Monogenea

They are small parasitic flatworms mainly found on skin or gills of fish and longer than
about 2 cm. A few species infecting certain marine fish are larger and marine forms are
generally larger than those found on fresh water hosts. They lack respiratory, skeletal and
circulatory systems and have no or weakly developed oral suckers. They attach to hosts
using hooks, clamps and a variety of other specialized structures.

They are often capable of dramatically elongating and shortening as they move.
Like all ectoparasites, monogeneans have well-developed attachment structures. The
anterior structures are collectively called as prohaptor, while the posterior ones are
collectively termed the opisthaptor, or simply haptor.

The posterior opisthaptor with its hooks, anchors, clamps etc. is typically the major
attachment organ. They are acoelomates having simple digestive system with mouth
opening, muscular pharynx and intestine without terminal opening (anus). They are
hermaphrodite, some are oviparous and few are viviparous. In addition to head region
containing sense organs and nervous tissue (brain) they contain three embryonic germ
layers such as endoderm, mesoderm, and ectoderm (Figure 3.9).

71
Figure 3.9: Cross section of Flatworm. An example of Class Monogenea. Digestive
system with mouth and pharynx. Posterior opisthaptor with attachment organ in the form
of hooks, or anchors.

3.2.3 Class Trematoda

Trematoda is a class of Platyhelminthes having two groups of parasitic flatworms,
known as flukes. They are internal parasites of molluscs and vertebrates. Most
trematodes have complex life cycle with at least two hosts. The primary host, where the
flukes sexually reproduce is vertebrate. The intermediate host, in which asexual
reproduction occurs, is usually a snail. The trematodes or flukes include 18,000 to 24,000
species, divided into two subclasses. Trematodes are flattened oval or worm-like animals,
usually no more than a few centimetres in length, although species as small as 1
millimetre (0.039 in among Monogeneans which Monogenea is not under the class
tremotoda and separate class under playthelminthes) and as large as 7 centimetres (2.8 in
as in Fasciolopsis) are known. Their most distinctive external feature is the presence of
two suckers, one close to the mouth and the other on the underside of the animal.

The body surface of trematodes comprises a tough syncitial tegument, which helps
protect against digestive enzymes in those species that inhabit the gut of larger animals as
well as the surface of gas exchange because there are no respiratory organs. The mouth
is located at the forward end of the animal and opens into a muscular, pumping pharynx.
The pharynx connects, via a short oesophagus, to one or two blind-ending caeca, which
occupy most of the length of the body. As in other flatworms, there is no anus and waste
material excreted out through the mouth. Although the excretion of nitrogenous waste
occurs mostly through the tegument mainly concerned with osmoregulation. This consists
of two or more protonephridia, with those on each side of the body opening into a

72
collecting duct. The two collecting ducts typically meet up at a single bladder, opening to
the exterior through one or two pores near the posterior end of the animal.

The brain consists of a pair of ganglia in the head region, from which two or three pairs
of nerve cords run down the length of the body. The nerve cords running along the
ventral surface are always the largest, while the dorsal cords are present only in the
Aspidogastrea. Most trematodes are hermaphrodites, having both male and female
organs. There are usually two testes, with sperm ducts that join together on the underside
of the front half of the animal. This final part of the male system varies structurally
among species, but may include sperm storage sacs and accessory glands in addition to
copulatory organ. There is usually only a single ovary. Eggs pass from it into an oviduct.
The distal part of the oviduct, called ootype, is dilated. It is connected via a pair of ducts
to a number of vitelline glands on either side of the body that produces yolk cells. After
the egg is surrounded by yolk cells, its shell is formed from the secretion of another gland
called Mehlis' gland or shell gland, the duct of which also opens in the ootype.

The ootype is connected to an elongated uterus that opens to the exterior in the genital
pore, close to the male opening. In most trematodes, sperm cells travel through the uterus
to reach the ootype, where fertilization occurs. The ovary is sometimes also associated
with a storage sac for sperm and copulatory duct termed Laurer's canal (Figure 3.10).

Figure 3.10: Generalized Fluke (Digenetic trematoda). Reproductive system is shown

here.

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3.2.2 (a) Sub-class Aspidogatrea
They consist of small group of flukes with species range in length from about one
millimeter to several centimeters. They are parasites of freshwater and marine mollusks
and vertebrates. Shared characteristics of the group are a large ventral disc with a large
number of small alveoli (suckerlets) or a row of suckers and a tegument with short
protrusions, so called microtubercles.

i. Larval Physiology
Larvae of some species have ciliated patches. Those of Multicotyle purvisi have
four patches on the anterior side of the posterior sucker and six at the posterior side,
those of Cotylogaster occidentalis have an anterior ring of eight and a posterior
ring of six, while larvae of Aspidogaster conchicola and some others lack cilia
altogether. Larvae of some species hatch from eggs while others do not.
ii. Excretory System
They use flame cells as an excretory mechanism. The two excretory bladders are
located dorsally, on the anterior side of the posterior sucker, connected to ducts,
and three flame cell "bulbs" on each side of the body; the ducts contain cilia to aid
the flow of excreta (Figure 3.11).

Figure 3.11: Excretory system of Fluke. Excretory bladders and flame cells are present.
Further, microfilaments further helping in the process of excretion.

iii. Nervous System

They have nervous system of extraordinary complexity, greater than that of related
free-living forms and many sensory receptors of many different types. They
consists of longitudinal nerves (connectives) connected by circular commissures.

74
 The brain (cerebral commissure) is located dorsally in the anterior part of the body
and eyes dorsally attached to it. A nerve from the main connective enters the
pharynx supplying the intestine while posteriorly enters the sucker. Sensory
receptors are scattered over the ventral and dorsal surface, the largest numbers
occurring on the ventral surface, at the anterior end and on the posterior sucker.
Electron-microscopic study revealed 13 types of receptors (Figure 3.12).

Figure 3.12: Larva of Lobatostoma manteri. False anterior sucker, the pharynx, the
blind ending caecum with two excretory bladder cells. The posterior end is drawn out
into a short appendage of unknown function.

iv. Life Cycle of Aspidogastrea

The life cycle is divided into two types. In one type, the entire life cycle completed
in molluscs although vertebrates may act as facultative (not obligate) hosts, in the
other, both mollusc and vertebrate are required for completion of the life cycle. An
example of the first kind is Aspidogaster conchicola (Figure 3.13).

Figure 3.13. Life cycle of Aspidogaster conchicola. Molluscs such as freshwater

bivalves or snails are necessary for the completion of the life cycle. Adult worms produce

75
eggs in which non-ciliated larva with an anterior and posterior sucker develops. The life
cycle repeats without the involvement of vertebrate host but if fish eats infected
mollusk then adults can produce eggs in it.

3.2.3 (b) Sub-Class Digenia

It consists of parasitic flatworms (known as flukes) with syncytial tegument and two
suckers, one ventral and one oral. Adults are particularly common in the digestive tract,
but occur throughout the organ systems of all classes of vertebrates. Around 6,000
species have been described. Because digenetic flukes requires atleast two different hosts
to complete their life cycles, these animals possess the most complex life cycles in the
entire animal kingdom. As adults, they are all endoparasites in the blood stream, digestive
tracts or other visceral organs in wide varierty of vertebrates serves as final hosts. They
have synctial tegument for the transport of nutrients, waste and gases across body wall
while complex structure includes microvilli, a glycocalyx, and majority of cell body
internal to basement membrane connected to outer zone by cytoplasmic bridges. The
anterior sucker is the oral sucker that sorrounds the mouth while other sucker, the
acetabulum, is located below the oral sucker on the middle portion of the body. Eggs of
digenetic trematodes are oval and have lid-like hatch called an operculum. When an egg
reaches freshwater, the operculum opens, and ciliated larva called miracidium swims out
which then penetrates the snail, loses its cilia, develops into sporocyst (alternatively
miracidium may remains in egg and hatch after eating by snail). Sporocysts are bag-like
structures having embryonic cells that develops into either daughter sporocysts or rediae
and produces hundreds of next larval stage, the cercariae having digestive tract, suckers
and a tail. When cercaria penetrates second intermediate or final host, which maybe
vertebrate, invertebrate or plant, it encysts as metacercaria. After eating of second
intermediate host by definitive host, the metacercaria excysts developing into an adult
(Figure 3.14).

Figure 3.14: Life Cycle of Digenea. Metacercaria stage of life cycle is shown. The life
cycle of a typical digenean trematode can be thought to begin when its egg is immersed

76
in water. When miracidium hatches it swims to find a mollusc host where it passes many
stages and at last emerging in the form of motile cercaria larvae.

Some Important Trematode Parasites of Humans

Chinese liver fluke, Clonorchis sinensis, is a common parasite of humans in Asia with 30
million infected people. Infection by liver flukes can cause chronic parasitic
inflammatory disease of the bile ducts. It occurs by ingestion of raw or undercooked
fluke-containing freshwater fish. The life cycle of C. sinensis involves both first
intermediate snail host and second intermediate fish host. Embryonated eggs are
discharged in the bile ducts and stool of human host. An adult fluke lays 2000 to 4000
eggs each day. If these eggs are ingested by a suitable intermediate snail host, the eggs
release miracidia, which go through several developmental stages: sporocyst, redia, and
cercaria. The cercariae are released from the snail and after a short period of free-
swimming time in water, they may come in contact with and penetrate the flesh of a
freshwater fish and encysting as metacercariae. Infection of humans occurs by ingestion
of undercooked, salted, pickled, or smoked freshwater fish. After ingestion, the
metacercariae excyst in the duodenum (first portion of the small intestine) and ascend the
biliary tract through the ampulla of Vater (hepatopancreatic ampulla, where the
pancreatic and bile ducts come together). Maturation takes approximately one month.
The adult flukes (which measure 10 to 25 mm by 3 to 5 mm) reside in small and medium-
sized bile ducts. In addition to humans, carnivorous animals can serve as reservoir hosts
(Figure 3.15).

Figure 3.15: Life cycle of Chinese liver fluke. Embryonated eggs are discharged in the
bile ducts and feces in water which are ingested by snail intermediate host releasing

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miracidia and passes developmental stages (sporocysts, rediae and cercariae ). The
cercariae are released from snail and travel in water to penetrate the flesh of freshwater
fish, where they encyst as metacercariae. After ingestion by humans, the metacercariae
excyst in the duodenum and ascend the biliary tract where adult flukes rests.

Fasciola hepatica, sheep liver fluke, all over the world attacks sheep causing liver rot
disease. Life cycle is completed within two hosts. The primary host is the sheep while the
intermediate host is snail. This type of life cycle involving two hosts is termed as
digenetic parasite. The adult lives in bileduct of liver. Eggs pass through bile duct to
intestine and eliminated. Eggs deposited in freshwater hatch and miracidia penetrates the
snail which then develops into sporocyst and then into rediae giving cercariae. After
about two months, cercariae emerges from snail and swims around to attach to grass. The
attached cercariae develops a protective membrane, forming cysts called metacercariae
that are able to survive dry and adverse climate conditions for many months. They encyst
on aquatic vegetation. Sheep or other animals becomes infected when they graze on such
aquatic vegetation. Following ingestion, metacercariae hatch and develops into early
immature flukes in the gut, cross the wall of gastrointestinal tract and migrates to the liver
through the body cavity. Upon reaching and penetrating the liver, the early immature
flukes migrate through the liver tissue causing acute fasciolosis. These flukes grow and
becomes immature causing sub-acute fasciolosis. After about eight weeks, flukes enters
bile duct and develops into adult, which then feeds on blood and start laying eggs causing
chronic fasciolosis. Humans may become infected with Fasciola hepatica by eating
freshwater plant called watercress that contains encysted metacercaria (Figure 3.16).

Figure 3.16: Life cycle of Fascoila hepatica (Sheep liver fluke). Adult fluke lays eggs
in the infected cattle or sheep bileduct in the liver which then enters intestinal tract and

78
expelled out with feces. After dropping into water, eggs hatch releasing miracidiae which
swims and find snail host. It multiplies and become cercariae which leaves snail after
about two months and attaches to grass. It forms a cyst wall around itself and known as
metacercariae and ingested by cattle while grazing. After ingestion, it hatches and
develops immature flukes in the gut.

Schistosomes commonly known as blood-flukes are parasitic flatworms causing

infections in humans termed schistosomiasis. The adult diocecious worms lives in the
human bloodstream. Male fluke is shorter and thicker than female with sides curved
under forming canal. Female fluke is long and slender which is carried in the canal of the
male. After copulation female produces thousands of eggs over her lifetime. Each egg
contains spine that mechanically aids it in moving through host tissue until it eliminates
in feces or urine. Eggs lacks operculum and miracidium release by slit that develops in
the egg when the egg reaches freshwater. The miracidium finds snail and penetrates it
developing into sporocyst and finally tailed cercaria. There is no redial generation. The
cercariae leaves the snail and penetrate human skin. Anterior glands that secrete digestive
enzymes helps in penetration. Once in a human, the cercariae lose their tails and develop
into adults in the intestinal veins skipping the metacercaria stage (Figure 3.17).

Figure 3.17: Life cycle of Schistosoma. Eggs are eliminated through feces. Under
optimal conditions, eggs hatch releasing miracidia which swims and penetrating snail
intermediate host. The stages in the snail include two generations of sporocysts and the
production of cercariae.

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 Activity 3: Study about various parasitic flukes causing diseases amomg humans.

3.2.4 Class Cestoidea: The Tape worms

The cestoda is the most highly specialized of the flat worm classes. All are endoparasites
and body is covered by tegument as in trematodes. The cestodes differ from the members
of other two classes in the complete absence of digestive tract. It is divided into two sub-
classes, the cestodaria and Eucestoda. They lack mouth and digestive tract in all of their
life cycle stages absorbing nutrients directly across their body wall. Majority of adult
tape-worms consists of long series of repeating units called proglottids.

3.2.4 (a) Sub-Class Cestodaria

They are all endoparasites in the Intestine and coelom of primitive fishes. They have
about 15 species. They show some trematode characters:
1. Only one set of both male and female reproductive organs are present in it.
2. Some animals bear suckers.
3. Their bodies are not divided into proglottids like other cestodes.

But they also show cestode characters like:

1. Absence of digestive system.
2. Presence or larval stages similar to eestodes
3. Presence of parenchymal muscle cells. These are not present in another
platyhelminths.

The presence of all these structures suggests strong phylogenetic affinities with other
cestodes.

3.2.4 (b) Sub-Class Eucestoda

They are called true tapeworms. They show high degree of specialization for parasitic
life. Their body is divided into three parts (Figure 3.18).
i. Scolex
It is present at one end. Scolex is a hold-fast structure. It contains circular or leaf
like suckers. Most of the times, they also have hooks. The tapeworm attaches
intestinal wall with the Scolex. Mouth is not present in them.
ii. Neck
The scolex narrows to from the neck.
iii. Strobila
It is the third body region. It consists of linearly arranged proglottids which
functions as reproductive units. During the growth of tapeworm, new proglottids
added in the neck region with older ones pushed posteriorly which mature and start
producing eggs. Anterior proglottids are immature and mature in the mid region.
The proglottids at the posterior end accumulate eggs. So they are gravid (ripened).

80
 Body wall
The outer body wall of tapeworms consists of a tegument. The tapeworms have no
digestive system. Therefore tegument plays an important role in nutrient absorption. The
tegument absorbs some of the host’s own enzymes. These enzymes help in digestion.

Figure 3.18: Tapeworm. An example of class Cestoidea.

 Nervous and Excretory System

The nervous system consists of a pair of lateral nerve cords which arises from nerve mass
in the scolex and extending along the length of strobili. A proto-nephridial system also
runs the length of the tapeworm (Figure 3.19).

Figure 3.19: Nervous system of tape worm. The main nerve center of cestoda is a
cerebral ganglion in its scolex or head. The Motor and sensor ability depends on the
number of nerves and scolex complexity.

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 Reproduction
Tapeworms are monoecious. They produce large number of eggs. Each proglottid
contains male and female reproductive organs.

i. Male Reproductive Organs

Numerous testes are scattered throughout the proglottids. Testes produce sperm.
These sperms move into the copulatory organ cirrus through a duct system. The
cirrus opens through a genital pore. The male system of proglottid matures before
than the female system. Therefore, copulation occurs with another mature
proglottlid of the same tapeworm or with another tapeworm in the same host. Thus
cross fertilization produce hybrid vigor.
ii. Female Reproductive Organs
Each proglottid contains ingle pair of ovary. It produces eggs. Sperm are stored in a
seminal receptacle. They fertilize eggs as the eggs move through the oviduct. Then
egg passes through the vitelline gland. Then the eggs pass into the ootype which is
an expanded region of oviduct. It forms capsules around the eggs. It is also
surrounded by Mehlis gland that helps in the formation of the egg capsule. Most
tapeworms have a blind- ending uterus. The eggs are stored in the uterus.
iii. Development
The reproductive organs degenerate after storage of egg. Therefore, gravid
proglottids are called as “bags of eggs” which breaks and get free from tapeworm
end. They pass from the host with the host’s feces. In some tapeworms, the uterus
opens to the outside of the worm. Thus the egg is released into the host’s intestine.
The proglottids are not continuously lost, So adults becomes very long.

Some Important Tape Worm Parasites of Humans

1. Beef Tape worm (Taeniarhynchus saginatus)

It is medically important tapeworm of humans. Adults lives in small- intestine. It may
reach the lengths of 25m. It has two hosts man and cattle (intermediate host). It produces
about 80, 000 eggs per proglottid. It forms following stages:

a) Onchosphere
The egg develops forming six-hooked larva called onchosphere which are ingested
by cattle when they graze in places contaminated with human feces becoming free
inside the body because of digestive enzymes of the cattle. Larva bore through
intestinal wall with its hooks and moves into the blood. Blood carries the larvae to
skeletal muscles.
b) Cysticercus
Onchosphere forms cyst in the muscles. It changes into a fluid-tilled ladder called a
cysticercus or bladder warm. When human eats raw or improperly cooked meat.
The cysticercus is released from the meat. Its scolex attaches to the human
intestinal wall and the tapeworm matures (Figure 3.20).

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Figure 3.20: Life Cycle of Beef tape worm. Adults attach to the intestinal wall with an
unarmed scolex. Worms mature in several weeks and begin shedding gravid proglottids.
Adults may have 2000 proglottids and be up to 25 meters in length.

2. Pork Tapeworm (Taenia solium)

Its intermediate host is the pig. The strobili are 10 m long. The oncospheres larvae hatch
and move through the intestinal wall and enter the bloodstream. Eggs or gravid
proglottids are passed with feces and can survive for days to months in the environment.
Cattle becomes infected by ingesting vegetation contaminated with eggs or gravid
proglottids. In the animal's intestine, the oncospheres hatch, invade the intestinal wall and
migrate to the striated muscles, where they develop into cysticerci which can survive for
several years in the animal. Humans become infected by ingesting raw or undercooked
infected meat. In the human intestine, the cysticercus develops over 2 months into an
adult tapeworm, which can survive for years. The adult tapeworms attaches to the small
intestine by means of scolex and resides in the small intestine.The adults produce
proglottids which mature, become gravid, detach from the tapeworm, and migrate to the
anus or are passed in the feces. Therefore, they are distributed throughout the body. This
causes disease cysticercosia. It can be fatal if the cysticerci encysted in the brain (Figure
3.21).

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Figure 3.21: Life cycle of Taenia solium. Humans are the only definitive hosts. The
oncospheres larvae hatch and move through the intestinal wall and enters the bloodstream
distributing throughout the body.

3. Broad Fish Tapeworm (Diphyllobothrium latum)

It is found mostly in the northern parts of North America and United States. It has a
scolex with two longitudinal grooves called bothria that acts as hold-fast structures. The
adult worm may attain length about 10m. Eggs released by proglottids through uterine
pores deposited in freshwater (Figure 3.22). It forms following larvae.
a) Coracidia
The eggs are hatched and a ciliated larvae coracidia (sing., coracidium) is formed.
These coracidia are ingested by copepods.
b) Procercoids
The coracida shed their ciliated coats in the copepods. It develops into procercoid
larvae. Fish eat the copepods.
c) Plerocercoid
The procercoids converts into plerocercoid larvae after burrowing in fish muscle.
When human eat raw or poorly cooked meat, it enters into the intestine where it
attaches and grows into adult worms.

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Figure 3.22: Life cycle of Broad Fish tape worm. (a) Definitive host are any fish eating
mammal (b) adult worm is in mammal small intestine (c) Shelled embryo passes in feces
at an early stage of development (d) Embryogenesis continues in water and free-
swimming coracidium hatches (e)Coracidium eaten by copepod and oncosphere
penetrates intestine into hemocoel (f) Procercoid develops in hemocoel (g) Copepod
eaten by fish where procercoid penetrates into muscle and develops into plerocercoid.

3.3 Phylum Nemertea

There are nine hundred species of nemerteans. They are long, flattened worms found in
marine mud and sand. They have long proboscis and also called as proboscis worms.
Adult worms range in size from a few millimeters to several centimeters in length. Most
nemerteans are pale yellow, orange, green or red. Its characteristics are (Figure 3.23)
1. They are triploblastic, acoelomate, bilaterally symmetrical unsegmented worms.
2. They possess ciliated epidermis having mucous glands
3. They have complete digestive tract with an anus
4. Excretory structures are protonephridia.
5. Nervous system is composed of cerebral ganglion, longitudinal nerve cords and
transverse commissures.
6. They have closed circulatory system.
7. Body musculature forms two or three layers.

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a. Body Parts
i. Proboscis
The nernerteans have long proboscis. It is the most distinctive feature of this group
This proboscis is held in a sheath called a rhynchocoel. Carnivorous Species use
the proboscis to capture annelid and crustacean prey.
ii. Digestive System
They have complete one-way digestive tract having mouth for food ingestion and
anus for excretion. The food moves from anterior to posterior part of digestive
cavity and mechanical breakdown of food, digestion, absorption and feces
formation take place. It is a great innovation in nemerteans.
iii. Circulatory System
They also have nervous system. It is another major innovation. Its circulatory
system consists of two lateral blood vessels. These lateral blood vessels have
branched tributary. Heart is absent in them. Blood dono circulate but large vessels
contractions propel it. It simply moves forward and backward through the
longitudinal vessels.

Evolutionary Advancement in Nemertea

The nemerteans have two advance characters. Therefore, they are much larger than the
most flat worms.
a) The presence of blood vessels. These blood vessels provide blood to local tissues.
b) They have one-way digestive system. It can process nutrient efficiency.

Reproductive System
Male and female reproductive structures develop from parenchyma. Larger species often
break up and fragments often grows into full individuals. All reproduce sexually and
most species are gonochoric (the sexes are separate) but all the freshwater forms are
hermaphroditic. Nemerteans often have numerous temporary gonads (ovaries or testes),
forming row down each side of the body in the mesenchyme. Temporary gonoducts
(ducts from which the ova or sperm are emitted), one per gonad are built when the ova
and sperm are ready. The eggs are generally fertilised externally. Some species shed them
into the water, some lay them in a burrow or tube, and some protect them by cocoons or
gelatinous strings. Some bathypelagic (deep sea) species have internal fertilization, and
some of these are viviparous, growing their embryos in the female's body.

The zygote (fertilised egg) divides by spiral cleavage and grows by determinate
development, in which the fate of a cell can usually be predicted from its predecessors in
the process of division. The embryos of most taxa develop either directly to form
juveniles (like the adult but smaller) or to form planuliform larvae, in which the larva's
long axis is the same as the juvenile's. The planuliform larva stage may be short-lived and
lecithotrophic ("yolky") before becoming a juvenile, or may be planktotrophic,
swimming for some time and eating prey larger than microscopic particles. However,
many members of the order Heteronemertea and the palaeonemertean family
Hubrechtidae form a pilidium larva, which can capture unicellular algae and which
Maslakova describes as like a deerstalker cap with the ear flaps pulled down. It has a gut

86
which lies across the body, a mouth between the "ear flaps", but no anus. A small number
of imaginal discs form, encircling the archenteron (developing gut) and coalesce to form
the juvenile. When it is fully formed, the juvenile bursts out of the larva body and usually
eats it during this catastrophic metamorphosis.

Figure 3.23: Phylum Nemertia. General Organization is shown over here. Section
showing tubular gut and proboscis.

3.4 Phylum Gastrotricha

They have about live hundred free- living marine and freshwater species living in space
between bottom sediments. Their size is from 0.01 to 4 mm in length. They use cilia on
their ventral surface to move over the substrate. They have following characteristics:
1. The dorsal cuticle often contains scales, bristles, or spines.
2. A forked tail is often present.
3. A syncytial epidermis is present beneath the cuticle.
4. Sensory structures are tufts of long cilia and bristles on the rounded head.
5. The nervous system includes a brain and a pair of lateral nerve trunks.
6. The digestive system is a straight tube. It has a mouth, a muscular pharynx, a
stomach-intestine, and an anus. They ingest microorganisms and organic detritus
by the pumping action of pharynx from the bottom sediment and water. Digestion
is mostly extracellular.
7. Adhesive glands are present in the forked tail. They secrete material that attaches
the animal to solid objects.
8. Paired protonephridia occur in freshwater species but absent in marine species. But
their protonephridia are morphologically different from other acoelomates. Each

87
 protonephridia possesses a single flagellum. But other acoelomates have cilia in
!lathe cells.
9. Most of the marine species reproduce sexually. They are hermaphroditic. Most of
the freshwater species reproduce asexually by parthenogenesis. The females lay
two kits of unfertilized eggs.
a) Thin-shelled eggs: It is hatched into females during favorable environmental
conditions
b) Thick-shelled resting eggs can withstand unfavorable conditions for long
periods. ‘then they are hatched into females. There is no larval stage.
Development is direct. The young have the same form as the adults (Figure
3.24).

Figure 3.24: Internal anatomy of gastrotrich. Animal body is about 3mm long.

3.5 Further Phylogenetic Considerations

There are different views about the evolution of platyhelminthes
1. Zoologists believe that the platyhelminthes body form is central to animal
evolution. They believe that an ancestral flatworm was similar to turbellarians. The
cladogram of platyhelminthes unites the Monogenea, Trematoda and Cestoidea.
2. Recent molecular data suggest that the acoelomate flatworms are not members the
Phylum Platyhelminthes. They are very close to the first ancestral bilateral
Animals.
3. More evidence links the parasitic flatworms to ancient, free-living ancestors. The
free-living and parasitic ways of life formed in the Cambrian period, 600 million

88
 years ago. The first flatworm parasites were associated with primitive molluscs,
arthropods and echinoderms. They later acquired the vertebrate hosts and complex
life cycles.
4. The gastrotriches show some distant relationships to the acoelomates. For example.
many gastrotriches lack a body cavity. They are Monoecious and small. Their
ventral cilia were derived from the same ancestral sources as those of the
turbellarians flatworms.

Key Points/Summary
1. The free-living platyhelminthes, mebers of class Turbellaria are small, bilaterally
symmetrical, acoelomate animals. Most turbellarians move by cilia and are
predators. Protonephridia involved in osmoregulation are present in mant
flatworms.
2. Monogenetic flukes are mostly ectoparasites of fishes. Class trematoda is divided
into two subclasses such as aspidogastrea and diginea and most are external or
internal parasites.
3. Cestodes are gut parasites of vertebrates. They are more specialized than flukes
having scolex with attachment organs, a neck region and strobili which consists of
chain of segments.
4. Nemerteans are much larger than platyhelminthes and prey on invertebrates.
5. Gastrotrichs are microscopic, aquatic animals with a head, neck and trunk. The
group is generally hermaphroditic, although males are rare and female
parthenogenesis is common in freshwater species.

Self Assessment Questions

Q: Answer the following questions:
1. Describe some key features of triploblastic, acoelomate animals.
Ans: See introduction.
2. Explain various morphological and developmental similarities and
differences among various classes of Phylum Platyhelminthes.
Ans: See 3.1
3. What are flame cells? How they play role in excretion.
Ans: See 3.1
4. Explain the life cycle of Chinese liver fluke.
Ans: See 3.1
5. Write note on some tape worm parasites of humans.
Ans: See 3.1

Q: Fill in the blanks with appropriate answers.

1. ………………….. are rod like cells which swells forming protective mucous
sheath around the body. (Rhabdites)
2. ………………….. are excretory organs in planaria. (Protonephridia)

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 3. Most turbellarians have two simple eyespots called ……………….. (Ocelli)
4. ………………………….., sheep liver fluke, all over the world attacks sheep
causing liver rot disease. (Fasciola hepatica)
5. ……………………………….. commonly known as blood-flukes are
parasitic flatworms. (Schistosomes)
6. ……………. is a hold-fast structure in tape worm. (Scolex)
7. Gravid proglottids are called as ……………………… which breaks and get
free from tapeworm end. (Bags of eggs)
8. Scientific name for beef tape worm is …………………………………
(Taeniarhynchus saginatus)
9. Flatworms larva became sexually mature by process known as
…………………….. (Paedomorphosis)
10. ………………… is turbellarian free swimming larvae. (Mullers Larva)

References
 Littlewood D T J; Bray R. A. (2000). "The Digenea". Interrelationships of the
Platyhelminthes. Systematics Association Special Volume. 60 (1 ed.). CRC. pp.
168–185. ISBN 978-0-7484-0903-7.
 Miller−Harley: (2001) Zoology. The McGraw−Hill Companies, Chapter Eight.
Animal like protista. Fifth Edition.Page144- 150.
 Poulin, Robert; Serge Morand (2005). Parasite Biodiversity. Smithsonian. p. 216.
ISBN 978-1-58834-170-9.
 Rohde, K.; Watson, N. A. (1990). "Paired Multiciliate Receptor Complexes in
Larval Multicotyle purvisi (Trematoda, Aspidogastrea)". Parasitology Research. 76
(7): 597–601

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UNIT-4

PESUDOCOELOMATE BODY
PLAN: ASCHELMINTHES

Written by: Dr. Sobia Mushtaq

Reviewed by: Arshad Mehmood Qamar

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 CONTENTS
Introduction ....................................................................................................... 93

Objectives ......................................................................................................... 93

4.1 Evolutionary Perspectives ..................................................................... 94

4.2 General Characteristics of Aschelminthes ............................................ 94

4.3 Classification up to Phyla with External Features ................................ 95

4.4 Some Important nematodes Parasites of Humans ................................. 109

4.5 Further Phylogenetic Considerations .................................................... 113

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Introduction

Aschelminths can be free-living or parasitic. The free-living organisms are extremely

abundant in soils and sediments and they feed on bacteria, while some others are plant
parasites and can cause disease in crops that are economically important. The others are
parasites that can be found in animals and human beings. Some of the parasitic worms
include hookworms, pinworms, Guinea worms, and intestinal roundworms.

Ascaris lumbricoides is the Giant Intestinal Roundworm that is an endoparasite living in

the human intestine. They are very common in children. These worms cause a disease
called ascariasis. Many adult roundworms live inside the intestine, causing obstruction to
the intestinal passage. This causes abdominal discomfort, colic-like pain, impaired
digestion, diarrhea and vomiting. Generally, deworming medicines are given to get rid of
these roundworms from the body.

Objectives
After completion of this unit, you will be able to:
 describe evolutionary perspectives of Aschelminthes.
 explain general characteristics of Pseidocoelomatic body plan.
 identify major features in pseudocoelm.
 write characteristics of feeding and digestive system and their organ systems.
 give a brief description on complete digestive tract, a muscular pharynx, constant
cell numbers, protonephridia, cuticle and adhesive glands.
 find some important nematode parasites of humans.

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4.1 Evolutionary Perspective
The group Aschelminthes has seven different phyla. They are very diverse animals. They
have unclear phylogenetic affinities very few fossils of these organisms are found. Two
hypotheses have been proposed for their phylogeny:

1. First Hypothesis: Monophyletic: It describes that these phyla are related due to
following common structures: Pseudocoelom, Cuticle, Muscular pharynx and
Adhesive glands.
2. Second Hypothesis: Polyphyletic: This hypothesis describe that these phyla are
not related to each other. Therefore they are polyphyletic. There is absence of any
single unique feature found in all groups. It strongly suggests that there is
independent evolution of each phylum. These animals are adapted to similar
environments. .lberetbre. similarities among them are due to convergent evolution.
3. Both Monophyletic and Polyphyletic: The correct phylogeny is in between the two
hypotheses. All phyla have some common anatomical and physiological features. Thus
they are distantly related to each other. Convergent evolution has also produced ome
analogous similarities. But each phylum arose from a common acoelomate ncestor. It
diverged very early in evolutionary history. Such ancestor is a primitive ciliated
acoelomate turbellarian. Therefore, it is concluded that the first ancestor was ciliated,
acoelomate, marine and monoecious. It lacked cuticle (Figure 4.1).

Figure 4.1: Evolutionary tree depicting major events of descent for pseudocoelomate

4.2 General Characteristics of Aschelminthes

Aschelminthes show following characters:
1. They are first invertebrates which posses body cavity. But they lack peritoneal linings
and mesenteries. These structures are found in more advanced animals. Therefore, the

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 various internal organs lie free in the cavity. Such a cavity is called a pseudococlom or
pseudocoel. Therefore, these animals are called Pseudocoelomates. The pseudococlom
is often fluid filled. Or it may contain a gelatinous substance with mesenchyma cells.
The pseudocoelom performs following functions:
i) This cavity is used for circulation.
ii) It helps indigestion.
iii) It acts as an internal hydrostatic skeleton. It is used in locomotion.
2. Most aschelminths have a complete tubular digestive system. It extends from an
anterior mouth to posterior anus. This complete digestive tract was first started in
the nemerteans. It is characteristic of almost all other higher animals. The
mechanical breakdown of food, digestion. absorption, and feces formation take
place in it from anterior to posterior direction. Most aschelminths also have a
specialized muscular pharynx. It is adapted for feeding.
3. Many aschelminths show eutely. In this case, numbers of cells are constant in the
entire animal and in each given organ in all the animals of that species. For
example, the number of body cells in cells in all adult Caenorhabditis elegans is
959 and the number of cells in the pharynx of every worm in the species is
precisely 80.
4. Most aschelminths are microscopic. But some are meter in length.
5. They are bilaterally symmetrical, unsegmented, triplohlastic animals They are
cylindrical in cross section.
6. Most aschelminths have an osmoreplator> system of protonephridia. There are
more osmotic problems in the fresh water animals. Therefore, this system is best
developed in freshwater forms.
7. No separate blood or gas exchange systems are present in them.
8. Some cephalization (head formation) is present in them. The anterror end contains
a primitive brain, sensory orgins, and a mouth.
9. Majority of aschelminths are dioecious. Reproductive systems are relatively
simple. Their life cycle is simple.
10. Cilia are absent on external surface. A thin. tough external cuticle is present. The
cuticle may have spines and scales. These structures protect the animal. Sonic
aschelminths shed this cuticle for growth. This process is called molting or
ecdysis. Beneath the cuticle is a syncytial (multinucleate) epidermis. It actively
secretes the cuticle. Several longitudinal muscle layers lie beneath the epidermis.
11. Most aschelminths are freshwater animals. Only a few Live in marine
environments. Many of the nematodes are parasite. The remaining aschelminths arc
mostly free-living. Some rotifers are colonial.

4.3 Classification up to phyla with External Features

Feeding and the digestive system; their organ systems, Reproduction and development of
phylum rotifer and phylum nematode; phylum kinorhyncha. Some important nematode
parasites of humans

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4.3.1 Phylum Rotifera
Characteristics of the phylum Rotifera are:
1. The rotifers derive their name from characteristic ciliated organ called corona.
Corona is present around lobes on the head. The cilia of the corona do not beat
simultaneously. Rather each cilium beat earlier than the next cilium. A wave by
beating cilia appears. It passes around the periphery of the ciliated lobes. It gives
the impression of a pair of spinning wheels. That is why the rotifers were earlier
called wheel animalcules.
2. Rotifers are small animals. They are abundant in most freshwater habitats. A few
species are marine. There are two thousand species of rotifera.”They are divided
into three classes.
3. The body has a thousand cells. The organs are eutely.
Rotifers are solitary, free- swimming animals. Some are colonial. Others occur
between grains of sand.
4. They are triploblastic, bilateral, unsegmented, pseudocoelomate
5. They have complete digestive system with specialized organs.
6. Anterior end has a ciliated organ called a corona.
7. Posterior end has toes and adhesive glands.
8. Well-developed cuticle is present.
9. Protonephridia with flame cells are present
10. Males are reduced in number or absent. Therefore, parthenogenesis is common in
them (Figure 4.2).

Figure 4.2: Anatomy of Rotifers

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 External Features
1. Cuticle: A cuticle covers rotifers external surface. In many species, the
cuticle thickens to form lorica. Lorica is case that covers the body. The
cuticle or lorica provides protection. The fluid in the pseudocoelom provides
hydrostatic support. But cuticle is the main supportive element.
2. Epidermis: The epidermis is syncytial. Plasma membranes are absent
between the nuclei.
3. Head: The head contains the corona, mouth, sensory organs and brain. The
corona surrounds buccal field. Buccal field is a large ciliated area.
4. Trunk: The trunk is the largest part of a rotifer. It is elongated and saclike.
The anus occurs dorsally on the posterior end of trunk.
5. Foot: The posterior narrow portion of rotifers is called foot. The terminal
portion of the fbot hears one or two toes. At the base of the foot are many
pedal glands. Their ducts open on the toes. The foot is attached to substratum
with the secretions of these glands.
 Feeding and the Digestive System
Most rotifers feed on small microorganisms and suspended organic material. The
corona! cilia create a current of water. This current brings food particles into the
mouth. Digestive system is composed of:
1. Pharynx: Pharynx contains a structure called the mastax (jaws). The mastax
is a muscular organ. It grinds food. The inner walls of the mastax contain
several sets of jaws called trophi. The trophi have different structures.
2. Stomach: Food passes from the mastax into a ciliated esophagus. It then
enters into the ciliated stomach. Salivary and digestive glands secrete
digestive enzymes into the pharynx and stomach. Digestion is extracellular
and absorption of food takes place in the stomach.
3. Intestine and cloacal bladder: In some species, ciliated intestine forms
cloacal bladder. It receives water from the protonephridia and eggs from the
ovaries. It also receives digestive waste. The cloacal bladder opens to the
outside by anus. Anus is present at the junction of the foot and trunk.
 Other Organ Systems
All visceral organs lie in a pseudocoelom. Pseudocoelom is filled with fluid and
interconnecting amoeboid cells. Protonephridia opens into cloaca! bladder. The
function of protonephridia is osmoregulation. Rotifers exchange gases and remove
nitrogenous wastes through body surfaces. The nervous system is composed of two
lateral nerves and t bilobed brain. Brain is present on the dorsal surface of the
mastax. Numerous ciliary :lusters and sensory bristles act as sensory structures.
They are concentrated on short antennae or the corona. One to five photosensitive
eyespots are present on the head.

Reproduction and Development

Some rotifers reproduce sexually. But parthenogenesis occurs in most species. Smaller
male may be produced in some classes. But in other classes no male are produced. In
some classes, fully developed males and females are present.

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Female reproductive organs
Mostrotifers have a single ovary. A syncytial (multinucleate) vitellarium is attached Nit
it. It produces yolk for eggs. The ovary and vitellarium fuse to form a single
germovitellarium. After fertilization, each egg travels through a short oviduct. It enters
into cloaca bladder and passes out through its opening.

Male reproductive organs

The mouth, cloacal bladder, and other digestive organs are absent in males. A single
testis produces sperm. Sperms travel through a ciliated vas deferens and enter into the
gonopore. Male rotifers have an eversible penis. It injects sperm into the pseudocoelom
of the female. It is called hypodermic impregnation (Figure 4.3).

Figure 4.3: Life cycle of Rotifers

Fertilization
1. In one class (Seisonidea), the females produce haploid eggs. This egg is fertilized
to develop into males or females.
2. In another class (Bdelloidea), all females are parthenogenetic. They produce
diploid eggs. These eggs hatch into diploid females.
3. The third class (Monogononta) produces two different types of eggs: Amictic and
mictic.

 Life Cycle
1) Amictic cycle: Amictic females produce amictic eggs by mitosis. These eggs
are diploid. These cannot be fertilized. Therefore, they develop directly into amictic
females. Therefore, first amictic cycle starts. These female develops large
populations quickly. Some other eggs become dormant. Another amictic cycle
starts more dormant eggs are produced. It occurs before the yearly cycle is over.
Winds or birds disperse dormant eggs.

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Mictic cycle: Some amictic females produce mictic eggs in summer by meiosis. These
are thin-shelled eggs. Mictic eggs are haploid. Some mictic eggs are not fertilized. These
develop parthogenetically into a male. Some mictic eggs are fertilized and secrete a thick,
heavy shell. These become dormant or resting winter eggs. Dormant egg is hatch in
melting snows and in spring rains. They develop into amictic females.

Most females lay amictic or mictic eggs but not both. The physiological condition during
of the female oocyte development determines whether her eggs will be amictic or mictic.

Self Assessment Questions

Q: Fill in the blanks.
i. Aschelminths can be free-living or…………. ( parasitic)
ii. ……………… is the Giant Intestinal Roundworm. (Ascaris lumbricoides)
iii. The rotifers derive their name from characteristic ciliated organ called
…………(Corona)
iv. The cuticle or ………. provides protection (Lorica)
v. The ……… is the largest part of a rotifer (Trunk)

Q: Answer the following.

i. What is wheel animalcule?
ii. What is molting?
iii. What do you know about pseudocoelem?
iv. Give few examples of aschelminthes.
v. Differentiate mictic and amictic cycle.

4.3.2 Phylum Kinorhyncha

Kinorhynchs are small, elongate bilaterally symmetrical worms. Their size is less than 1
mm. They are found exclusively in marine environments. They live in mud and sand.
They have no external cilia or locomotory appendages. Therefore, they simply burrow
through the mud and sand with their snouts. Therefore, the phylum takes its name from
this method of locomotion. The phylum Kinorhyncha contains about 150 known species.

 External Features
The cilia are absent on the body surface of kinorhynch. It is composed of 13 or 14
definite units called zonites. The head, is zonite 1. It bears mouth, an oral cone. and
spines. The neck is Anire 2. It contains spines called scalids and plates called placids.
The head can he retracted into the neck. The trunk consists of the remaining 11 or 12
zonites. It ends with the anus. Each trunk zonite bears a pair of lateral spines and one
dorsal spine (Figure 4.4).

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Figure 4.4: External anatomy of Kinorhynch

 Internal Features
The body wall consists of a cuticle, epidermis and two pairs of muscles. These muscles
are dorsolateral and ventrolatcral. The pseudocoelom is large and contains ambeboid
cells.

Feeding and Digestive System

A complete digestive system is present in them. It consists of a mouth, buccal cavity,
muscular pharynx, esophagus, stomach, intestine and anus. Most kinorhynchs feed on
diatoms, algae, and organic matter.

Other Organ Systems

Apair of protonephridia is present on zonite 11. The nervous system consists of brain and
single ventral nerve cord. This nerve chord has a ganglion in each zonite. Some species
have eyespots and sensory bristles.

Reproduction and Development

Kinorhynchs are dioecious with paired gonads. Several spines surround the male
gonopore. These spines are used in copulation. The young hatch into larvae. The larva
does not have all of the zonites. As the larvae grow and molt, the adult morphology
appears. Molting no longer occurs in adult.

4.3.3 Phylum Nematoda

Nematodes are roundworms. They are some of the most abundant animals on earth.
Number or round worm species is from 16, 000 to 500,000. There are following
characteristic of these animals:

Roundworms feed on every source of organic matter. They feed on rotting. substances in
the living tissues of other invertebrates, vertebrates and plants.

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They are from microscopic to several meters long.

Many nematodes are parasites of plants or animals. Some are free- living in marine,
freshwater, or soil habitats. Some nematodes play an important role in recycling nutrients
in soils and bottom sediments.

There are two common characteristics between nematodes and arthropods:

a) Nematodes and arthropods lack cilia.
b) The sperm of nematodennd arthropods are amoeboid.

They are triploblastic, bilateral, vermiform (resembling a worm in shape; long and
slender), unsegmented, pseudocoelomate.

Their body is rounded in cross section.

Body is covered by a cuticle. Growth takes place by molting.

They have complete digestive tract. Mouth is surrounded by lips Sense organs are present
on lips.

They have unique excretory system. It is composed of one or two cells or a set of
collecting tubules. Their body wall contains only longitudinal muscles.

 Exernal Features
The body nematode is slender, elongate, cylindrical, and tapered at both ends. It has
following external structures:
1. Cuticle: The nematodes are most successful due to presence of cuticle on body
wall. This cuticle is non cellular and collagenous.. Cuticle is also present in the
foregut, hindgut, sense organs and parts of the female reproductive system. The
cuticle may be smooth. Or it may contain spines, bristles, papillae, warts or
smooth. Such structures have taxonomic importance. Cuticle is composed of three
primary layers. These layers are cortex, matrix layer, and basal layer. The cuticle
maintains internal hydrostatic pressure. It provides mechanical protection. It resists
digestion by the host in parasitic species. The cuticle is molted four times during
maturation.
2. Epidermis: Epidermis or hypodermis is present beneath the cuticle. It surrounds the
pseudocoelom. The epidermis may be syncytial. Its nuclei are present in four epidermal
cords (one dorsal, one ventral, and two laterals). These nuclei project inward.
3. Muscles: They have only longitudinal muscles. They are the principal means of
locomotion in nematodes. Contraction of these muscles produces undulatory
waves. These waves pass from the anterior to posterior end of the animal. It causes
characteristic thrashing movements. Nematodes lack circular muscles. Therefore,
they cannot crawl.

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4. Lips and skeleton: Some nematodes have lips surrounding the mouth. Some
species develop spines or teeth on the lips. In others, the lips have disappeared.
Some roundworms have head shields for protection.
5. Sense organs: Sensory organs are amphids, phasmids and ocelli. The nematods are
lassi lied on the basis of presence or absence of these sense organs.

a) Amphids are anterior depressions in the cuticle. It contains modified cilia. It

functions in chemoreception.
b) Phasmids are near the anus. It also functions in chemoreception.
c) Paired OceIli (eyes) are present in aquatic nematodes.

 Internal Features
The pscudocoelom of nematode is a spacious, fluid-filled cavity. It contains visceral
organs. It forms a hydrostatic skeleton. The body muscles contracts against the
pseudocoelom. Thus fluid generates an equal outward force in all directions. Therefore
all the nematodes are round.

 Feeding and the Digestive System

Nematodes can feed on a wide variety of foods. They may be carnivores, herbivores,
omnivores, or saprobes (saprotrophs), or parasitic species. The parasitic species feed on
blood and tissue fluids of their hosts.

Nematodes have a complete digestive system. It consists of a mouth. buccal cavity;

muscular pharynx; long tubular intestine. Mouth may have teeth, jaws, or stylets (sharp,
pointed structures). Digestion and absorption occur in intestine. They have short rectum
and anus. Hydrostatic pressure in the pseudocoelom and the pumping action of the
pharynx push food through the alimentary canal (Figure 4.5).

Figure 4.5: Phylum Nematoda. Internal anatomical features of (a) Female and (b) Male
Rhabditis. (c) Section through nematode cuti showing the various layers. (d) Cross
section through the region of the muscular pharynx of a nematode.

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 Other Organ Systems

Excretion
The osmoregulation and exeretion of nitrogenous aste products (ammonia, urea) take
place by two nique systems.
a) Glandular system: It is present in aquatic species. It consists of ventral gland cells
called renettes. It is present posterior to the pharynx. Each gland absorbs wastes
from the pseudocoelom. It opens outside through an excretory pore.
b) Tubular system: Parasitic nematodes have a more advanced system. It is called
the tubular system. It is developed from the renette system. In this system, the
renettes unite to form a large canal. This canal opens outside through an excretory
pore (Figure 4.6).

Figure 4.6: Excretory system (a) Glandular (b) Tubular

Nervous System
The nervous system consists of an anterior nerve ring. Nerves extend anteriorly and
posteriorly. They may connect to each other via commissures. Certain neuroendocrine
secretions are involved in growth, molting, cuticle formation by metamorphosis.

Reproduction and Development

Most nematodes are dioecious and dimorphic (different shapes). The males are smaller
than the females. The gonads are long and coiled. They lie freely in the pseudocoelom.

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Female Reproductive Organs
The female system consists of a pair of convoluted ovaries. Each ovary is continuous
with an oviduct. The proximal end of oviduct is swollen to form a seminal receptacle.
Each oviduct becomes a tubular uterus. The two uteri unite to form a vagina. Vagina
opens to the outside through a genital pore.

Male Reproductive System

The male system consists of a single testis. It is continuous with a vas deferens. Vas
deferens opens into a seminal vesicle. The seminal vesicle connects to the cloaca. Males
contain a posterior flap of tissue called a bursa. The bursa helps in the transfer of sperm
to the female genital pore during copulation (Figure 4.7).

Figure 4.7: Nematode Reproductive Systems The reproductive systems of (a) female and
(b) male nematodes, such as Ascaris

 Fertilization and Development

Fertilization takes place during copulation. The hydrostatic forces in the pseudocoelom
move each fertilized egg to the gonopore. The number of eggs produced varies with the
species. Some nematodes produce only several hundred eggs. But others may produce h
indreds of thousands daily. Some nematodes give birth to larvae (ovo-viviparity).
External factors like temperature and moisture influence the development and hatching of
the eggs. Hatching produces a larva. The larva has most adult structures. The larva
(juvenile) undergoes four molts. In some species, the first one or two molts may occur
before the eggs hatch.

Self Assessment Questions

Q: Fill in the blanks.
i. The cilia are absent on the body surface of kinorhynch. It is composed of 13
or 14 definite units called ……………. (Zonites).
ii. Kinorhynchs are ……………… with paired gonads (dioecious).

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 iii. The hydrostatic forces in pseudocoelom move each fertilized egg to the
………….. (Gonopore).
iv. Nematodes have a complete ………….. system (Digestive).
v. Nematodes are …………… (Round worms).

Q: Answer the following.

i. What are two common characteristics between nematodes and arthropods?
ii. What are amphids?
iii. What are scalids and placids?
iv. What are zonites?
v. What do you know about commissures?

4.3.4 Phylum Nematomorpha

Nematomorphs are a small group of about 250 species. They are elongated worms. They
are commonly called horsehair worms or Gordian worms.

Habitat: The adults are free-living. But juveniles are all parasitic in arthropods.They
have a worldwide distribution. They are found in both running and standing water.

Body form: The nematomorph body is extremely long and threadlike. They have no
distinct head.

Body wall: The body wall has a thick cuticle, a cellular epidermis and longitudinal cords.
It also has a muscle layer of longitudinal fibers.

Nervous system: The nervous system contains an anterior nerve ring and a ventral cord.

Reproduction: Nematomorphs have separate sexes. Two long gonads extend the length
of the body. The eggs are deposited in water.

Development: A small larva is hatched. It has a protrusible proboscis armed with spines.
Terminal stylets are also present on the proboscis. The larva must quickly enter an
arthropods (e.g. a beetle cockroach) host. It enters the host by penetrating the host or it is
eaten by arthropod. Larva lacks a digestive system. The larva absorbs material directly
across its body wall. The larva becomes mature. It leaves its host only when the
arthropod is near water. The free-living adult form becomes sexual mature (Figure 4.13).

105
Figure 4.8: (A) An entire Gordius (B) Transverse section of female Gordius

4.3.5 Phylum Acanthocephala

1. Habitat: Adult acanthocephalans are endoparasite in the intestinal tract of
vertebrates (especially fishes). They complete their life cycle in two hosts. The
juveniles are parasites of crustaceans and insects. Zoologists have identifiedabout a
thousand species.
2. Size: Acanthocephalans are generally small. They are less than 40 mm long).
However, one species Mucracamhorhynchus hirudinaccus can be up to 90 cm long.
It occurs in pigs.
3. Body form: The body of the adult is elongate. Body is composed of a short
anterior proboscis, a neck region and a trunk. Trunk is covered with recurved
spines. Therefore, they are named as spiny-headed worms. The retractable
proboscis is used for attachment in the host’s intestine. Females are always larger
than males.
4. Body wall: Body wall is covered by a living syncytial tegument. It is an adaptation
to the parasitic way of life. A glycocalyx covers the tegument. It consists of
mucopolysaccharides and glycoproteins and protect against host enzymes and
immune defenses.
5. Digestive system: Digestive system is absent in acanthocephalans. They absorb
food directly through the tegument from the host. Protonephridia may be present
6. Nervous system: The nervous system is composed of a ventral, anterior ganglionic
mass. Anterior and posterior nerves arise form it. Sensory organs are poorly
developed.
7. Reproduction: The sexes are separate. The male has a protrusible penis.
Fertilization is internal. Eggs develop in the pseudocoelom.
8. Development: The eggs pass out of the host with the feces. These eggs must be
eaten by certain insects like cockroaches or grubs (beetle larvae). They are also
eaten by aquatic crustacean (e.g. amphipods. isopods, ostracocis). The larva
emerges from the egg in the host. It is now called an acanthor. It burrows through

106
 the gut wall and reaches the hemocoel. It develops into an acanthella in hemocoel.
It finally changes into a cystacanth. Mammal, fish or bird eats the intermediate
host. The cystacanth comes out of cyst. It attaches to the intestinal wall with its
spiny proboscis and develops into adult (Figure 4.8).

Figure 4.9: Phylum Acanthocephala

4.3.6 Phylum Loricifera

The phylum Loricifera is a recently described animal phylum. Zoologists have described
about 14 species of Loriciferans. Its first members were identified and named in 1983.
Loriciferans live in spaces between marine gravel (sand + silt). A characteristic species is
Name/oricits mysticav. It is a small, bilaterally symmetrical worm. It has a spiny head
called introvert, a thorax and an abdomen. Abdomen is surrounded by a lorica.
Loriciferans can retract both introvert and thorax into the anterior end of the lorica. The
introvert contains eight oral styles that surround the mouth. The lorical cuticle is
periodically molted. A pseudocoelom is present. It contains a short digestive system,
brain, and several ganglia. Loriciferans are dioecious with paired gonads (Figure 4.15).

Figure 4.10: Phylum Loricifera

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4.3.7 Phylum Priapulida
1. Habitat: The priapulids are a small group. It has only 16 species of marine worms
found in cold waters. They lived in the mud of sand or seafloor. They feed on small
annelids and other invertebrates. Priapulus cuudalus is most common specie.
2. Body Form and Size: The priapulid body is cylindrical in cross section. They
range in length from 2 mm to about 8 cm.
3. Body Parts: The anterior part of the body is introvert(proboscis).
They can draw this introvert into longer posterior trunk. The muscles introvert is
surrounded by spines. It is used for burrowing.
4. Body Wall: A thin cuticle covers the body. This cuticle has spines. The trunk
bears superficial annuli.
5. Pseudocoelom: A straight digestive tract is suspended in a large pseudocoelom.
Pseudocoelom acts as a hydrostatic skeleton. In some species, the pseudocoelom
contains Excretory amoeboid cells. These organ cells are used for transport of gas.
6. Nervous System: The nervous system consists of a nerve ring around the pharynx
and a single midventral nerve cord.
7. Reproduction: The sexes are separate. A pair of gonads is suspended in the
pseudocoelom. It shares a common duct with the protonephridia. The duct opens
near the anus and gametes are shed into the sea. Fertilization is external. The eggs
finally sink to the bottom. Here larvae develop into adults. The Cuticle is repeatedly
molted throughout life (Figure 4.16).

Figure 4.11: Phylum Priapulida

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Self Assessment Questions
Q: Fill in the blanks.
i. The Giant Intestinal Roundworm of Humans is ……………. (Ascaris
lumbricoid).
ii. ……………. are the most common roundworm parasites in the United States
(Pinworms).
iii. A filarial worm …………… is common in the United States (Dirofilaria
immitis)
iv. The nematomorph body is extremely long and ……………… (Threadlike).
v. The phylum …………… is a recently described animal phylum (Loricifera).

Q: Answer the following.

i. Name various nematode parasites in humans.
ii. Where flariform larva is formed?
iii. Write few characters about phylum priapulida.
iv. What do you know about trichinosis?
v. What are spiny headed worms?

4.4 Some Important Nematodes Parasites of Humans

Parasitic nematodes have a number ‘of evolutionary adaptations. These adaptations are:
1. They have high reproductive potential.
2. Their life cycles increase the chance of transmission from one host to another.
3. They develop enzyme resistant cuticle, resistant eggs and encysted larvae.
4. Only one host is involved in the life cycle Of nematods. Therefore Nematode life
cycles are not as complicated like cestodes or tremoteds.

Ascris lumbricoides: The Giant Intestinal Roundworm of Humans

Approximately 800 million people are infected with Ascaris throughout the world.
Ascaris live in the small intestine of humans. They produce large numbers of eggs. These
eggs pass out with feces. A first-stage larva develops rapidly in the egg. It molts and
forms second-stage larva. Second larva is the infective stage. Human may ingests
embryonated eggs. They are hatched in the intestine. The larvae penetrate the intestinal
wall. Blood carries it to the lungs. They molt twice in the lungs and move up into the
trachea, and are swallowed. The worms become sexually mature in the intestine. It mates
and begins egg production (Figure 4.12).

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Figure 4.12: Life cycle of Ascaris Lumbricoide

Enterobius vermicularis: The Human Pinworm (enteron, intestine + bios, life)

Pinworms are the most common roundworm parasites in the United States. Adult
Enterobius present in the lower region of the large intestine. The gravid females move out
of the caecum at night. It reaches into the perianal area (area around anus). They deposit
egg there. These eggs develop first stage larva. The eggs then fall. The human ingest the
eggs and they are hatched. The larva molt four times in the small intestine. It moves into
large intestine. Mating takes place between the male and female and again egg production
starts (Figure 4.9).

Figure 4.13: Life cycle of Enterobius vermicularis

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Necator americanus: The New World or American hookworm, Necator americanus is
found in the southern United States. The adults live in the small intestine. They hold the
intestinal wall with teeth and feed on blood and tissue fluids. Female may produce as
many as 10,000 eggs daily. These eggs pass out of the body with feces. An egg hatches
on warm and moist soil. It releases a small rhabditiform larva. It molts and becomes the
infective filariform larva. Filariform larva penetrates the human’s skin between the toes.
The larva burrows through the skin. It enters into the circulatory system. It reaches the
small intestine and becomes adult (Figure 4.14).

Figure 4.14: Life cycle of Necator americanus

Trichinella spiralls: The Porkworm (Or. Trichinos, Hair)

Adult Trichinella live in the mucosa of the small intestine of humans and other omnivores
Lille pigs. The adult females produce young larvae in the intestine. It then enters into the
circulatory system. It then reaches in skeletal muscles. The young larva forms cyst in the
skeletal muscles. The muscle remains infective for many years. It causes disease called
trichinosis. Another host ingests infective meat (muscle). Humans are infected by eating
improperly cooked pork (flesh of pig) products. After ingestion the larvae form cyst in
the stomach. It reaches in to the small intestine. They molt four times in intestine and
develop into adults (Figure 4.15).

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Figure 4.15: Life cycle of Trichinella spiralls

Wuchereria spp: The Filarial Worms (L fihium, thread)

Over 250 million humans are infected with filarial worms in tropicalcountries. Two
species of human filarial worms are W bancrofti and W. malayi. These are elongated
thread like nematodes. They live in the lymphatic system and block the lymph vessels.
The lymphatic vessels return tissue fluids to the circulatory system. Therefore, fluids and
connective tissue accumulate in peripheral tissues. This accumulation causes the
enlargement of various appendages. This condition is called elephantiasis.

Life Cycle of Wuchereria

1. The adult filarial nematodes copulate in the lymphatic vessels. They produce larvae
called microfilarie. The microfilariae are released into the bloodstream of the
human host. They migrate to the peripheral circulation at night.
2. A mosquito feeds on a human and ingests the microfilariae. The microfilariae
migrate to the thoracic muscles of mosquito. There they molt twice and become
infective.
3. The mosquito injects it proboscis into a healthy person. It transfers the infective
third-stage larvae into the blood of the healthy human. The larvae enter the
lymphatic vessels. Final two molts take place in it and it becomes adult.

A filarial worm Dirofilaria immitis is common in the United States. It is a parasite of

dogs. The adult worms live in the heart and large arteries of the lungs. Its infection is
called heartworm disease. Filarial worms are difficult to eliminate and they can be fatal.
All dogs should be given heart- worm medicine (Figure 4.12).

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Figure 4.16: Life cycle of Wuchereria

4.5 Further Phylogenetic Considerations

The aschelminths are diverse animals. They have common cuticle, pseudocoelom,
muscular pharynx and adhesive glands. But no distinctive features in every phylum.

1. Origin of Rotifers
The rotifers and acoelomates have certain common features. The protonephridia of
rotifers closely resemble with some of the freshwater turbellarians. The zoologists
giznerally believed that rotifers originated in freshwater habitats. Both flatworms
and rotifers have separate ovaries and vitellaria. Rotifers may have originated from
the curliest acoelomates. Both these groups may have common bilateral, metazoan
ancestor.
2. Origin of Other Phyla
The kinorhynchs, acanthocephalans, loricIferans and priapulids all have a spiny
anterior end. This end can be retracted.Therefore, they are related to each other.
Lioriciferans and kinorhynchs are most closely related.
3. Origin and Affinities of Nematoda
The affinities of the nematodes with other phyla are not clear. No other living
group is closely related to these worms. Nematodes evolved in freshwater habitats.
Then they established in the oceans and soils. The ancestral nematodes were
sessile. They were attached at the posterior end. The anterior end protrude upward
into the water. The nematode cuticle, feeding structures, and food habits have

113
 adapted these worms for parasitism. Therefore, free-living species can become
parasitic without anatomical or physiological changes.

Nematomorphs are more closely related to nematodes due to cylindrical shape, cuticle,
doecious and sexually dimorphic. But the larval form of some nematomorphs resembles
plapulids. Therefore, their exact affinity to the nematodes is questionable.

Key Points/Summary
1. The aschelminthes are seven phyla having well-defined pseudocoelem, constant
number of body cells, protonephridia and complete digestive system with well
developed pharynx.
2. Majority of rotifers inhabit fresh water. The head of these animals bears unique
ciliated corona used for locomotion and food capture.
3. Kinorhynchs are minute worms living in marine habitats. Their bodies are
comprised of 13 zonites which have circular plates and spines.
4. Nematodes live in aquatic and terrestrial environments. Many are parasitic and of
medical and agricultural importance.
5. Nematomorpha are thread like and free living in fresh water.
6. Acanthocephalans are also known as spiny headed worms and all are endoparasites
in vertebrates.
7. Phylum loricifera have microscopic animals having spiny head and thorax.
8. Phylum priapulida contains only 16 known species of cucumber shaped, worm like
animals that lived buried in bottom sand and mud in marine habitats.

References
 Dudek, Ronald W.; Fix, James D. (2004). "Body Cavities". Embryology.
Lippincott Williams & Wilkins.
 Evers, Christine A., Lisa Starr. Biology:Concepts and Applications. 6th ed. United
States:Thomson, 2006.
 Hall, B.K.; et al. (2008). "Animals Based on Three Germ Layers and a Coelem".
Strickberger's evolution: the integration of genes, organisms and populations. Jones
& Bartlett Learning.
 Overhill, Raith, ed. (2006). "What are the advantages of the coelem and
metamarism?". An introduction to the invertebrates (2nd ed.). Cambridge
University Press.
 Ruppert, Edward E.; Fox, Richard, S.; Barnes, Robert D. (2004). Invertebrate
Zoology, 7th edition. Cengage Learning. p. 205.

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UNIT-5

MOLLUSCAN SUCCESS

Written by: Dr. Sobia Mushtaq

Reviewed by: Arshad Mehmood Qamar

115
 CONTENTS
Introduction ....................................................................................................... 117

Objectives ......................................................................................................... 118

5.1 Evolutionary Perspective Relationship to other Animals; Origin of

the Coelom ............................................................................................ 119

5.2 Molluscan Characteristics ..................................................................... 121

5.3 Classification up to Class ...................................................................... 123

5.4 The Characteristics Features ................................................................. 125

5.5 Further Phylogenetic Considerations .................................................... 142

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Introduction

Mollusc is the second-largest phylum of invertebrate animals after the arthropoda.

Around 85,000 extant species of molluscs are recognized. The number of fossil species is
estimated between 60,000 and 100,000 additional species. The proportion of undescribed
species is very high. Many taxa remain poorly studied.

Molluscs are the largest marine phylum, comprising about 23% of all the named marine
organisms. Numerous molluscs also live in freshwater and terrestrial habitats. They are
highly diverse, not just in size and anatomical structure, but also in behaviour and habitat.
The phylum is typically divided into 8 or 9 taxonomic classes, of which two are entirely
extinct. Cephalopod molluscs, such as squid, cuttlefish, and octopuses, are among the
most neurologically advanced of all invertebrates and either the giant squid or the
colossal squid is the largest known invertebrate species. The gastropods (snails and slugs)
are by far the most numerous molluscs and account for 80% of the total classified
species.

The three most universal features defining modern molluscs are a mantle with a
significant cavity used for breathing and excretion, the presence of a radula (except for
bivalves), and the structure of the nervous system. Other than these common elements,
molluscs express great morphological diversity, so many textbooks base their
descriptions on a "hypothetical ancestral mollusc". This has a single, "limpet-like" shell
on top, which is made of proteins and chitin reinforced with calcium carbonate, and is
secreted by a mantle covering the whole upper surface. The underside of the animal
consists of a single muscular “foot”. Although molluscs are coelomates, the coelom tends
to be small. The main body cavity is a hemocoel through which blood circulates; as such,
their circulatory systems are mainly open.The “generalized” mollusc’s feeding system
consists of a rasping “tongue”, the radula, and a complex digestive system in which
exuded mucus and microscopic, muscle-powered “hairs” called cilia play various
important roles. The generalized mollusc has two paired nerve cords, or three in bivalves.
The brain, in species that have one, encircles the esophagus. Most molluscs have eyes,
and all have sensors to detect chemicals, vibrations and touch. The simplest type of
molluscan reproductive system relies on external fertilization, but more complex
variations occur. All produce eggs, from which may emerge trochophore larvae, more
complex larvae, or miniature adults. The coelomic cavity is reduced. They have an open
circulatory system and kidney-like organs for excretion.

Good evidence exists for the appearance of gastropods, cephalopods, and bivalves in the
Cambrian period, 541 to 485.4 million years ago. However, the evolutionary history both
of molluscs emergence from the ancestral Lophotrochozoa and of their diversification
into the well-known living and fossil forms are still subjects of vigorous debate among
scientists.

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Objectives
After completion of this unit, you will be able to:
 describe origin of coelom.
 discuss characteristics of Mullascans
 classify phylum Mollusca upto class level
 present characteristics of shell and associated structures, feeding, digestion, gas
exchange and locomotion.
 compare the reproduction among members of different classes.
 find common characteristics of different organisms of different classes.
 explain maintenance functions and diversity in gastropods.
 explain different aspects of Bivalves and Cephalopods.

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5.1 (A) Evolutionary Perspective: Relationship to Other
Animals; Origin of the Coelem
Some of the molluscs like octopuses, squids and cuttlefish (the cephalopods) are most
adapted predators. The evolution of large brains in them takes place due to predatory
lifestyles. They have complex sensory structures, rapid locomotion, grasping, tectacles
and tearing mouthparts. But cephalopods are not present in large number. Once their
number of species was nine thousand. Now this class includes only about 550 species.
Zoologists can explain the reason of their decline. There can be two reasons of their
decline:

The vertebrates appeared in prehistoric seas. Sonic vertebrates acquired active, predatory
lifestyles. It may be possible that vertebrates have outcompeted cephalopods.

The cephalopods may be declined simply because of random evolutionary events. This is
not the case for all molluscs. Overall, this is very successful group. The molluscs have
twice the number of vertebrates. Majority of the nearly 100,000 living species of
molluscs belongs to two classes:
a) Gastropoda: the snails and slugs
b) Bivalvia: the clams and their close relatives.

Molluscs are triploblastic. They are the first animals that possess a coelom. But the
coelom of molluscs is only a small cavity. This is called pericardial cavity. It surrounds
art and gonads. A coelom is a body cavity that arises in mesoderm and is lined by a
sheet of mesoderm called peritoneum (Figure 5.1).

Figure 5.1: Evolutionary relationship of molluscs

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5.1 (B) Relationships to Other Animals
Molluscs are protostomes. In protosotme, blastopore form mouth. Therefore, there are
great similarities in the embryological development of the molluscs and other proto tomes
like annelids (segmented worms). These similarities are:
1. Both have similar trochophore larvae.
2. Certain adult structures of Molluscs and annelids are similar. For example, the
excretory organs and their duct systems are similar in them.

Most zoologists accept the protostomes relations of the molluscs. But the relationship
between the members of this phylum and other protostomes is distant. Therefore, the
ancestral evolutionary pathways are speculative.

5.1 (C) Origin of the Coelom

There are a number of hypotheses about the origin of the coelom. These hypotheses
relationships among triploblastic phyla.

1. Schizocoel Hypothesis: (Gr. schizen. to split + koilos, hollow)

In this case, coelom is formed by the splitting of mesoderm. It is found in all
protostomes. Mesoderm fills the area between ectoderm and endoderm. Coelom is
formed by the splitting of this mesoderm. It indicates that mesodermally derived tissues
are formed before the coelom formation. It suggests that a triploblastic acoelomate
(flatworm) body form is the ancestor of the coelomate body form.

2. Enterocoel Hypothesis: (Gr. enteron. gut + koilos, hollow)

It suggests that the coelom have arisen as out pocketing of a primitive gut. This pattern of
coelom formation is present in deuterostomes. The animals in which blastopore forms
anus and mouth formed as a secondary opening are called deutrostomes. This hypothesis
suggests that mesoderm and the coelom are formed from the gut of a diploblastic animal.
It means that the mesoderm fills the body cavity of a coelomate and triploblastic
acoelomate body was formed. Therefore, acoelomate body form was secondarily derived
from the coelomate body form.

Conclusion
Unfortunately, zoologists do not know which hypothesis is correct. Some zoologists
believe that the coelom was formed more than once in different evolutionary lineages.
Therefore, more than one explanation can be correct (Figure 5.2).

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Figure 5.2: Molluscan body organization

5.2 Molluscan Characteristics

The size of molluscs varies greatly. The largest body form is the giant squid 18 m in
length. The smallest garden slug is less than 1 cm long. They have following common
characteristics:
1. Body has two parts: head-foot and visceral mass.
2. Mantle secretes a calcareous shell. It covers the visceral mass.
3. Mantle cavity functions in excretion, gas exchange, elimination of digestive wastes.
and release of reproductive products
4. They have bilateral symmetry.
5. The develop protosotme characteristics like trochophore larvae, spiral cleavage and
schizocoelous coelom formation.
6. Coelom reduced to a cavity. This cavity surrounds the heart, nephridia and gonads.
7. They have open circulatory system except in one class (Cephalopoda).
8. Radula is present. It is used in scraping of food.

Regions of the Body

The body of molluscs has three main regions: the head- foot, the visceral mass and the
mantle.

a) Head- Foot Region: It is elongated with an anterior head. Head contains mouth
and certain nervous and sensory structures. It has elongated foot. It is used for
attachment and locomotion.
b) Visceral Mass: Visceral mass contains the organs of digestion, circulation,
reproduction, and excretion. It is presented dorsal to the head-foot.
c) Mantle: Mantle of molluscs is attached to the visceral mass. It enfolds most of the
body. It secretes a shell that overlies the mantle.

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Shell
Th shell of the molluscs is secreted in three layers:

1. Periostracum: The outer layer of the shell is called the l’eriostracum. It is protein
in nature. Mantle cells at the mantle’s outer margin secrete this layer.
2. Prismatic Layer: The middle layer of the shell is called the prismatic layer. It is
the thickest of the three layers. It consists of calcium carbonate mixed with organic
materials. Cells at the mantle’s outer margin also secrete this layer.
3. Nacreous Layer: The inner- layer of the shell is called nacreous layer. It forms
thin sheets of calcium carbonate alternating with organic matter. The epithelial
border cells of the mantle secrete the nacreous layer. Nacre secretion thickens the
shell (Figure 5.3).

Figure 5.3: Molluscan Shell and Mantle

Mantle cavity
The space between the mantle and the foot is a called the mantle cavity. The mantle
enmity opens to the outside. It functions in gas exchange. excretion. elimination of
digestive wastes and release of reproductive products.

Radula
The mouth of most molluscs possess a rasping organ called radula. Radula consists of a
chitinous belt and rows of posteriorly curved teeth. The radula lies over a fleshy tongue
like structure. It is supported by a cartilaginous odontophore. Muscles and odontophore
help the radula to protrudeout from the mouth.

Muscles move the radula back and forth over the odontophore. Food is scraped from a
substrate and passed posteriorly to the digestive tract (Figure 5.4).

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Figure 5.4: Radula structure

Self Assessment Questions

Q: Fill in the blanks.
i. …………….. is the second-largest phylum of invertebrate animals after the
arthropoda (Mollusc).
ii. The ……………….may be declined simply because of random evolutionary
events (Cephalopods).
iii. Muscles move the radula back and forth over the ………….(Odontophore).
iv. The space between the mantle and the foot is a called the ………….(Mantle
cavity).
v. Molluscs are ……………….. (Protostomes).

Q: Answer the following.

i. What is peritoneum?
ii. What do you know about Enterocoel hypothesis?
iii. What is the function of radula?
iv. What are three main regions of body of molluscs?
v. Write few examples of molluscs.

5.3 Classification up to Class

1. Class Caudofoveata
1. They are wormlike molluscs.
2. They have cylindrical, shell-less body and scale like, calcareous spicules.
3. They lack eyes, tentacles, statocysts, crystalline style, foot and nephridia.
4. They are deep-water and marine burrowers.
Examples: Chaetoderma
2. Class Aplacophora
1. They lack shell, mantle and foot.

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 2. They are also wormlike
3. Head is poorly developed
4. They are burrowing molluscs They are marine.
Example: Neomenia.

3. Class Polyplacophora
1. They are elongated and dorsoventrally flattened.
2. Head is reduced in size.
3. Shell consisting of eight dorsal plates.
4. They are marine, on rocky intertidal substrates.
Example: Chiton.

4. Class Monoplacophora
1. They have single arched shell.
2. Their foot broad and flat.
3. They are marine.
Example: zeopilina.

5. Class Scaphopoda
1. Body enclosed in a tubular shell. It is open at both ends.
2. Tentacles are used for deposit feeding.
3. They have no head.
4. They are marine.
Example: Dentalium.

6. Class Bivalvia
1. Body enclosed in a shell consisting of two valves. Valves are hinged
2. They have no head or radula.
3. They have wedge-shaped foot.
4. They are marine and freshwater.
Eample: Anodonta, Mytilus, Venus

7. Class Gastropoda
1. Shell is coiled.
2. Their body symmetry distorted by torsion.
3. Some monoecious species.
4. They are marine, freshwater and terrestrial.
Example: Nerita, Helix

8. Class Cephalopoda
1. Foot modified into a circle of tentacles and a siphou.
2. Shell is reduced or absent.
3. Head is in line with the elongated visceral Mass.
4. They are marine.
Example: Octopus, Loligo, Sepia, Nautilus.

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5.4 The Characteristics Features
(Shell and Associated Structures, Feeding, Digestion, Gas Exchnge,
Locomotion, Reproduction & Development and Other Maintenance Functions
in Selected Classes)

5.4.1 Caudofoveata
The exclusively sea-living class Caudofoveata is among the least known molluscs. Only
by the end of the 19th century it became known that they are molluscs; before they had
been placed among the holothurians, worm like echinoderms, related to starfish. So were
the Solenogasters (Solenogastres). Caudofoveatans generally are very small, mostly only
few centimetres long. They live in a depth of more than 20 m, where they can appear in a
density of up to 4 to 5 individuals per squarmetre, as far down to the deep sea. Externally,
caudofoveatans resemble worm-like beings. They have no recognizable shell, which is
why they are counted among the so-called shell-less molluscs ("Aplacophora"). With a
closer look, some characters typical for molluscs can be recognized. Though
caudofoveatans do nohave a shell, they do have a sturdy exterior tissue, the cuticula. This
cuticula is additionally protected by calcareous scales, which are estimated to be a
predecessor of the more highly developed molluscs' shell. Chitons (Polyplacophora) also
have this cuticula, but only as a rudimentary protection around their sides, the so-called
girdle or perinotum. The rest of the molluscs' back is protected by shell plates, that are
thought to have evolved from calcareous scales we find today among caudofoveatans. It
is well imaginable that the first molluscs have evolved from worm-like ancestors, such as
the caudofoveatans, which lived on the ocean floor or dug in it, and whose sturdy skin
was protected by calcareous scales, which in time would change into the astounding shell
of most of today's mollusc groups. The worm-like form of today's caudofoveatans,
though, is the result of a secondary reduction of the foot, more advanced in this group,
than, for example, in the solenogasters.

The Evolution of the Mollusc Shell

At their head end, caudofoveatans have a hard shield studded with sense cells helping the
creature's orientation. Caudofoveatans live on the ocean floor, where they look for food,
either crawling or digging. On the one hand they are detritus eaters, so they live on
decaying organic matter, or they feed on monocellular organisms, such as foraminifers
and diatoms (silicate algae).

As in all molluscs, also in caudofoveatans, feeding takes place with the help of a rasp
tongue, the radula, which may have as many as 1,000 toothlets. Caudofoveatans, though,
do not yet possess the ribbon-like radula of higher molluscs; theirs grows from the
oesophagus wall. So the radula is a very old character, which seems to have appeared
with the very first molluscs long ago. Among caudofoveatans, the radula is quite variable,
comparable to the radula of a gastropod. In that regard, caudofoveatans are different from
scaphopods, similar in their digging way of life, and chitons (Polyplacophora).
As do other molluscs, caudofoveatans also have a pallial cavity, only it is very small and
located at the body's end. In the pallial cavity, there are the paired comb-gills or ctenidia.
The gonad, the sexual organ, is located on the dorsal side and from it a efferent duct leads

125
to the pericardium (the heart bag) and from there several ducts open into the pallial
cavity, also a very ancient character. The dorsal gonad is also present in chitons
(Polyplacophora), so it is a character which both groups have in common.
Caudofoveatans have separate sexes, so there are males and females; fertilization takes
place externally in the water. As it does among the solenogasters, larval development in
caudofoveatans takes place passing a planktontic trochophora stage.

The neural system of the Caudofoveata is simple and built like a rope ladder. There is,
though, a recognizable cerebral ganglion to provide the cerebral shield with neurons.

5.4.2 Class Aplacophora

Members of the class Aplacophora are called solenogasters. They have approximately
250 species. These are cylindrical molluscs. They lack a shell and crawl on their ventral
surface. Their nervous system is ladder like. It resembles the nervous system of flatworm.
It suggests that this group may be closely related to the ancestral molluscan stock. One
small group of aplacophorans contains burrowing species. These species feed on
microorganism and detritus. They possess a radula and nephridia. Most aplacophorans
lack nephridia and radula. They are surface dwellers on corals and other substrates. Some
are carnivores and feed on cnidarian polyps.

5.4.3 Class Polyplacophora

1. Habitat: The Class Polyplacophora contains the Chitons. Chitons live on hard
substrates in shallow marine water. Early Native Americans ate Chitons. Chitons
have a fishy flavor. But they are touch to chew and difficult to collect.
2. Body Parts: Chitons have a reduced head, a flattened foot and a shell. Shell
divides extends beyond the margins of the shell and foot. The mantle cavity
Islamabad restricted to the space between the margin of the mantle and the foot.
3. Locomotion: Chitons crawl over their substrate like gastropods. Their body
attaches to a substrate with the help of muscular foot. It allowed chitons to
withstand strong waves and tidal currents. Sometimes, chitons are disturbed. In this
case, the edges of the mantle rightly grip the substrate. The foot muscles contract
and raise the middle of the foot. This action creates a vacuum that holds the chiton
in place. There are articulations in the shell. Chitons roll into a ball due to this
articulation when dislodged from the substrate.
4. Gills and Mantle Cavity: A linear series of gills are present in the mantle cavity
on each side of the foot. Cilia are present on the gills. These cilia create water
currents. Water enters below the anterior mantle margins and exit posteriorly. The
digestive, excretory and reproductive tracts open near the exhalant area of the
mantle cavity. Exhalant water carries products of these systems away.
5. Nutrition: Most chitons feed on attached algae. A subradular organ extends from
the mouth. It produces a chemireceptor and detects food. The radula rasps this food
from the substrate. Mucus traps food. Food then enters the esophagus by ciliary
action. Extracellular digestion and absorption occur in the stomach. The wastes
move in to the intestine and pass out fan LIS.

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6. Nervous System and Sense Organs: The nervous system is ladder like. It is
composed tour anteroposterior nerve cords and many transverse nerves. A nerve
ring encircles the esophagus. Sensory structures are osphradia tactile receptors,
chemoreceptors and statocysts. Tactile receptors are present on the mantle margin.
Chemoreceptors are present near the mouth. Statocysts are present in the foot.
Photoreceptors are present on the surface of the shell in some chitons.
7. Reproduction: Sexes are separate in chitons. External fertilization takes place. The
zygote develops to –form a swimming trochophore. This larva settles and
metamorphoses into an adult. Veliger stage is absent in them (Figure 5.5).

Figure 5.5: Class Polyplacophora

5.4.4 Class Monoplacophora

The members of the class Monoplacophora have an undivided, arched shell. They have a
broad flat foot. They have serially repeated pairs of gills and foot-retractor muscles. They
are dioecious. Nothing is known of their embryology. This group of molluscs was known
only from fossils. But limpet like its member Neopilina was discovered in 1952. It was
renovated from a depth of 3,520 meter off the Pacific coast of Costa Rica.

5.4.5 Class Scaphopoda

1. Habitat: Members of the class Scaphopoda are called tooth shells or tusk shells.
This class has over three hundred species. All are burrowing marine animals living
in moderate depths.
2. Shell and Body Parts: A conical shell is their most distinctive characteristic. This
shell is open at both ends. Head and throat project from the wider end of the shell.
The rest of the body and mantle is greatly elongate. It extends the length of the
shell.
3. Respiration and Nutrition: Scaphopods mostly buried in the substrate. Their head
and foot oriented down. The apex of the shell projects into the water. There are
openings at the apex of the shell. Incurrent and excurrent water enters and leaves

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 the mantle through these openings. Functional gills are absent. Gas exchange
occurs across mantle folds. Scaphopods have a radula and tentacles. These are used
for feeding on foraminiferans.
4. Reproduction: Sexes are separate. Trochophore and veliger larvae are produced.

5.4.6 Class Bivalvia

There are 30,000 species of this class. It is the second largest molluscan class. This class
includes the clams, oysters, mussels and scallops.
1. They have sheet like mantle.
2. Shell consists of two valves. Shell covers the laterally compressed animals (Figure
5.6).

There are many uses of bivalves.

1. Many bivalves are edible.
2. Some bivalves form pearls.
3. Most bivalves are filter feeders Therefore they are used in removing bacteria from
polluted water.

Figure 5.6: Inside view of Bivalve shell

Shell and Associated Structures
Shell: The two convex halves of the shell are called valves. A proteinaceous hinge is
present at the dorsal margin of the shell. A series of tongue-and-groove modifications

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called teeth are also present at dorsal margin. Hinge and teeth prevent the valves from
twisting. The oldest part of the shell is called Limbo. Limbo is a swollen area near the
shell’s anterior margin. The shell forms as a single structure in embryo.

Hinge ligament: The shell is continuous along its dorsal margin. But the mantle secretes
greater quantities of protein in the region of the hinge and secretes relatively little
calcium carbonate. Thus it produces an elastic hinge ligament. The muscles relax and
hinge ligament opens the valves due to elasticity.

Adductor muscles: These muscles are present at both end of the dorsal half of the shell.
They contract and close the shell. This is important for bivalves. These valves are
primary defense against predator like sea stars. The bivalve does open their shells and
remain save.

Pearl formation: The mantle attaches to the shell around the adductor muscles and near
the shell margin. Sometimes, a sand grain or a parasite comes within the shell. The
mantle secretes nacre around it and form pearl. The Pacific Oysters, Pinctuda
margarirfera Pincoysters, Pinctuda margaririfera forms highest-quality pearls (Figure
5.7).

Figure 5.7: a. Bivalve anatomy b. Stomach

Exchange, Filter Feeding and Digestion
Respiration
Bivalves are sedentary and filter-feeding.. Therefore, they have lost the head and radula.
The cilia cover gills. Gills forms folded sheets called lamellae. One end of gill is attached

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to the foot and the other end attached to the mantle. The mantle cavity ventral to the gills
on is inhalant region. The cavity dorsal to the gills is the exhalant region.

Mechanism
a) Cilia moves and water enters into the mantle cavity through an incurrent opening of
the mantle. This opening is present at the end of a siphon. Siphon is an extension of
the mantle. A bivalve can extend its siphon to the surface. Therefore, it can feed
and exchange gases when it is buried in the substrate.
b) Water moves from the mantle cavity into small pores of the gills. Then water
moves into vertical channels in the gills. called water tubes. Blood is present
around the water tube. Therefore gases are exchanged by diffusion.
c) Water leaves the bivalve through suprabranchial chamber and ex- current opening
in the mantle. Supra branchial chamber is part of the mantle cavity. It is present at
the dorsal side of the gills (Figure 5.8).

Figure 5.8: Respiration in Bivalves

Nutrition
1. Ingestion
The gills trap food particles. Zoologists thought that cilia action was responsible for the
trapping of food. But recent study indicates that cilia and food particles have little
contact. The food-trapping mechanism is unclear. Cilia move the particles to the gills
after trapping of food. Cilia along the ventral margin of the gills move food toward the
mouth. Cilia also cover the labial palps. The labial pains are present on both side of the
mouth. The cilia of laibial palps also filter food particles. Cilia carry small particles into
the mouth. They move larger particles to the edges of the palps and gills. This rejected
material called pseudofeces. Pseudofeces are transferred to the mantle. The ciliary of the

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mantle transports the pseudofeces posteriorly. Valves are forcefully closed. Therefore,
water rushes out. This water washes pseudofeces from the mantle cavity.

Digestion
The digestive tract of bivalves is similar to other molluscs.

a) Oesophagus: Food enters into the esophagus. It is covered in mucous. It forms

mucoid food string. Cilia line the digestive tract. They move this mucoid string in
to the stomach.
b) Stomach: A diverticulum called the style sac open into stomach. A crystalline
style mucoid mass) moves into the stomach from style sac. Enzymes for the
digestion of carbohydrate and fat are present in the crystalline style. Stomach has a
chitinous astric shield. Cilia of the style sac rotate the style against the gastric
shield. The friction and acidic conditions in the stomach release the enzymes from
crystalline style. The crystalline style rotates. Therefore, mucoid food string winds
around it. It pulls the food string into the stomach from the esophagus. The action
and the acidic pH in the stomach remove food particles from the food string.
Further rocess separates fine particles from the indigestible materials.
c) Intestine: Indigestible materials are sent to the intestine. Partially digested food
enters into a digestive gland for intracellular digestion.

3. Egestion
Cilia carry undigested wastes from the digestive gland back to the stomach and then to
the intestine. The intestine release wastes through the anus. The anus is present near the
excurrent opening. The excurrent water carries feces away.

Other Maintenance Functions

Blood Vascular System
Blood flows from the heart to tissue sinuses, nephridia and gills. It then moves back to
the heart. The mantle is an additional site for oxygenation. Therefore, a separate aorta
supplies blood directly to the mantle.

Excretion
Two nephridia are present below the pericardial cavity (the coelom ). Their duct system
connects to the coelom at one end. It opens at nephridiopores in the anterior region-of the
suprabranchial chamber.

Nervous System
Their nervous system consists of three pairs of interconnected ganglia. These ganglia are
present near oesophagus, foot and posterior adductor muscle.
Sense Organ: The margin of the mantle is the principal sense organ. It always has
sensory cells. It has sensory tentacles and photoreceptors. In some species photoreceptors
are present in the form of complex eyes. Eye has a lens and a cornea. Other receptors are
statocysts. Statocysts are present near the pedal ganglion. Another receptor osphradium is
presentin the mantle beneath the posterior adductor muscle.

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Reproduction and Development
Gonads
Most bivalves are dioecious. A few species are monoecious. Some of the monoecious
species are protandric. Gonads are present in the visceral mass. They surrounded the
looped intestine. Ducts of these gonads open directly in to the mantle cavity. Or it open
by the nephridiopore in to the mantle cavity.

Fertilization
External fertilization takes place in most bivalves. Gametes are released through the
suprabranchial chamber of the mantle cavity and exhalant opening. The sperm enter into
mental cavity through inhalant water. Fertilization occurs in the mantle cavity.

Development
Trochophore and veliger stages are formed during development. The veliger settles on
the substrate and it becomes adult.
1. Trochophore and veliger larvae: Most freshwater bivalves brood (internal
development) their young. Some brood their young in maternal gills. They have
reduced trochophore and veliger stages. Young clams are shed from the gills.
2. Glochidium larvae: Others brood their young to a modified veliger stage called a
glochidium. Cilochidium is parasitic on fishes. These larvae possess two tiny
valves (Figure 5.9).

Some species have tooth like hooks. Larvae moves out through the exhalant aperture and
sink to the substrate. The giocnidium. Attaches to the gills. Fins, or another body part of
fish. It begins to feed on host tissue. The fish may form a cyst around the larva. The
mantles of some freshwater bivalves produce a fish like lure to attract predatory fish. The
fish try to feed on the lure and the bivalve ejects glochidia onto the fish. After several
weeks the glochidium starts developing adult structures. A mall clam falls from its host.
It takes up its filter-feeding lifestyle.

Figure 5.9: Trocophore and glichidium larvae

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Bivalve Diversity
Bivalves live in nearly all aquatic habitats. They may completely or partially bury
themselves in sand or mud. They may be attached to solid substrates. Or they may bore
into submerged wood, coral or limestone.

1. Boring Bivalves: The margins of mantle of burrowing bivalves are fused to form
an opening in the mantle cavity. This opening is called siphons. Water enters
through this siphon and washes the mantle cavity during burrowing. Therefore, the
sediments do not accumulate in the mantle cavity.

Boring bivalves lie beneath the surface of limestone, clay, coral, wood, and other
substrates. The larvae settle to the substrate and boring starts. The anterior margin
of their valves mechanically removes the substrate. Acidic secretions dissolve
limestone. The most recently bored portions of the burrow are larger in diameter
than portions bored earlier. Therefore as the bivalve grows, it is often entangles in
its rocky burrow.
2. Surface-dwelling Bivalves: They are attached to the substrate by proteinaceous
strands called byssal threads. It is secreted by a gland in the foot. It is found in
common marine mussel.

Self Assessment Questions

Q: Fill in the blanks.

i. The class ……………….includes the snails, limpets, and slugs (Gastropoda).
ii. In some snails a proteinoceous covering ……………….. is present on the
dorsal, posterior margin of the foot (Operculum).
iii. …………. are lost or reduced in land snails (Gills).
iv. Gastropods have an ……….. circulatory system (Open).
v. The nervous system of primitive gastropod consists of ……… ganglia (Six).

Q: Answer the following.

i. What are Osphradia?
ii. How trocophore larva is produced in gastropods?
iii. What do you know about pseudofeces?
iv. What are Surface-dwelling bivalves?
v. Write few uses of bivalves

5.4.7 Class Gastropoda

The class Gastropoda includes the snails, limpets and slugs. It has 35,000 living species.
It is the largest class of molluscs. Its members occupy a wide variety of marine,
freshwater; and terrestrial habitats. Gastropods are intermediate hosts for some medically
important trematode parasites of humans.

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Torsion
Torsion is 1800, counterclockwise twisting of the visceral mass, mantle and mantle
cavity. It occurs during early development. Torsion brings of gills, anus, and openings
from the excretory and reproductive systems just behind the head and nerve. It also twists
the digestive tract into a U shape (Figure 5.10).

Figure 5.10: Different Stages of Torsion A. Before torsion B. Process of torsion C. After
Torsion
Advantages of Torsion
There are three advantages of torsion.

1. First Advantage: There is problem in withdrawal of body into shell without

torsion. The foot enters first and the head enters in the last. Thus head can be
attacked by predator. After torsion the head enters the shell first. Therefore, it does
not expose the head to predators. In some snails a proteinoceous covering
operculum is present on the dorsal, posterior margin of the foot. It enhances
protection. The gastropod draws the foot into the mantle cavity and the operculum
closes the opening of the shell. Thus it prevents the desiccation (dehydration) when
the snail is in drying habitats.
2. Second Advantage: Water enters through posterior opening without torsion. The
snail’s crawling contaminates water with silt. After torsion, opening comes at
anterior end. Therefore, the anterior opening of the mantle cavity allows clean
water to enter into the mantle cavity front the front of the snail.
3. Third Advantage: Mantle’s sensory organs are twisted due to torsion. These sense
organs come around the head. It makes the snail more sensitive to stimuli coming
ioni the direction in which it moves.

Disadvantages of Torsion
The anus and nephridia open dorsal to the head after torsion. It creates folding problems.
However number of evolutionary adaptations solves this problem.

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1. Various modifications produce notches or openings in the mantle and shell
posterior o the head. Therefore, the mantle cavity removes the water and wastes
through these opening.
2. Some gastropods undergo detorsion. In this case the embryo undergoes a full 180°
torsion and then untwists approximately 90°. The mantle cavity thus opens on the
tight side of the body behind the head.

Shell Coiling
The shell in the earliest fossil gastropods coiled in one plane. The growth has developed
an increasingly huge shell. Therefore, this arrangement is not common in later fossils.
Most modern snail shells are asymmetrically coiled into a more compact form. The
successive coils or whorls are slightly larger then the preceding whorl. This pattern leaves
less space on one side of the visceral mass for certain organs. It means that the organs
that are now single were paired in ancestor.

Locomotion
All gastropods have a flattened foot. It is often ciliated and covered with gland cells. Foot
is used to creep across the substrate. The smallest gastropods use cilia to propel
themselves over a mucous path. Larger gastropods use waves of muscular contraction
that move over the foot. The foot of some gastropods is modified for clinging and
swimming.

Feeding and Digestion

Mot gastropods scrap algae or other small, attached organisms from their substrate. Some
are herbivores. They feed on larger plants. Others are scavengers, par sites or predators.

The anterior portion of the digestive tract is modified into an extensible proboscis. This
proboscis contains radula. This structure is important for some predatory snails. Thee
snails extract animal flesh from hard-to-reach areas: The digestive tract of gastropods is
ciliated. Mucous trap the food. It forms a mucoid mass called protostyle. It is rotated by
cilia and extends to the stomach. Digestive gland in the visceral mass releases enzymes
and acid into the stomach. These enzymes free the food from protostyle and digested.
Wastes form fecal pellets in the intestine.

Other Maintenance Functions

Respiration
Mantle cavity is involved in the gas exchange. Primitive gastropods had two gills.
Modern gastropods have lost one gill because of coiling. Some gastropods have a rolled
extension of the mantle called siphon. It acts as an inhalant tube. Burrowing species
extend the siphon in to the substrate to bring water. Gills are lost or reduced in land snails
(pulmonates). These snails have a richly vascular mantle for gas exchange between blood
and air. Mantle contractions circulate air and water through the mantle cavity.

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Circulatory system
Gastropods have an open circulatory system. Blood leaves the vessels and directly bathes
cells in tissue spaces called sinuses. Molluscs have a heart. It consists of a shgle.
muscular ventricle and two auricles. Most gastropods have lost one auricles due to
coiling. Blood transports nutrients, wastes and gases.

Locomotion
The blood of molluscs acts as a hydraulic skeleton. In hydraulic skeleton blood in tissue
spaces provides support. A mollusc contracts muscles and uses its hydraulic skeleton to
extend body. For example, snails have sensory tentacles on their heads. If a tentacle is
touched, retractor muscles can rapidly withdraw it. However, no antagonistic muscles
exist to extend the tentacle. The snail contract muscles and squeeze blood into the
tentacle. Thus these tentacles are slowly extended.

Nervous System
The nervous system of primitive gastropod consists of six ganglia. These ganglia are
located in the head-foot and visceral mass. In primitive gastropods, torsion twists the
nerves that link these ganglia. Evolution in gastropod nervous system took place. It
untwisted the nerves. Therefore, concentration of nervous tissues takes place in larger
ganglia, especially in the head.

Sense Organ: Gastropods have well-developed sense organs. These are:

1. Eyes: Eyes are present at the end of tentacles. Eyes may be simple pits of
photoreceptor cells. Or it has a lens and cornea.
2. Statocysts are present in the foot.
3. Osphradia are chemoreceptors. They are present in the anterior wall of the mantle
cavity. It detects chemicals in inhalant water or air. The osphradia of predatory
gastropods detect prey.

Excretory System
Primitive gastropods possessed two nephridia. The right nephridium has disappeared in
modern species due to shell coiling. The nephridium consists of a sac. The sac has highly
folded walls. It is connected to pericardial cavity. Excretory wastes are formed from
fluids filtered and secreted into the coelom from the blood. The nephridium selectively
reabsorbs certain ions and organic molecules and modifies this waste. The nephridium
opens in to the mantle cavity. In land snails, it opens on the right side of the body
adjacent to the mantle cavity and anal opening.

a) Aquatic Gastropods: Toxic ammonia is diluted in excess water in aquatic

gastropod. Therefore, they excrete ammonia.
b) Terrestrial Gastropods: Terrestrial snails convert ammonia to a less-toxic uric
acid. Uric acid is insoluble in water and less toxic. Thus it can be excreted in a
semisolid form. It conserves water.

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Production and Development

Reproductive Organ
Many marine snails are dioecious. Gonads are present in spirals of the visceral mass.
Ducts discharge gametes into the sea. External fertilization takes place in them. Many
other snails are monoecious. But cross fertilization take place. Fertilization is internal.
There is mutual sperm transfer during copulation. Or one snail may act as the male and
other as the female. A penis is formed by the folding of the body wall. The portion of the
female reproductive tract is glandular. It secretes mucus or a capsule around the fertilized
egg. Some monoecious snails are protandric. In this case, testes develop first, a after
they degenerate. Then ovaries become mature.

Development
Eggs are shed singly or in masses for external fertilization. Fertilized eggs are deposited
in gelatinous masses. The large, yolky eggs of terrestrial snails are deposited in moist
environments like forest-floor leaf litter.

Two larvae are produced in gastropods:

a) Trochophore Larva: In marine gastropods spiral cleavage take place. It produces
a free-swimming trochophore larva.
b) Veliger larva: Trochophore larva then develops into veliger larva. Veliger larva is
free-swimming larva with foot, eyes, tentacles and shell. Sometimes, the
trochophore is not produced. Therefore, the veliger is the primary larva. Torsion
occurs during the veliger stage. It then settle and metamorphosed in to the adult.

Gastropod Diversity
Subclass Prosobranchia is the largest group of gastropods. It has 20,000 species. They
are mostly marine. A few are freshwater or terrestrial. Most members of this subclass are
herbivores or deposit feeders. Some are carnivorous. Some carnivorous species inject
venom into their fish, mollusc or annelid prey with a radula. Their radula is modified into
a hollow, harpoon like structure. Gastropoda have following subclasses:

1. Prosobranchia: They are most familiar marine snails and abalone. This subclass
also includes the heteropods. These animals are voracious predators. They have
very small shells or no shells. The foot is modified into an undulating fin. It propels
the animal through the water.
2. Opisthobranehia: This subclass includes sea hares, sea slugs, and their relatives.
They are mostly marine. This class has two thousand species. The shell, mantle
cavitn and gills are reduced or lost in these animals. Many of these animals acquire
undischarged nematocysts from their cnidarian prey. They use these namatocyts to
save themselves from the predators. The preropods have a foot modified into thin
lobes. It is used for swimming.
3. Pulmonata: It contains about 17, 000 species. Most of these species are freshwater
or terrestrial. The snails are mostly herbivores. They have a long cedilla for
scraping plant material. The mantle cavity of pulmonate astropods is highly

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 vascular. It acts as a lung. Air or water moves in or out of the opening of the mantle
cavity. The pulmonates include terrestrial slugs.

5.4.8 Class Cephalopoda

The class Cephalopoda includes the octopuses. Squid, cuttlefish and nautili. They are the
most complex molluscs.
1. The anterior portion of their foot is modified into a circle of tentacles or arms. It is
used for prey capture, attachment, locomotion and copulation.
2. The foot with mantle cavity is modified into funnel. This funnel is used for jet like
locomotion.
3. Their head is in line with the visceral mass.
4. Cephalopods have a highly muscular mantle. Mantle encloses all of the body
except the head and tentacles. The mantle acts as a pump to bring large quantities
of water into the mantle cavity (Figure 5.11).

Figure 5.11: General anatomy of Cephalopod

Shell
Different types of shell are present in cephalopods:
1. External Shell: The ancestral cephalopods had a conical shell. The only living
cephalopod that possesses an external shell is the nautilus. Septa subdivide its
coiled shell. As the nautilus grows, it moves forward. It secrets a new shell around
itself and leaves an empty septum behind. Only the last chamber is occupied. These
chambers are fluid filled. A cord of tissue called a siphuncle perforates the septa. It
absorbs fluids by osmosis and replaces this fluid with metabolic gases. The amount
of gas in the chambers is regulated. Therefore, it changes the buoyancy of the
animal.

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2. Reduced Internal Shell: The shell is reduced or absent in all other cephalopods. In
cuttlefish, the shell is internal. It is laid down in thin layers. Therefore, it leaves
small, as-filled spaces. It increases buoyancy. Cuttlefish shell is called cuttlebone.
It is used to make powder for polishing. It is also fed to pet birds to supplement
their diet with calcium.
3. Pen: The shell of a squid is reduced. It has internal chitinous structure called the
pen. quid also have cartilaginous plates in the mantle wall, neck and head. These
plates upport the mantle and protect the brain.
4. Without Shell: The shell is absent in octopuses (Figure 5.12).

Figure 5.12: Internal structure of Squid

Locomotion
Cephalopods are predators. Therefore, cephalopods depend on their ability to move
quickly. They move by jet-propulsion system. The mantle of cephalopods contains radial
and circular muscles. The circular muscles contract. It decreases the volume of the mantle
cavity. It also closes collar like valves. This closing of valve prevents water from moving
out of the mantle cavity between the head and the mantle wall. Water is thus forced out of
a narrow funnel. Muscles attached to the funnel control the direction of the animal’s
movement. Radial muscles of mantle increase the volume of cavity. It brings water into
the mantle cavity. Posterior fins act as stabilizers in squid. It also helps in propulsion and
steering in cuttlefish. Octopuses are more sedentary animals. They may use jet propulsion
to escape. It normally crawls over the substrate using their tentacles.

In most cephalopods, the use of the mantle in locomotion takes place at same time with
the loss of an external shell. Therefore, a rigid external shell comes before the jet-
propulsion method.

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Feeding and Digestion

Ingestion
Cephalopods legate their prey by sight. They capture prey with tentacles. The tentacles
have adhesive cups. In squid, the margins of cups have tough protein. They sometimes
possess small hooks. All cephalopods have jaws and a radula. The jaws are powerful,
beak like structures. They are used for tearing food. The radula rasps food and forces it
into the mouth cavity.

Cuttlefish and nautili feed on small invertebrates on the ocean floor. Octopuses are
nocturnal hunters. They feed on fish and crustaceans. Octopuses have salivary glands.
Salivary glands inject venom into prey. Squid feed on fishes and shrimp. They kill them
by biting across the back of the head.

Digestion
The digestive tract of cephalopods is muscular. Peristalsis has replaced the ciliary action
in moving food. Most digestion occurs in a stomach and a Cecum. Digestion is primarily
extra cellular. They have digestive glands supplying enzymes. An intestine ends at the
anus, near the funnel. The exhalant water carries wastes out of the mantle cavity.

Other Maintenance Functions

Blood Vascular System

Cephalopods have a closed circulatory system. Blood remains in vessels during its
circulation in the body. Capillaries connect arteries and veins. The exchanges of gases,
nutrientsand metabolic wastes take place through capillary wall. The heart of
cephalopods consists of two auricles and one ventricle. The cephalopods have contractile
arteries and branchial hearts. The branchial hearts arc present at the base of each gill. It
helps in movement of blood through the gill. These modifications increase blood pressure
and the rate of blood flow. Blood pressure is necessary for active cephalopods with high
metabolic rates. Large quantity of water circulates over the gills at all times. Cephalopods
show greater excretory efficiency due to close circulatory system. There is a close
association of blood vessels with nephridia. It allows wastes to filter and secrete directly
from the blood into the excretory system.

Nervous System
The cephalopod have specialized nervous system. Cephalopods are predatorv animals.
Therefore, they need efficient nervous system. Cephalopod brains are large. The brain is
formed by a fusion of ganglia. Large areas of brain are used to control muscle
contraction, sensory perception and memory and decision making.

Sense Organ
1. Eye: The structure of eye of cephalopod is similar to the structure of vertebrate
eyes. This is an example of convergent evolution. But the nerve cells leave the eye
outside of the eyeball. Therefore, no blind spot exists in their eye. The cephalopods

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 locus by moving the lens back and forth. Cephalopods can form images of different
shapes. They differentiate between some colors. The nautiloid eye is less complex.
It lacks a lens. Its interior is open to seawater. Thus it acts as a pinhole camera.
2. Statocysts: Cephalopod statocysts detect gravity and acceleration. Statocysts are in
the form of cartilages next to the brain.
3. Osphradia: They are present only in Nautilus.
4. Tactile receptors and additional chemoreceptors are widely distributed over the
body.

Body Colour
1. Chromatophores: Cephalopods have pigment cells called chromatophores. Tiny
muscles attached to these pigment cells. These muscles contract and the
chromatophores quickly expand. It changes the color of the animal. Color changes
functions in alarm responses. Color changes may spread in waves over the body. It
forms a large, flickering, patterns. The cephalopods blend with their background by
changing color changes. Color changes are also involved with courtship displays.
Some species combine chromatophores displays with bioluminescence.
2. Ink gland: All cephalopods possess all ink gland. It opens just behind the anus. Ink
is a brown 9r black fluid. It contains melanin and other chemicals. This ink
confuses a predator. Thus it allows the cephalopod to escape. For example, Sepiola
reacts to danger by darkening itself with chromatophore before releasing ink. After
ink discharge. Sepiola changes to a lighter color again. This change of colour helps
in escape from the predator.

Reproduction and Development

Cephalopods are dioecious. Their gonads are present in the dorsal portion of the visceral
mass.

Reproductive Organs
The male reproductive tract consists of testes and spermatophores. Spermatophores are
used for covering the sperm in packets. The female reproductive tract produces large,
yolky eggs. Glands secrete gel-like cases around eggs. These cases become hard in
seawater.

Fertilization
One tentacle of male cephalopods is called hectocotylus. It is modified for
spermatophore transfer. The hectocotylus has several rows of smaller suckers in Loligo
and Sepia,. These suckers pick up spermatophores. Male and female tentacles intertwine
during copulation and the male removes spermatophores from his mantle cavity. The
male inserts his hectocotylus into the mantle cavity of the female. It deposits a
spermatophore near the opening to the oviduct. Spermatophores have an ejaculatory
mechanism. It frees sperm from the capsule. Eggs leave the oviduct and get fertilized.
Fertilized eggs are deposited singly or in string like masses. They attach to some
substrates. Octopuses clean developing egg with their arms.

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Development
Cephalopods develop in the egg membranes. The hatchings are similar to adults. Young
are not cared after hatching.

Self Assessment Questions

Q: Fill in the blanks.
i. The only living cephalopod that possesses an external shell is the ……….
(Nautilus
ii. Cephalopods have pigment cells called ……………….. (Chromatophores).
iii. Cephalopods have ………… circulatory system (Closed).
iv. The digestive tract of cephalopods is ………… (Muscular).
v. Cephalopods have a highly muscular ………… (Mantle).

Q: Answer the following.

i. What are statocysts?
ii. How trocophore larva formation takes place?
iii. What do you know about chitons in Class Polyplacophora?
iv. What do you know about solenogasters?
v. What is class scaphopoda?

5.5 Further Phylogenetic Considerations

Molluscs are over 500 million years old according to fossil record. There are different
views about the evolution of molluscs.

Protosotme Ancestry
The molluscs have protosotme ancestry. But zoologists do not know the exact
relationship of this phylum with other animal phyla. There are following evidences which
support protosotme ancestry of molluscs.
1. A mollusc Neopilina (Monoplacophora) was discovered in 1952. It helped greatly
to determine the position of molluscs. Neopilina has segmental arrangement of
gills, excretory structures and nervous system. •IIhe annelids and arthropods also
have a segmental arrangement of body parts. Theretbre, monoplacophorans are
“missing link” between other molluscs and the annelid-arthropod evolutionary
line.
2. The molluscs, annelids, and arthropods share certain protosotme characteristics
3. Chitons also show a repetition of some body parts.

Origin from Triploblastic Stock

Following characteristics does not support the protosotme ancestry of molluscs:
1. But the segmentation in some molluscs is very different from annelids and
arthropods

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2. The serially repeating structure in any other mollusc does not develop like annelid-
arthropod.
3. Segmentation is also not an ancestral molluscan characteristic.
Therefore many zoologists now believe that molluscs diverged from ancient triploblastic
stocks. Other zoologists still believe that molluscs are tied to the annelid-arthropod line.
But it is clear that the relationship of the molluscs to other animal phyla is distant.

History of Evolution of Molluscs

A diversity of body forms and lifestyles is present in this phylum. Therefore, mollusca
are an excellent example of adaptive radiation. Molluscs began in Precambrian times.
They were slow-moving, marine bottom dwellers. Then unique molluscan features deli–
eloped. It allowed them to diversify quickly. By the end of the Cambrian period, some
were filter feeders, some were burrowers. and others were swimming predators. Later,
some molluscs became terrestrial. They adapted in many habitats like tropical rain forests
to arid deserts.

The classes Caudofoveata and Aplacophora lack shell. The zoologists believe that the
lack of a shell in a primitive character. All other molluscs have a shell or they are derived
from shelled ancestors. The multipart shell distinguishes the Polyplacophora from other
classes. They have extensive adaptive radiation. Therefore, it is difficult to develop
higher taxonomic relationships of this phylum.

Key Points/Summary
1. Molluscs are protostomes. They share embryological stages with other
protostomes,
2. especially annelids. In spite of these similarities, relationship between molluscs and
other protostomes are distant.
3. Theories regarding the origin of coelem influence how zoologists interpret
evolutionary relationships among triploblastic animals.
4. Members of class gastropoda chracterised by torsion and coiled shell.
5. Class bivalvia includes clams, oysters. They lack head and covered by sheet-like
mantle and shell consisting of two valves.
6. Members of class cephalopoda are octopuses having closed circulatory system and
highly developed nervous and sensory systems.
7. Some zoologists believe molluscs are derived from annelid-arthropod lineage.
Adaptive radiation in molluscs has resulted in their presence in most ecosystems of
the earth.

143
References
 BUNJE, P.: The Aplacophora - the naked mollusks (University of California
Museum of Palaeontology).
 Henry, J.; Okusu, A.; Martindale, M. (2004). "The cell lineage of the
polyplacophoran, Chaetopleura apiculata: variation in the spiralian program and
implications for molluscan evolution". Developmental Biology. 272 (1): 145–
160.
 Monks, N. "A Broad Brush History of the Cephalopoda". Retrieved 2009-03-21.
 MULCRONE, R.S.: Aplacophora (Animal Diversity Web, University of
Michigan).
 Parkhaev, P. Yu. (2007). The Cambrian 'basement' of gastropod evolution.
Geological Society, London, Special Publications. 286. pp. 415–421
 Rosenberg, Gary (2014). "A New Critical Estimate of Named Species-Level
Diversity of the Recent Mollusca". American Malacological Bulletin. 32 (2):
308–322.
 Taylor, P.D., & Lewis, D.N. (2005). Fossil invertebrates. Harvard University
Press, 208 pp.
 Zong-Jie, F. (2006). "An introduction to Ordovician bivalves of southern China,
with a discussion of the early evolution of the Bivalvia". Geological Journal. 41
(3–4): 303–328

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UNIT-6

ANNELIDA: THE
METAMERIC BODY FORM

Written by: Arshad Mehmood Qamar

Reviewed by: Dr. Sobia Mushtaq

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 CONTENTS
Introduction ....................................................................................................... 147

Objectives ......................................................................................................... 147

6.1 Evolutionary Perspectives ..................................................................... 148

6.2 Metamerism and Tagmatization ........................................................... 149

6.3 Classification up to Class ...................................................................... 123

6.4 Further Phylogenic Consideration ........................................................ 168

146
Introduction

Having over 17,000 species, Phylum Annelida is a large phylum. Annelids are also
known as ringworms or segmented worms. They exist in various environments including
marine waters, fresh waters and also in moist terrestrial areas. The size of the annelids
can range from a few millimetres to an amazing three metres in length. The Australian
earthworm measures around 3 metres. Furthermore some species from this phylum
exhibit some unique shapes and brilliant colours.Annelids exhibit bilateral symmetry and
are invertebrate organisms. They are coelomate and triploblastic. The body is segmented
which is the most distinguishing feature of annelids.

Objectives
After completion of this unit, you will be able to:
 describe evolutionay perspectives of Annelida
 tell the relationship of annelid with other animals
 differentiate between tagmatization and metamerism
 classify phylum annelid upto class level
 comapare the characterstics of class Polychaeta and class ologochaeta.
 describe external structures, feeding and digestive system. Gas exchange, nervous
system and sensory functions.
 explain further phylogenetic considerations.

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6.1 Evolutionary Perspective
At the time of the November full moon on island near Samoa in the South Pacific, natives
rush about preparing for one of their biggest yearly feasts. In just one week, the sea will
yield a harvest that can be scooped up in nets and buckets. Worms by the millions
transform the ocean into what one writer called “vermicelli soup!” Celebrants gorge
themselves on worms that have been cooked or wrapped in breadfruit leaves. The
Samoan palolo worm (Eunice viridis) spends its entire adult life in coral burrows at the
sea bottom. Each November, one week after the full moon, this worm emerges from its
burrow and specialized body segments devoted to sexual reproduction break free and
float to the surface, while the rest of the worm is safe on the ocean floor. The surface
water is discolored as gonads release their countless eggs and sperms. The natives’ feast
is short-lived; however these reproductive swarms last only 2 days and do not recur for
another year.

The Samoan palolo worm is a member of the phylum annelid (ah-nel’i-dah) (L. annellus,
ring). Other members of this phylum include countless marine worms (Figure 6.1) the
soil-building earthworms, and predatory leeches.

Characteristics of the bilaterally symmetrical wormlike

1. Body metameric, bilaterally symmetrical and wormlike
2. Protosome characteristics spiral cleavage, trochophore larvae (when larvae are
present) and schizocoelous coelom formation
3. Paired, epidermal setae
4. Closed circulatory system
5. Dorsal suprapharyngeal ganglia and ventral nerve cord(s) with ganglia
6. Metanephridia (usually) or protonephridia

6.1.1 Relationships to other Animals

Annelids are protostomes (Figure 6.3). Protosomes characteristics, such as spiral
cleavage, a mouth derived from an embryonic blastopore, schizocoelous coelom
formation, and trochophore larvae are present in most members of the phylum (Figure
6.2). This diverse phylum, like most other phyla originated at least as early as
precambrain times, more than 600 years ago. Unfortunately, little evidence documents
the evolutionary pathways that resulted in the first annelids.

A number of hypotheses accounts for annelid origins. These hypotheses are tied into
hypotheses regarding the origin of the coelom. If a schizocoelous origin of the ceolom is
correct, as many zoologists believe, then the annelids evoloved from ancient flatworm
stock. On the other hand, if an enterocoelous ceolom origin is correct, then annelids
evolved from ancient diploblastic animals, and the triploblastic, acoelomate body may
have been derived from a triploblastic, coelmate ancestor. The recent discovery of a
worm, Lobatocerebrum that shares annelid and flatwarm characteristics has lent support
to the enterocoelous origin hypothesis. Lobatocerebrum is classified as an annelid based
on the presence of

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Figure.6.1: Phylum Annelida. The phylum annelid includes about nine thousands
species of segmented worms. Most of these are marine members of the class polychaeta.
The fanworm (sriro branchus) is shown here. The fan of this tube-dwelling polychaete is
specialized for feeding and gas exchange. Certain segmentally arranged excretory organs,
an annelid-like body covering, a complete digestive tract, and an annelid-like nervous
system. However it has a ciliated epidermis and is acoelo-illustrates how the triploblastic,
acoelomate design could have been derived from the annelid lineage.

6.2 Metamerism and Tagmatization

Earthworms bodies are organized into a series of ring like segments that is not externally
obvious, however, is that the body is divided internally as well. Segmental arrangements
of body parts are an animal is called Metamerism (Gr. Meta, after + mere, part).

Metamerism profoundly influences virtually every aspect of annelid structures and

functions, such as the anatomical arrangement of organs that are coincidentally associated
with Metamerism. For example, the compartmentalization of the body has resulted in
each segment having its own excretory, nervous and circulatory structures. Two related
functions are probably the primary adaptive features of Metamerism: flexible support and
efficient locomotion. These functions depend on the metameric arrangement of the
coelom and can be understood by examining the development of the coelom and the
arrangements of body-wall muscles.

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Figure 6.2: Evolutionary Relationship of the Annelida. Annelida (shaded in orange)
are protostomes with close evolutionary ties to the arthropods.

During embryonic development, the body cavity of annelids arises by a segmental

splitting of a solid mass of mesoderm that occupies the region between ectoderm and
endoderm on either side of the embryonic gut tract. Enlargement of each cavity forms a
double-membraned septum on the anterior and posterior margin of each coelomic space
and dorsal and ventral mesenteries associated with digestive tract. (Figure 6.3)

Muscles also develop from the mesodermal layers associated with each segment. A layer
of circular muscles lies below the epidermis, and a layer of longitudinal muscles, just
below the circular muscles, runs between the septa that separate each segment. In
addition, some polychaetes have oblique muscles, and the leeches have dorsoventral
muscles.

One advantages of the segmental arrangement of coelomic spaces and muscles is the
creation of hydrostatic compartments, which allow a variety of advantageous locomotor
and supportive functions not possible in nonmetameric animals that utilize a hydrostatic
skeleton. Each segment can be controlled independently of distant segments, and muscles
can act as antagonistic pairs within a segment. The constant volume of coelomic fluid
provides a hydrostatic skeleton against which muscles operate. Resultant localized
changes in the shape of groups of segments provide the basic for swimming, crawling,
and burrowing.

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Figure 6.3: Developments of Metameric, Coelomic Spaces in Annelids.

a) A solid mesodermal mass separates ectoderm and endoderm in early embryonic

stages.
b) Two cavities in each segment form from the mesoderm splitting on each side of the
endoderm (schizocoelous coelom formation)
c) These cavities spread in all directions. Enlargement of the coelomic sacs leaves a
thin layer of mesoderm applied against the outer body wall (the parietal
peritoneum) and the gut tract (the visceral peritoneum), and dorsal and ventral
mesenteries form. Anterior and posterior expansion of the coelom in adjacent
segments forms the double-membranced septum that separates annelid metameres.

Figure 6.4: Possible origins of Annelids and Arthropods.

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Possible sequence in the evolution of the annelid/arthropods line from a hypothetical
worm-like ancestor (a) Wormlike prototype (b) paired, metameric appendages develop.
(c) Divergence of the annelid and arthropod lines. (d) Paired appendages develop into
parapodia of ancestral polychaetes. (e) Extensive tagmatization results in specialization
characteristics of the arthropods. A head is a sensory and feeding tagma, and an abdomen
contains visceral organs.

A second advantage of Metamerism is that it lessens the impact of injury. If one or a few
segments are injured, adjacent segments, set off from injured segments by septa, may be
able to maintain nearly normal functions, which increase the likelihood that the worm, or
at least a part of it, will survive the trauma.

A third advantage of Metamerism is that it permits the modification of certain regions of

the body for specialized functions such as feeding, locomotion, and reproduction. The
specialization of body regions in a metameric animal is called tagmatization. (Gr. tagma
arrangement). Although it is best developed in the arthropods, some annelids also display
tagmatization. (The arthropods include animals such as insects, spiders, mites, ticks, and
crayfish)

Virtually zoologists agree that because of similarities in the development of metamarism

in the two groups, annelids and arthropods are closely related. Other common features
include triploblastic coelomate organization, bilateral symmetry, a complete digestive
tract, and a ventral nerve cord. As usual, fossil evidence documenting ancestral pathways
that led from a common ancestor to the earliest representative of these two phyla is scant.
Zoologists are confident that the annelids and arthropods evolved from a marine,
wormlike, bilateral ancestor that possessed metameric design. Figure 6.4 depicts a
sequence of evolutionary changes that may have given rise to these two phyla.

Self Assessment Questions

1. What are two hypotheses regarding the origin of phylum Annelida?
2. What is metamerism?
3. What is tagmatization?
4. Write commonalities and differences in metamerism and tagmatization.
5. Elaborae three advantages of metameric organization.
6. Which other phylum is closely relted to annelids? And why?

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6.3 Classification of the Phylum Annelida
Phylum Annelida (ah-nel’I dah)
The phylum of triploblastic coelomate animals whose members are metameric
(segmented), elongate, and cylindrical or oval in cross section. Annelids have a complete
digestive tract; paired, epidermal setae; and a ventral nerve cord. The phylum is divided
into three classes.

6.3.1 Class Polychaeta (pol’’e-ket’ah)

The largest annelid class, mostly marine, head with eyes and ten taceles, parapodia bear
numerous setae; monoecious or dioecious; Nereis, Arenicola, Sabella. More than 5,300
species.

6.3.2 Class Oligochaeta ( ol’’i-go-ket’ah)

Few setae and no parapodia; no distinct head; monoecious with direct development;
primarily fresh water or terrestrial. Lumbricus, tubifex. Over 3,000 species.

6.3.3 Class Hirudinea (hi’ru-din’’e-ah)

Leeches; bodies with 34 segments; each each segment subdivided into annuli; anterior
and posterior suckers present ; monoecious with direct development; parapodia absent;
setae reduced or absent. Fresh water marine, and, terrestrial. Hirudo. Approximately 500
species.

6.3.1 Class Polychaeta

Members of the class Polychaeta (pol’’-e-ket’ah) (Gr. polys, many + chaite, hair) are
mostly marine, and are mostly marine, and are usually between 5 and 10cm long (see
table 22.1/6.1). With more than 5300 species Polychaeta is the largest of the annelid
classes. Polychaeta have adapted to a variety of habitats. Many live on the ocean floor,
under rocks and shells, and within the crevices of coral reefs. Other Polychaeta are
burrowers and move through their substrate by peristaltic constrictions of the body wall.
A bucket of intertidal sand normally yields vast numbers and an amazing variety of these
burrowing annelids. Other Polychaeta construct tubes of cemented sand grains or secreted
organic materials. Mucus-lined tubes serve as a protective retreats and feeding stations.

1. External Structure and Locomotion

In addition to metamerism, the most distinctive feature of polychaetes is the presence of
lateral extensions called parapodia. (Gr. para, besides + podion, little foot) (Figure 6.5).
Chitinous rods support the parapodia, and numerous project from the parapodia giving
them their class name. Setea (L. saeta, bristle) are bristles secreted from invagination of
the distal ends of parapodia. They aid locomotion by digging into the substrate and also
hold a worm in its burrow or tube.

The prostomium (Gr. pro, before + stoma mouth) of a polychaete is a lobe that projects
dorsally and anteriorly to the mouth and contains numerous sensory structures, including
eyes, antennae, palps, and ciliated pits or grooves called nuchal organs. The first body

153
segment, the peristomium (Gr. peri, around), surrounds the mouth and bears sensory
tentacles or cirri.

The epidermis of polychaetes consists of a single layer of columnar cells that secrete a
protective, nonliving cuticle. Some polychaetes have epidermal glands that secrete
luminescent compounds.

Various species of polychaetes are capable of walking, fast crawling, or swimming. To

do so, the longitudinal muscles on one side of the body act antagonistically to the
longitudinal muscles on the other side of the body so that undulatory waves move along
the length of the body from the posterior end toward the head. The propulsive force is the
result of parapodia and setae acting against the substrate or water. Parapodia on opposite
side of the body are out of phase with one another. When longitudinal muscles on one
side of a segment contract, stiffening the parapodium and protruding the setae for the
power stroke (figure 6.5a/6.6a). As a polychaete changes from a slow crawl to
swimming, the period and amplitude of undulatory waves increase. (figure 6.5/6.6b)

Burrowing polychaetes push their way through sand and mud by contractions of the body
wall or by eating their way through the substrate. In the latter, polychaetes digest organic
matter in the substrate, and eliminate absorbed and undigestible materials via the anus.

Figure 6.5: Class Polychaeta. External structure of Nereis virens. Note the numerous
parapodia

2. Feeding and the Digestive System

The digestive tract of polychaetes is a straight tube that mesenteries and septa suspend in
the body cavity. The anterior region of the digestive tract is modified into a proboscis that
special protractor muscles and coelomic pressure can evert through the mouth. Retractor
muscles bring the proboscis back into the peristomium. In some, when the proboscis is

154
everted, paired jaws are opened and may be used for seizing prey. Predatory polychaetes
may not leave their burrow or coral crevice. When prey approaches a burrow entrance,
the worm quickly extends its anterior portion, averts the proboscis, and pulls the prey
back into the burrow. Some polychaetes are herbivores and scavengers, and use jaws for
tearing food. Deposit-feeding polychaetes (e.g., Arenicola, the lugworm) extract organic
matter from the marine sediments they ingest. The digestive tract consists of a pharynx
that, when everted, forms the proboscis; a storage sac, called a crop. A grinding gizzard
;and a long straight intestine. They are similar to the digestive organs of earthworm.
Organic matter is digested extracellularity, and the inorganic particles are passed through
the intestine and released as “castings.”

A number of sedentary and tube dwelling polychaeters are filter feeders. They usually
lack a proboscis but possess other specialized feeding structures. Some tube dwellers,
called fanworms, possess radioles that form a funnel shaped fan. Cilia on the radioles
circulate water through the fan, trapping food particles. During transport finest particles
are transported to the mouth. Another filter feeder, Chaetopterus, lives in U-shaped tube
and secretes a mucous bag that collects food particles, which may be so small as I
micrometer. The parapodia of segments 14 through 16 are modified into fans that create
filtration currents. When full, the entire mucous bag is injested.

Elimination of digestive waste can be problem for tube-dwelling polychaetes. The

organisms having tube which open both sides carry wastes away by water circulations.
But those which have one opening of the tube must use cilaiary tracts along the body wall
to carry faeces to the tube opening.

Fig: 6.6: Digestive systems in Polychaete

Activity: Study different part of a polychaete and discuss with your class fellow.

155
Figure 6.7: Feeding and Digestive system in Polychaetes

3. Gas Exchange and Circulation

Respiratory gases of more annelids simply diffuse across the body wall, and parapodia
increase the surface area for these exchanges. In many polychaetes, parapodia gills
further increase the surface area for gas exchange.

Polychaetes have a closed circulatory system. Oxygen is usually carried in combination

with molecules called respiratory pigments, which are usually dissolved in the plasma
rather than contained in blood cells. Blood may be colourless, green or red, depending on
the presence and /or type of respiratory pigment.

Contractile elements of polychaetes circu;atory system consists of a dorsal aorta that lies
just above the digestive tractand propels blood from rear to front, and a ventral aorta that
lies ventral to the digestive tract and propels blood from front to rear. Running between
these two vessels are two or three sets of segmented vessels that receive blood from the
ventral aorta and break into capillary beds in the gut and body wall. Capillaries coalesce
again int segmental vessels that deliver to the dorsal aorta.

Figure 6.8: Circulatory system of polychaete.

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4. Nervous and Sensory Functions
Nervous systems are similar in all three classes of annelids. It includes a pair of
suprapharyngeal ganglia, which connect to a pair of subpharyngeal ganglia by
circumpharyngealconnectives that run dorsoventrally along either side of the pharynx. A
double ventral nerve cord runs the length of the worm along the ventral margin of each
coelomic space, and a paired segmental ganglion may fuse to varying extents in different
taxanomic groups. Lateral nerves emerge from each segmental ganglion, supplyingthe
body wall musculature and other structures of that segment.

Segmented ganglion coordinate swimming and crawling movements in isolated segments.

Each segment separately from, but a closely coordinated with neighboring segments. The
sub-pharyngia helps mediate locomotor functions requiring coordination of distant segments.
The supra-pharyngial ganglia probably control sensory and motor functions involved with
feeding, and sensory functions associated with forward locomotion.

Polychaete have various sensory structures. Two or four pairs of eye are on the surface of
the prostomium. They vary in complexity from a simple cup of receptor cells to structure
made of a cornea, lens and vitreous body. Most polychaete reacts negatively to increased
light intensities. Fanworms, however, react negatively to decreasing light intensities. If
shadows cross them, fanworms retreat into their tubes. This response is believed to help
protect fanworm from passing predators. Nuchal organs are pairs of ciliated sensory pits
or slits in the head region. Nerves from the superapharyngeal ganglia innervate nuchal
organs, which are thought to be chemoreceptors for food detection. Statocysts are in the
head region of polychaetes, and ciliated tubercles, ridges, and hands, all of which contain
receptors for tactile senses, cover the body wall.

Figure 6.9: Nervous system of a polychaete

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5. Excretion
Annelids excrete ammonia, and because ammonia diffuse readily into the water, more
nitrogen excretion probably occurs across the body wall. Excretory organs of annelids are
more active in regulating water and ion balances, although these abilities are limited.

The excretory organs of annelids, like those of many invertebrates are called nephredia.
Annelids have two types of nepherida. A protonepheridium consists of a tubule with a
closed bulb at one end and a connection to the outside of the body at the other end.
Protonepheridia have a tuft of flagella in their tubular end that drives fluids through the
tubule. Second type of nepheridium is called metanepheridium. It consist of an open,
ciliated funnel, called a neprostome, that projects through an anterior septum into the
coelom of an adjacent segment. At the opposite end , a tube opens through the body wall
at a nephridiopore . There is usually one pair of nepheridia per segment, and tubules may
be extensively coiled, with one portion dilated into the bladder.

Figure 6.10: Annelids Excretory System

6. Regeneration, Reproduction and Development

Annelids are a large and diverse group of invertebrates, with species capable of
regenerating a new tail or head or both simultaneously. Certain species can regenerate a
whole worm even from a tiny fragment (Berrill, 1952; Herlant Meewis, 1964; Korotkova,
1997; Bely, 2006). In addition, most annelids reproduce asexually, mainly by architomy
type of transverse fission (a worm splits into fragments which then restore missing heads
and tails) or by paratomy (new heads and tails emerge before splitting) (Fig. 1). Modified
forms of paratomy and architomy can be called by specific terms in different animal

158
groups, for example ctenodrilization, stolonization, schizometamery,and so on.
Correlation between abilities to reproduce agametically and to regenerate lost body parts
is shown for annelids as well as for other phyla (Morgan, 1901; Vorontsova and Liosner,
1960; Bely, 1999; Kharin et al., 2006)

All polychaetes have remarkable ability of regeneration. They can replace lost parts snd
some species have break points that allow worms to serve themselves when a predator
grab them. Lost segments are later regenerated.

Some polychaetes reproduce asexually by budding.

6.3.2 Class Oligochaeta

More than 3000 species of oligochaetes are found in a great variety of sizes and habitats.
They include the familiar earthworms and many species that live in fresh water. Most are
terrestrial or freshwater forms, but some are parasitic, and a few live in marine or
brackish water. The class Oligochaeta has over three thousand species. They are found
throughout the world in freshwater and terrestrial habitats. A few oligochaetes are
estuarine, some are marine. Aquatic species live in shallow water, where they burrow in
mud and debris. Terrestrial species live in soils some live in hot, dry weather. The depths
of their burrow are 3 m below the surface.

1. External and Internal Structure of Oligochaeta

With few exceptions, oligochaetes bear setae, which may be long or short, straight or
curved, blunt or needlelike, or arranged singly or in bundles. Whatever the type, they are
less numerous in oligochaetes than in polychaetes, as is implied by the class name, which
means “few long hairs.” Aquatic forms usually have longer setae than do earthworms.

Like the polychaetes, oligochaetes have bodies divided into segments. However, they
lack parapodia and, with a few exceptions, have relatively few and inconspicuous setae.
The setae are usually arranged in four bundles on each segment; those of aquatic forms
are longer than those of land forms. The setae of an earthworm may be felt as a roughness
if one rubs a finger along its side.

Oligochaetes are less varied in their external form than the polychaetes, but are much
more numerous. As many as 4,000 oligochaetes have been counted in 1 square meter of
lake bottom, and about 9,000 in 1 square meter of meadow soil. In almost all
oligochaetes, the head is a simple cone-shaped structure without sensory appendages.
Light is detected by photoreceptor cells in the skin, usually concentrated toward the front
of the animal.

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Figure 6.11: External Struture of Earth worm

The fluids of marine polychaetes have the same salt balance as (i.e., are isosmotic with)
the surrounding seawater and thus can tolerate no more than a moderate change in the salt
(i.e., ion) content of the salt water. Coelomic fluids contain little or no protein. Certain
aquatic oligochaetes, however, which live exclusively in fresh water, are capable of
regulating the internal medium because, although their coelomic fluid contains fewer
salts than does that of polychaetes, it contains more proteins. Freshwater leeches have
osmoregulatory mechanisms similar to those of oligochaetes.

Figure 6.12: Internal Structure of Earthworm

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Figure 6.13: Internal Structure of Earthworm

The body wall of a typical marine polychaete, such as Perinereis cultrifera, which cannot
adapt to salinity fluctuations of seawater, swells and bursts if salinity is reduced to 20
percent that of seawater because the worm has no physiological mechanism for the
control of water intake. On the other hand, certain individual Nereis diversicolor worms
are capable of tolerating intertidal changes of salinity because they have enlarged
nephridia that enable them to excrete excess water.

2. Locomotion in Polycaetes
Locomotion takes place by antagonistic contraction of circular and longitudinal muscles.
The waves of contraction move from rear to front. The longitudinal muscles contract and
segments bulge and setae protrude out. Therefore, the setae penetrate into the soil. The
contraction of circular muscles retracts the setae. It elongates the segments and pushes
them forward. Contraction of longitudinal muscles in segments behind a bulging region
pulls those segments forward. The waves of muscle contraction move anteriorly on the
worm. Therefore the segments move forward.

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Figure 6.14: Locomotion in Earthworm an Oligochate

Locomotion in free-moving polychaetes is accomplished by circular, longitudinal, and

parapodial muscles and by coelomic fluid. When a worm such as Nereis moves slowly,
the contractual force comes from the sweeping movement of the parapodia. The
parapodia of each segment are not aligned, and the effective stroke is the backward one,
in which the aciculae (needlelike processes) are projected beyond the parapodium and
come in contact with the crawling surface. In the recovery, or forward, stroke, the
aciculae retract, and the parapodium lifts free of the surface. When a parapodium ends its
backward stroke, the next parapodium initiates one. Body undulations, which help the
worm to move rapidly, are produced by the contraction of longitudinal muscles
stimulated by the backward stroke of parapodium of a particular segment.

Locomotion in the burrowing polychaetes, especially those forms lacking anterior

appendages, is similar to that of the earthworm. In tube-dwelling sedentary forms, such as
the Sabellidae, locomotion is restricted to movement within the tube. In this group, the
parapodia are reduced or absent; specialized setae, the uncini, function in much the same
way as do parapodia in free-moving forms

3. Burrowing
The coelomic hydrostatic pressure helps in burrowing. This pressure is transmitted
toward the prostomium. Earthworm uses its expanded posterior segments. It pushes

162
through the soil during burrowing. It uses its protracted setae to anchor itself to its burrow
wall. Contraction of circular muscles makes the prostomium a conical wedge. It is I mm
in diameter at its tip. Contraction of body-wall muscles generates coelomic pressure. This
pressure forces the prostomium through the soil.The earthworms swallow a considerable
quantity of soil during burrowing.

4. Feeding and the Digestive System

Oligochaetes are scavengers. They feed on fallen and decaying vegetation.
They drag these decay food into their burrows at night. The digestive tract of
oligochaetes is tubular and straight.

1. Mouth: The mouth opens into a muscular pharynx.

2. Pharynx: The pharyngeal muscles attach to the body wall. The pharynx acts as a
pun p tbr ingesting food. The pharynx pumps the food into the oesophagus.
3. Oesophagus: The esophagus is narrow and tubular. It expands to form a stomach,
crop or gizzard. Crop and gizzard are common in terrestrial species.
4. Crop: A crop is a thin-walled storage structure
5. Gizzard: A gizzard is a muscular structure. Its inner wall is lined with cuticle.
Gizzard is grinding structure. Calciferous glands are present in the invagination of
esophageal wall. They are used to remove excess calcium absorbed from food.
They also help to regulate the pH of body fluids.
6. Intestine: The intestine is a straight tube. It is the principal site of digestion and
absorption. A dorsal fold of the epithelium is presentin the intestine. It is called
typhlosole. It increases the surface area of the intestine. The intestine ends at the
anus.

5. Gas Exchange and Circulation

Respiratory gases simply diffuse across the body wall. The parapodia increase the surface
area for gases exchange. Polychaetes have a closed circulatory system. Oxygen is carried
by respiratory pigments. The respiratory pigments are dissolved in the plasma. They are
present in the blood cells in other animals. Blood is colorless. green, or red, depending on
the respiratory pigment. It has following circulation pattern:

The dorsal aorta of polychaetes circulatory systems acts as contracting elements. Dorsal
aorta lies just above the digestive tract. It propels blood from rear to front. In the front,
the blood moves into ventral aorta. Ventral aorta is present ventral to the digestive tract.
It propels blood from front to rear.

Hearts: Some segmental vessels expand and they are contractile the earthworm. “Fhese
are sometimes called hearts. They transfer the blood from dorsal to ventral blood vessels.
Two or three sets of segmental vessels are present between dorsal and ventral vessels.
Segmental vessels receive blood from the ventral aorta and break into capillary beds in
the gut and body wall. Capillaries unite again into segmental vessels. These vessels
deliver blood to the dorsal aorta.

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6. Nervous and Sensory Functions
The ventral nerve cords and all ganglia of oligochaetes show a high degree of fusion. The
giant fibers produce escape responses. An escape response results from the stimulation of
anterior or posterior end of a worm.Oligochaetes are burrowing animals. Therefore, they
lack well-developed eyes. Animals living in continuous darkness do not have well-
developed eyes. Other oligochaetes have simple pigmented cup called ocelli.
Photoreceptor cells scattered over the dorsal and lateral surfaces of the body. Therefore,
they all have a “dermal light sense”. Photoreceptor cells produce a negative phototaxis in
strong light and a positive phototaxis in weak light. Oligochaetes are sensitive to different
chemical and mechanical stimuli. Receptors of these stimuli are scattered over the body
surface, especially around the prostomium.

7. Excretion
Oligochaetes use metanephridium for excretion and osmoregulation (ion water balance).
The nephrostome of metanephridium is present in anterior segment. Their tubule and
nephridiopore are present in the posterior segment. Nitrogenous wastes are ammonia and
urea. Oligochaetes excrete large amounts of very dilute urine. But they retain vital ions. It
is important for them. They live in environments where water is plentiful but essential
ions are hinted.

Oligochaetes possess chloragogen tissue. These tissues surround the dorsal blood vessel.
They lie over the dorsal surface of the intestine. Chloragogen tissue acts as vertebrate
liver. It is a site of amino acid metabolism. Chloragogen’ tissue deaminates amino acids
and converts ammonia to urea. It also converts excess carbohydrates into glycogen and
Fat.

8. Reproduction and Development

Oligochaetes are monoecious. They exchange sperm during copulation. One or o pairs of
tested and one pair of ovaries are present on the anterior septum of some anterior
segment. Both the sperm ducts and the oviducts have ciliated funnels at their proximal
end. It i. used to draw gametes into their male or female tubes.

Reproductive Organs
Testes are associated with three pairs of seminal vesicles. Seminal vesicles are used for
maturation and storage of sperm. Seminal receptacles in female receive sperm during
copulation. A pair of very small ovisac is present. It is associated with oviducts. Egg is
stored and become mature in ovisacs.

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Figure 6.15: Reproduction inEarthworm.

Two worms line up facing opposite directions during copulation. Their ventral surfaces
and their anterior ends are in contact with each other. This arrangement lines up the
clitellum of one worm with the genital segments of the other worm. Clitellum secretes a
mucous sheath. This sheath covers the anterior halves of both worms. This sheath holds
the worms in place. Some species also have penile structures and genital setae. These
structures maintain contact between worms. The Sperm duct releases sperm. Sperms
travel in a groove in the external ventral body wall. This groove is formed by the
contraction of special muscles. Muscular contractions along this groove propel the sperm
toward the openings of the seminal receptacles. In other Oligochaetes, there is direct
transfer of sperm into seminal receptacle during copulation. Copulation lasts for two to
three hours and both worms exchange sperms.

The clitellum forms cocoon for the deposition of eggs and sperm. The cocoon consists of
mucoid and chitinous materials. The clitellum secretes albumen into the cocoon. The
worm begins to come out of the cocoon. The cocoon passes through the openings of the
oviducts and eggs are deposited in it. The sperm are released as the cocoon passes the
openings to the seminal receptacles. Fertilization occurs in the cocoon. The worm come
out from the cocoon. The ends of the cocoon are sealed. The cocoons are deposited in
moist soil.

Spiral cleavage is modified. Larva is not formed in them. Hatching occurs in one to few
weeks. The young worms emerge from one end of the cocoon.

Asexual Reproduction
Freshwater Oligochaetes also reproduce asexually. Asexual reproduction takes place by
the transverse division of a worm. The missing segments are regenerated

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Self Assessment Questions
Q.1 How does an Earthworm move across substrate?
Q.2 How do earthworm reproduve?
Q.3 Describe internal structure of Oligochates.
Q.4 In what ways do sensory functions differe in Oligochates and polychaetes?

6.3.3 Class Hirudinea

The class Hirudinea contains five hundred species of leeches. Most leeches are fresh
water. Some are marine. Some are completely terrestrial. Leeches prey on small
invertebrates or they feed on the body fluids of vertebrates.

1. External Structure and Locomotion

Leeches lack parapodia and head appendages. Setae are absent in most of leeches. Setae
occur in anterior segments in few leeches. Leeches are dorsoventrally anteriorly. They
have 34 segments. The segment are divided by annuls. Therefore, it is difficult to
distinguish the segment externally. Anterior to and posterior segments are modified into
suckers. The leeches have modified patterns of body-wall muscles and coelom. It
influences locomotion of leech.

The musculature of leeches is more complex than annelids. A layer of oblique muscles is
present between the circular and longitudinal muscles layers. These muscles are
responsible for the flattening of leech. The coelom of leech has lost its metameric
partitioning. Septa are lost. Connective tissue has filled the coelom. It produces many
interconnecting sinuses.

These modifications have changed the patterns of locomotion in leeches. Therefore,

leeches do not use independent coelomic compartments. The leech has a single
hydrostatic cavity. It uses a looping type of locomotion. Leeches also swim by
undulations of the body.

2. Feeding and the Digestive System

Many leeches feed on body fluids. They may eat the bodies of other invertebrates. Some
feed on the blood of vertebrates, including human blood. Leeches are parasites. But the
association between a leech and its host is very brief. Therefore, leeches are described as
predator. Leeches are also not specie specific.

The mouth of a leech opens in the middle of the anterior sucker. In some leeches, the
anterior digestive tract is modified into a protrusible proboscis. It is lined inside and
outside by a cuticle. In other leeches, the mouth is armed with three chitinous jaws. A
leech attaches to its prey by anterior sucker during feeding. They extend their proboscis
into the prey. Or they use its jaws to cut through host tissues. Salivary glands secrete an
anticoagulant called hirudin. Hirudin prevents blood from clotting.

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There is muscular pharynx behind the mouth. It pumps body fluids of the prey into the
leech. The pharynx opens into esophagus and oesophagus open into a large stomach.
Stomach has lateral cecae. Most leeches ingest large quantity of blood or other body
fluids. They store it in their stomachs and lateral cecac. It increases their body mass 2 to
10 times. The leech can tolerate periods of fasting after storage of this fluid. This period
may last for months. The digestive tract ends in a short intestine and anus.

3. Gas Exchange and Circulation

Leeches exchange gases though the body wall. Some leeches retain the basic annelid
circulatory pattern. But most leeches have coelomic sinuses in place vessels. Coelomic
fluid has taken over the function of blood. Respiratory pigments are absent except in to
orders.

4. Nervous and Sensory Functions

The leech nervous system is similar to other annelids. Ventral nerve cords are not fused.
The suprapharyngeal and subpharyngeal ganglia and the pharyngeal connectives all fuse
to forms a nerve ring. This nerve ring surrounds the pharynx. Ganglia of posterior end of
the animal are also fused.

5. Sense organs
Different types of epidermal sense organs are scattered over the body. Most leeches
have photoreceptor cells in pigment cups. Pigments cups are present along the dorsal
surface of the anterior segments. Leeches are negatively phototactic. But the behavior of
some leeches changes during searching of food. Thus they become positively phototactic.
It helps in search of prey.

Hirudo medicinalis has a well-developed temperature sense. It helps it to detect the

higher body temperature of mammalian prey. Other leeches are attracted by the prey
tissues. All leeches have sensory cells with terminal bristles. They are present in a row
along the middle annulus of each segment. These sensory cells are called sensory
papillae. Their function is unknown. But they are taxonomically important.

6. Excretion
Leeches have 10 to 17 pairs of metanephrindia. One nephridium is present per segment in
the middle segments of the body. Their metanephridia are highly modified. It possesses a
capsule. This capsule is involved in the production of coelomic fluid. Chloragogen tissue
spread through the body cavity of most leeches.

7. Reproduction and Development

All leeches reproduce sexually. They are monoecious. Asexual reproduction or
regeneration is absent in them. They have a single pair of ovaries. They have four to
many testes. Leeches have a clitellum. Clitellum is composed of three body segments.
The clitellum is present only in the spring. Spring is breeding season of leeches.

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A penis is used to transfer sperm between individuals. A few leeches transfer sperm by
expelling spermatophore from one leech into the integument of another. It is a form of
hypodermic impregnation. Special tissues are connected to the ovaries by short ducts.
Cocoons are deposited in the soil. or they attached to underwater objects. There are no
larval stages. The offspring become mature in next spring.

6.4 Further Phylogenetice Considerations

It suggests that the ancestral polychaetes gave rise to modern polychaetes through
adaptive radiation. Then a group of annelids adapted in freshwater. The fresh water
developed the ability to regulate the salt and water content of body fluids. Then the
Oligochaetes are evolved from this group. Then some of the early Oligochaetes have to
the Hirudinea. Cladistics analysis suggests that the phylum Annelida is not a
monophyletic suggests that: The Polychaetes arose from a metameric ancestor. It has
independently of the Oligochaetes and leeches.

The oligochoetes and leeches form a single They have many common important
characteristics. A clitellum is present in groups. No unique synapomorphies (derived
characteristics) are present in the oligochaetes. Rather, oligochaetes are defined by the
absence of leech characteristics. These facts support the idea that the oligochoetes and
leeches should be combined into a single group called Clitellata. It means that the
Polychaeta, Clitellata. Arthropoda and Pogonophora have a common ancestor. Their
ancestor is a metameric species. Therefore, phylum annelida should be discarded.

A group of annelids hat e been discovered recently. It is called Archiannelida. These

annelids lack coelom, setae, and some other annelid characteristics. The group is poorly
known. Its taxonomic relationships to other annelids cannot be established. Some of these
worms are closed ancestral form. Some are derived from the polyphyletie lineage. Work
is still going on these animals.

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Figure 6.17: Phylogenetic Considerations

Self Assessment Questions

Q.1 How are earthworms helpful to farmers?
Q.2 Where do annelids live?
Q.3 Where you can find leeches?
Q.4 On a rainy day, you notice a long, brown, bilaterally symmetric organism in your
garden. This organism has a body that is divided into segments, from the head to
the tail. Under which phylum will you classify the organism, looking at its
features? What do you think is the organism?

Activity: Please go to this site, open link and answer the questions.
https://www.mcqslearn.com/study/phylum/phylum-annelida-multiple-choice-
questions.php

Activity: Watch a video on Phylum Annelida by clicking the link and write a summary of
the video?
https://study.com/academy/lesson/phylum-annelida-characteristics-classes-examples.html

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Key to Question
Ans.1: To a farmer, earthworms are the most helpful and friendly creatures. They
are burrowing creatures and play an important role in improving the soil
texture and enriching the soil. Earthworms plough the soil by eating their
way through the soil. They digest the soil along with the dead leaves and any
other organic material. By doing this, they constantly loosen the upper layer
of soil. This results in good water percolation and air penetration. The
droppings of the earthworm also enrich the soil, making it good for plants to
grow.
Ans.2: Annelids can live in marine or freshwater habitats or even in moist terrestrial
environments.
Ans.3: Freshwater leeches can be found in freshwaters. Marine leeches can be found
in oceans. Leeches crawl well and are good swimmers.
Ans.4: Since the organism is long, brown and bilaterally symmetrical, with a
segmented body, it can be classified under phylum Annelida. The
distinguishing characteristic is the segmented body. And Annelids also live
in moist terrestrial areas. This annelid is an earthworm, which can be found
in moist soils.

References
 Ackermann C, Dorresteijn A, Fischer A (2005) Clonal domains in postlarval
Platynereis dumerilii (Annelida: Polychaeta). J Morphol 266:258–280
 Beesley, P. L., Ross, G. J. B., and Glasby, C. J. 2000. Polychaetes and Allies: The
Southern Synthesis. Fauna of Australia. Vol. 4 Polychaeta, Myzostomida,
Pogonophora, Echiura, Sipuncula. Melbourne, Australia: CSIRO Publishing.
 Siddall, M. E., Borda, E., and Rouse, G. W. 2004. Towards a Tree of Life for the
Annelida.
 In, Assembling the Tree of Life (Cracraft, J. and Donoghue, M., eds.), pp. 237-251.
New York, NY: Oxford University Press.
 Rouse, G. W. and Pleijel, F. 2001. Polychaetes. Oxford, UK: Oxford University
Press
 Rouse, G. W. and Pleijel, F. 2001. Polychaetes. Oxford, UK: Oxford University
Press

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UNIT-7

ARTHROPODS

Written by: Dr. Tauseef Anwar

Reviewed by: Dr. Muhammad Waseem

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 CONTENTS
Introduction ....................................................................................................... 173

Objectives ......................................................................................................... 173

7.1 Phylum Arthropoda: General characteristics and its Classification .... 174

7.2 Phylogenetic Consideration .................................................................. 174

7.3 Relationships to Other Animals ............................................................ 176

7.4 The Arthropods: Blueprint of Success .................................................. 177

7.5 Diversity among Copepods ................................................................... 178

7.6 Classification of Phylum Arthropoda ................................................... 179

7.7 Exoskeleton – Composition, Modification & Molting ......................... 190

7.8 Metamerism and Tagmatization ........................................................... 192

7.9 Metamorphosis ...................................................................................... 192

7.10 External Structure and Locomotion ...................................................... 193

7.11 Insect Behaviour ................................................................................... 205

7.12 Social Insects ........................................................................................ 205

7.13 Insects and Humans .............................................................................. 206

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Introduction
The phylum Arthropoda, which includes crabs, shrimps, spiders, millipedes, centipedes and
insects, is invariably described with superlatives. More than 85% of animal species are
arthropods, over one million species of insects have already been described and estimates of
the total number of species range up to 30 million (based on extrapolation from tropical forest
studies). The phylum has radiated into every habitat, from the ocean floor to high altitudes,
and its members are extraordinarily diverse in their structure and life histories. The biomass
of arthropods is colossal; some calculations suggest it is greater than all other animals
combined. Arthropods are characterized by a segmented body covered in a resistant covering
of cuticle which is often hardened to form an exoskeleton. Flexible cuticle between sections
of the limbs and body segments forms joints and allows movement by muscles attached to the
cuticle. The principal component of the cuticle is the carbohydrate chitin, the second most
abundant polymer on earth, after cellulose. Other distinguishing features of arthropods
include a segmented body showing bilateral symmetry and a variable number of body
segments with paired appendages having different functions depending on the part of the
body from which they originate. The circulatory system comprises a dorsal heart and vascular
spaces making up a haemocoel, and the central nervous system consists of an anterior supra-
oesophageal centre or brain and a ganglionated ventral nerve cord. The muscles are
principally of the striated fibre type and there is a general absence of ciliated epithelial cells.
The arthropods are an ancient group, for example the trilobites had already widely radiated to
become the dominant arthropods in the Early Palaeozoic seas (about 550 million years ago)
and then became extinct by 280 million years ago. It is presumed that the arthropods had a
long Precambrian period of development during which they differentiated from their annelid
ancestors. The earliest arachnids appeared about 400 million years ago and the insects,
millipedes and seaspiders appeared in the Devonian period, about 350 million years ago.

Objectives
After completion of this Unit, you will be able to:
 Define terms related to phylum arthropoda.
 Write characteristics ofArthropods.
 Identify different arthropods out of classes.
 Tell diversity among copepods.
 Differentiate between metamerism and tagmatization.
 Elaborate different aspects of metamorphosis.
 Develop relationship of arthropods with other animals.
 Classify arthropods upto class level
 Describe structure of arthropods.
 Explain different maintenance function for life in arthropods.
 Determine chemical regulation in insects
 Differentiate different behaviours among insects
 Identify uses of insects for human beings
 Identify harmful effects of insects for human beings and crops.

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7.1 Phylum Arthropoda: General Characteristics and its
Classification
The term “Arthropoda” is derived from two Greek words ‘arthros’ meaning ‘joint’ and
‘podos’ meaning ‘leg’. Hence the word “Arthropoda” means “jointed legs”. Arthropods
are protostomic, eucoelomic organisms and they have functional segmentation body with
a pair of the jointed appendage. They also have a bilaterally symmetrical body with
external chitinous cuticle. Some species of arthropods bear wings for aerial movement.

Phylum Arthropoda is the largest animal group which constitutes the largest percentage of the
world’s organisms. It is estimated that about 84% of all known species of animals belong to
this phylum and number of known species vary between 1,170,000 and 5 to 10 million.
Besides these, many arthropods are not yet unidentified. Some prominent groups of
arthropods are insects, crabs, spiders, bees, ants, millipedes, shrimps, centipedes, etc. Among
them, insects form the single largest class of phylum Arthropoda. According to the
entomologist, globally there are over 10 quintillion insects on earth, of which, more than
300,000 species are Coleoptera, 180,000 species belong to Lepidoptera, over 90,000 dipteran
species and over 100,000 species belong to Hymenoptera.They are the successful animal
groups which show a great variety of adaptations. Among them, some live in aquatic
environments, some inhabit in terrestrial habitat and others are adapted for aerial habitats.

Characteristics of the phylum Arthropoda include:

 They show metamerism. Metamerism is modified by the specialization of body
regions for specific functions (tagmatization)
 Chitinous exoskeleton provides support and protection. It is modified to form
sensory structures.
 They have paired jointed appendages.
 Ecdysis or molting takes place during growth.
 They have ventral nervous system.
 Coelom reduced to cavities. These cavities are reduced to gonads and sometimes
excretory organs.
 They have open circulatory system. Blood is released into tissue spaces (haemocoel
derived from the blastocoel.
 They have complete digestive tract
 Metamorphosis is often present. It reduces competition between immature and
adult stages.

7.2 Phylogenetic Consideration

There is a fundamental question about the evolution of arthropod. The question is that
whether or not the arthropod taxa represent different evolutionary lineages.

 Polyphyletic origin: Many zoologists believe that the living arthropods should be
divided into three separate phyla: C’helicerata, Crustacea and Uniramia. There are

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 similar arthropods features in all three phyla. Therefore a polyphyletic origin of
these groups shows convergent evolution. There can be dual origins of tracheae,
mandibles and compound eyes. But man zoologists believe that convergence in all
other arthropod traits is not possible.
 Monophyletic origin: Memberers of all four subphyla are present in the fossil
record from the early Paleozoic era. There are currently no known fossils of
arthropods from Precambrian times. Therefore, the fossil record does not help in
discovering the evolutionary relationships among the arthropod subphyla. Thus
zoologists rely on comparative anatomy, comparative embryology and molecular
studies.

These are two important issues in the phylogeny of arthropods.

Issue 1: The question is whether or not the biramous limbs of crustaceans and
trilobites are homologous structures. Their homology shows that trilobites
are closely related to the crustaceans. Therefore, many zoologists take
trilobites as an important ancestral group. Thus crustaceans and arachnids
have arises from trilobites. But it is difficult to evolve Myriapoda and insects
form trilobites.
Issue 2: The question is whether or not the mandible of crustaceans and trilobites are
homologous structures. The mandibles in the groups of arthropods are
structurally and functionally similar. But they have different muscle
arrangements and articulation in mandibles. Therefore, some zoologist does
not take mandible in these groups as homlogous structures. Therefore, the
zoologists are working on the origin and homologies of compound eyes,
tracheal systems and malpighian tubules.

Number of hypothesis have been given on the basis of these issues. These hypotheses
show the relationship between the different sub-phyla of the arthropods.
 First hypothesis shows that Uniramia and Crustacea have close relationship. These
hypotheses develop homologies between these two groups. The homologous
organs between these two groups are mandibles, compound eyes and some other
structures.
 Second hypothesis shows that the Uniramia diverge independently. It has not
common ancestor with Chelicerata and Crustacea. Chelicerata and Crustacea are
closely related to trilobites. Therefore, the mandible of the crustaceans and
Uniramia are not homologous. On the other hand the biramous appendages of the
crustaceans and trilobites are homologous.

This discussion shows that it is difficult to develop evolutionary relationship between the
different groups of arthropods. More fossils and molecular evidences are required to
develop relationship between them.

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Figure 7.1: Traditional Interpretation of Arthropod Phylogeny

Phylogeny of Chelicerates: The subphylum Chelicerata is a very important group of

animals from an evolutionary point of view, They have lesser numbers of species. But
they have more individual than many of the crustacean groups. Their exoskeleton and the
evolution of excretory and respiratory systems minimize water loss. These characteristics
shown that ancestral members of this subphylum are the ancestor of terrestrial animals.
Chelicerates are not the only terrestrial arthropods. In fact, the chelicerates have lesser
numbers of species and numbers of individuals than the insects and their relatives.

7.3 Relationships to Other Animals

The arthropods and annelids are closely related. They share many protosome
characteristics. These are;
1. Both have schizocoelous coelom formation.
2. Their mouth is developed from blastopore.
3. Paired ventral nerve cords are present in them.
4. Both have metamerism.
5. Tribitomorpha: All members or this subphylum are extinct.

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Figure 7.2: Relationship for Arthropods with other animals

These characteristics show that the arthropods and annelids have common ancestor. But
zoologists disagree about the evolutionary relationships among the arthropods. Many
zoologists believe that it is not one phylum. But three living arthropods are divided into
three sub- phyla:
i. Chelicerata
ii. Crustacea
iii. Uniramia

7.4 The Arthropods: Blueprint of Success

7.4.1 The Hexapods and Myriapoda
The insects are most successful animals. Zoologists have described approximately 750
100 species of insects. The actual number of insect species is 30 million. Most of the
described species are in tropicalrain forests. The insect species comprise 75% of all living
species. There are numerous freshwater and parasitic insect species. But the most
successful insects are terrestrial. The success of these insect is due to their ability to
exploit terrestrial habitats.

Evolution of insects: Animals were absent in terrestrial environments of late Silurian and
early Devonian periods (about 400 million years ago). The herbaceous plants and the firs
forests began to flourish. Enough ozone accumulated in the upper atmosphere. It filters
ultraviolet radiation from the sun. Therefore, a large amount of photosynthetic production
was available for animal. Animal started living in terrestrial environment. The animals
faced little competition from other animals for resources. But they placed many problems

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in terrestrial life. Support and movement was difficult on land. Similarly regulation of
water, ion (electrolyte) and temperature was also a problem on land.

Adaptations in insects: A number of factors are responsible for dominance of insects on

terrestrial habitats. These are:
 The exoskeleton adapted the insects on land.
 The evolution of a waxy epicuticle increases the water conserving properties of
exoskeleton.
 The evolution of flight also played a big role in success of insects. The ability to fly
helps them in many ways. It allows them to use wide resources of food. They can
migrate to new habitat.
 They develop desiccation-resistant eggs.
 Their larvae undergo metamorphosis.
 They have high reproductive potential.
 They have different types of mouthparts and feeding habits.

7.5 Diversity among Copepods

The species of copepods have greatest number of individuals. The size of copepod is 1 to
2 mm. These are crustaceans. Copepods are very successful marine animals. They feed
on the photosynthetic production (panktonic algae) in the open oceans. If plankton net is
thrown behind a moving boat in the ocean, three million copepods are collect after 20
minutes. It can fill 2 gallon pail.

Copepods are food for fish, whale, sharks, blue whale and its relatives. Humans eat these
fishes. Therefore, human also directly benefited from it. Unfortunately, we do not get this
food directly. We use them as a feed of poultry. Therefore, we lose 90% of the original
energy present in the copepods.

Copepods are belonged to phylum Arthropoda. Crayfish, lobsters, spiders, mites,

scorpions and insects are also arthropods. Zoologists have described about one million
species arthropods. It is estimated that 30 to 50 million species are still unsubscribed.

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Figure 7.3: Evolutionary Relationships of Arthropods

7.6 Classification of Phylum Arthropoda

The phylum Arthropoda is divided into the following five sub-phyla:
 Trilobitomorpha
 Chelicerata
 Myriapoda
 Hexapoda and
 Crustacea

7.6.1 Sub Phylum-I: Trilobitomorpha [Gk. tri = three, lobos = lobe, morphe = shape
= three-lobed form]
 Extinct marine arthropods are included in this subphylum.
 They were plentiful during the Paleozoic era.
 The body is divided into three regions: head or cephalon, trunk or thorax, and
pygidium.
 Their body size ranges from 10 mm to 60 cm.
 Their exoskeleton or carapace was un-jointed which covered the head and
pygidium.
 Head was distinct with a pair of compound eyes and antennae on the anterior part
of the body.
 Post-antennal appendages were uniform, unspecialized and biramous.
 Legs were 8 segmented and anal opening was present on the last segment of the
pygidium.
 The subphylum included about 4000 species under 5 classes. Among them, class
Trilobita includes the largest number of species.

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Examples: Triarthrus eatoni, Dalmanites limulurus and Triarthrus eatoni

Figure 7.4: Triarthrus eatoni and Dalmanites limulurus

7.6.2 Sub Phylum-II: Chelicerata (Gr. chele=claw, keros=horn)

 The body is divided into cephalothorax and abdomen with no distinct head. In this
case, the abdomen has two regions such as anterior mesosoma and the posterior
metasome with a telson.
 Cephalothorax bears six pairs of appendages with no antennae or antennules.
Among them, the first pair appendage is chelicerae or feeding appendage.
 Chelicerates do not contain jaws or mandibles and each chelicera is jointed and
bears a terminal chela.
 The abdomen bears 12-13 segments and a telson where the second abdominal
segment bears genital aperture.
 Median simple eyes are present but in most cases compound eyes are degenerated
 This subphylum includes the sea spiders, arachnids, scorpions and potentially
horseshoe crabs.
 The development is usually direct.

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Subphylum Chelicerata is again divided into the following two classes:
 Class-I: Xiphosura
 Class-II: Arachnida
Class I: Xiphosura (Gr. xiphos=sword, oura=tail)
 They are commonly called horseshoe crabs. All are bottom dwellers and inhabit
marine environment.
 The cephalothorax is covered by a broad, smooth, and horse-shoe shaped carapace.
 One pair of chelicerae, four pairs of walking legs and one pair of pusher legs are
present in the cephalothorax.
 They have lateral compound eyes and median ocelli.
 5 pairs of book gills are present in the abdomen.
 At caudal region, an elongated, slender and pointed spine-like structure is present.
 In the abdominal region, the dorsal ridge is visible.
 Excretion occurs through a four-lobed coxal gland.
 Indirect development with a distinct larval stage (trilobite larva).Limulus
polyphemus

Examples: Limulus polyphemus (horseshoe crabs), Tachypleus gigas

Figure 7.5: Limulus polyphemus (horseshoe crabs) and Tachypleus gigas

Class II: Arachnida (Gr. Arachne =spider)

 This class contains about 74,000 species. Among them, notable species are
scorpions, ticks, mites and spiders.
 Mostly they are terrestrial organisms and carnivorous.
 The body is divided into two regions: cephalothorax and abdomen.
 Cephalothorax bears 6 pairs of appendages. Among them, the first pair of preoral
chelicerae, 2nd pair of pedipalps which are postoral and remaining four pairs of
winged legs (Thoracic legs).
 In some species such as spiders, each chelicea contains fangs with poisonous
glands.

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 Generally, abdominal appendages are changed into book lungs but in spiders, it
forms spinnerets while in scorpions it is modified into pectin.
 Most of the species do not contain telson but in scorpions, it is present as a sting.
 Excretion occurs through malpighian tubules, coxal glands or both.
 They show separate sexes; they lay centrolecithal and yolky eggs and their
development is direct.
 Generally, the simple eye is present but sometimes degenerated compound eye is
also found.

Examples: Heterometrus cyaneus

Figure 7.6: Scorpio and Spider

7.6.3 Sub Phylum-Myriopoda

 This subphylum contains over 16,000 species, most of which are terrestrial and few
are aquatic.
 They have an elongated body which is divided into head and trunk with numerous
segments.
 The body length ranges from 0.5–300 mm.
 Generally, they live in humid environments where caves are available.
 They have simple eyes with one pair of antennae but in some cases, they have large
and well-developed compound eyes (Scutigera).
 They perform respiration through spiracles which connect to a tracheal system.
 Mouth bears one pair of mandibles.
 Excretion occurs through Malpighian tubules.

Sub Phylum Myriopoda is divided into the following four classes:

 Class-I: Chilopoda (centipedes)
 Class-II: Diplopoda (millipeds)
 Class-III: Pauropoda
 Class-IV: Symphyla (pseudocentipedes)

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Class-I: Chilopoda (centipedes)
 This class contains about 2800 species.
 The body is dorso-ventrally flattened with 15–173 segments.
 Each segment bears one pair of legs.Scutigera coleoptrata
 They can live in most terrestrial environments including desert fringes.
 They have poisonous fangs (forcipules) which enable centipedes to kill and
consume insects and other organisms.
 Their body is covered with a chitinous and non-calcified exoskeleton.

Example: Scolopendra galapagoensis

Figure 7.7: Centipede

Class-II: Diplopoda (Millipedes)

 This class contains over 10000 species.
 They are relatively long-lived and can live up to 7 years.
 They have a cylindrical body with 11–90 segments.
 Each segment bears two pairs of legs.
 The body bears segmental plates which are constructed to roll up or coil for
preventing them when threatened.
 They are slow-moving herbivore organisms which eat decaying leaves and wood.
 They have no poisonous fangs but they keep away predators by producing volatile
poison from repugnatorial glands.

Example: Julus terrestris

Figure 7.8: Millimpede (Julus terrestris)

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Class-III: Pauropoda
 This class contains around 830 species under 12 families.
 They are small and pale millipede-like organisms.
 They prefer to live in soil and leaf mold.
 They have a soft cylindrical body with 0.5 to 2 mm in length.
 Each body segment bears long sensory hairs.
 They have a distinctive anal plate.
 Each body segment bears ventral tracheal pouches which form apodemes.
 They have branching and segmented biramous antennae

Example: Pauropus amicus

Figure 7.9: Pauropus amicus

Class-IV: Symphyla (Pseudocentipedes)

 They are also known as pseudocentipedes or garden centipedes.
 They have a soft body with two regions: head and trunk.
 They can grow up to 30 mm in length.
 The head is long with segmented antennae.Scutigerella immaculata
 The trunk contains 15–24 segments, of which, ten or twelve segments contain legs.
 They can live up to four years or more.
 They prefer to consume decaying vegetation.

Example: Scutigerella immaculata (garden symphylan)

Figure 7.10: Scutigerella immaculata (Garden Symphylan)

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7.6.3 Sub Phylum: Hexapoda
 They constitute the largest number of species of Arthropods which includes insects
and wingless arthropods such as Collembola, Protura, and Diplura.
 Their body is divided into three regions: head, thorax, abdomen with six thoracic
legs.
 The body length ranges from 0.5 mm to 300 mm.
 The head bears presegmental acron which usually contains eyes.
 The body is segmented and covered by chitinous cuticle called an exoskeleton.
 Thorax contains three pairs of jointed legs which give the name Hexapoda.
 They perform respiration through trachea and excretion through Malpighian
tubules.
 The development includes metamorphosis.

Examples: Musca domestica (housefly), Lepisma saccharina (silverfish)

SubphylumHexapoda is divided into the following twoclasses:
 Class-I: Insecta (insects)
 Class-II: Entognatha

Class-I: Insecta (insects)

 Class Insecta is the largest and diverse group under the phylum Arthropoda.
 They are also known as hexapod invertebrates.
 The word "insect" is derived from the Latin word insectum, meaning divided body.
 The body is divided into three parts: head, thorax, and abdomen.
 They have a chitinous exoskeleton.
 The body contains three pairs of jointed legs, one pair of compound eyes and one
pair of antennae.
 They are mostly terrestrial and few are aquatic.
 The abdomen contains 7-11 segments without appendages.
 Respiration occurs through the trachea, gills, etc.
 Excretion performs through malpighian tubules.

Examples: Pierisbalcana (Butterfly) Apis serana

Figure 7.11: Apis serana &Butterflies

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Class-II: Entognatha
 They are the wingless and ametabolous arthropods.
 They have entognathous mouthparts because all parts of the mouth are retracted
within the head.
 Their mouthparts protected inside the head and they only project them during
feeding.
 Most of them are herbivores or detritivores.
 They are small-sized (<10mm) and inconspicuous organisms.

Example: Entomobrya albocincta

Figure 7.12: Entomobrya albocincta

7.6.4 Subphylum-V: Crustacea (Crusta: shell)

Class crustacean makes the large group of organisms under phylum Arthropoda. The
notable crustaceans include the crabs, crayfish, lobsters, shrimp, barnacles, krill,
copepods, ostracods, and mantis shrimp, etc. This class contains over 67,000 known
species which are found in freshwater and sea. The American lobster is the largest
crustaceans which can grow up to 20 kg in weight while the giant Japanese crab which
has legs that can span up to 12 feet (3.7 meters).
 They have a segmented and bilaterally symmetrical body.
 The body is divided into three regions: Head, thorax, and abdomen.
 Generally, they are mostly aquatic some are terrestrial and very few leads parasitic
life.
 Head and thoracic segments are fused to form a cephalothorax. In this case,
cephalothorax is covered by the large carapace.
 Each segment of thorax and abdomen bear a pair of biramous appendages.
 Head bears two pairs of sensory antennae, two pairs of maxillae and one pair of
mandibles
 They perform respiration through gills and general body surface.
 They perform excretion through a pair of green glands which is situated near the
base of the antennae.

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Classification of Crustacea
The subphylum Crustacea is divided into the following six classes;
 Class-I: Malacostraca
 Class-II: Maxillipoda
 Class-III: Branchipoda
 Class-IV: Ostracoda
 Class-V: Remipedia
 Class-VI: Cephalocarida

Class-I: Malacostraca
This class is the largest and extremely diverse class which contains over 40,000 species.
The notable species include pill bugs, lobster, amphipods, crabs, crayfish, krill, mantis
shrimp, etc.
 They are plentiful in all marine habitats but also inhabit in freshwater and terrestrial
habitats.
 They have a segmented body with up to 21 segments.
 The body is divided into three regions: head, thorax, and abdomen.
 The head is six segmented which contains two pairs of antennae, of which, the first
pair of antennae are biramous.
 The body is covered with exoskeleton which is made up of chitin and calcium.
 They have stalked or sessile eyes.

Example: Nennalpheus sibogae Lysmata amboinensis

Figure 7.13: Nennalpheus sibogae & Lysmata amboinensis

Class-II: Maxillipoda
 This class contains over 23,000 described species.
 They are mostly small crustaceans and their body length ranges from 0.5-2.0 mm.
 The notable species include freshwater and marine copepods, marine barnacles, and
a few additional groups like fish lice, etc.
 They have a reduced abdomen with few or no appendages.
 The head bears five pairs of appendages while the trunk is up to 11 segmented.
 They have single and simple median eye and antennules are uniramous.

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Example: Aglaodiaptomus leptopus & Tigriopus brevicornis

Figure 7.14: Aglaodiaptomus leptopus & Tigriopus brevicornis

Class-III: Branchipoda
 This class contains roughly 800 species and comprises brine shrimp, tadpole
shrimp, fairy shrimp, water fleas, etc.
 They chiefly inhabit in freshwater environments.
 They have compound eyes.
 Thoracic legs are flat, unbranched and edged with setae and have no distinct
segments.Artemia salina
 Their body length ranges from 0.25 mm to 10 cm (fairy shrimp: Branchinecta
gigas).

Example: Daphnia pulex

Figure 7.15: Daphnia pulex

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Class-IV: Ostracoda
 This class contains about 800 described species and sometimes they are known as
seed shrimp.
 They have a small, flattened body which is protected by a bivalve-like calcareous
shell.
 The body is divided into two regions: head and thorax.
 Generally, they are small aquatic organisms and their body length ranges from 0.1
to 32 mm.
 The head bears five pairs of appendages but body bears only 1-3 pairs of
appendages.
 They have two pairs of well-developed antennae which help to swim through the
water.
Example: Stenocypris hislopi & Herpetocypris reptans

Figure 7.16: Stenocypris hislopi & Herpetocypris reptans

Class-V: Remipedia
 This class contains about 17 species.
 The body is divided into head and elongated trunk.
 The trunk bears up to 32 similarly shaped segments.
 Generally, they have no eyes (except Enantiopoda).
 The trunk segments bear paddle-like appendages which are directed out to the side.

Example: Tesnusocaris goldichi , Cryptocaris hootchi

Figure 7.17: Tesnusocaris goldichi, Cryptocaris hootchi

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Class-VI: Cephalocarida
 It is a very small class which contains about 12 species.
 They have an elongated body with short head and their body size ranges from 2 to
4 mm in length.
 The body is divided into three regions: head, thorax, and abdomen.
 Thorax bears 8 segments that possess biramous, paddle-like
appendages.Hampsonellus brasiliensis
 The abdomen consists of eleven segments that do not contain any appendages.
 They have no eyes and the head bears two pairs of antennae and two pairs of jaws.
In this case, the first pair of antennae are uniramous while the second pair is
biramous.
 The tail end or telson bears one pair of long, thread-like, uniramous appendages.

Example: Chiltoniella elongate, Hampsonellus brasiliensis

Figure 7.18: Chiltoniella elongate, Hampsonellus brasiliensis

7.7 Exoskeleton – Composition, Modification & Molting

Arthropods have an external jointed skeleton. It is composed of cuticle. Exoskeleton
encloses the arthropods. The exoskeleton is major reason or the success of arthropods.
Exoskeleton performs following functions:
 It provides structural support.
 It protects the body.
 It is impermeable for water. Therefore, it prevents the loss of water.
 It acts as a system of levers for muscle attachment and movement.

The exoskeleton covers all body surfaces. It also covers the invaginations of the body
wall like anterior and posterior portions of the gut. It is nonliving. It is secreted by a
single layer of epidermal cells. The outside of arthropods is covered by exoskeleton. Thus
its epidermis is not directly exposed to air or water. Therefore their epidermal laver is
called the hypodermis.

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Figure 7.19: Modification of Exoskeleton
7.7.1 Composition of Exoskeleton
The exoskeleton has two lavers:
 Epicuticle: It is the outermost layer. It is made up of a waxy lipoprotein. It is
impermeable for water. It also acts as a barrier to microorganisms and pesticides.
 Procuticle: It forms the bulk of the exoskeleton. It is present below the epicuticle.
Procuticle is called the endocuticle in crustaceans. The procuticle is composed of
chitin and several kinds of proteins. Chitin is a tough leathery polysaccharide. The
procuticle hardens by a process called sclerotization. It is sometimes impregnated
with calcium carbonate. Sclerotization is a tanning process in which layers of protein
is chemically cross-linked with one another. It hardens and darkens the exoskeleton.
This bonding occurs in the outer portion of the procutile in insects and most other
arthropods. The exoskeleton of crustaceans hardens by sclerotization and bv the
deposition of calcium carbonate in e middle regions of the procuticle. Some proteins
give the exoskeleton resiliency for flapping wings and jumping. The inner portion of
the procuticle does not harden.

Figure 7.20: Events of Ecdysis; (a,b) During preecdysis, (c,d): secreting a new epicuticle,
(e): ecdysis occurs when the animal swallows air or water, (f): after ecdysis
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7.7.2 Modification in the exoskeleton
 The hard procuticle protects the arthropods as armor. It also causes many
modifications and adaptations in arthropods:
 Invaginations of the exoskeleton confirm firm ridges and bars. These ridges and bars
are used for muscle attachment.
 Exoskeleum forms joints. A flexible membrane articular membrane is present in the
inner and less hardened areas. The cuticle is modified to form sensilla. Sensilla is
present in the form of pegs, bristles and Lenses.
 There are modifications of the exoskeleton for gas exchange.

7.7.3 Molting or Ecdysis

The periodic shedding of exoskeleton for growth in arthropods is called molting process
or ecdysis. Ecdysis is divided into four stages:
 Hypodermal glands secrete enzymes. These enzymes start digesting the old
procuticle and separate the hypodermis and the exoskeleton.
 New procuticle and epicuticle are secreted.
 The animal stretches by air or water intake. Therefore, the exoskeleton splits along
ecdysal lines. Pores in the procuticle secrete additional epicuticle.
 Finally calcium carbonate deposits and sclerotization harden the new
exoskeleton.The arthropod is vulnerable (weak) to predators during ecdysis.
Therefore, it remains hidden. The nervous and endocrine systems control all these
changes.

7.8 Metamerism and Tagmatization

Three characteristics have contributed in the success of arthropods. One of is
metamerism. Metamerism of arthropods is apparent externally. The arthropod body is
composed of similar segments. Each segment bears a pair of appendages. Similarly most
organ systems are not metamerically arranged. Different zoologists give different reasons
for the loss of internal metamerism. It is believed that the arthropods do not need
metamerically arranged hydrostatic compartments. They arthropods are enclosed by an
external skeleton. Therefore, they don’t require hydrostatic compartments or support or
locomotion or animal. Metamerism produces specialization in regions of the body for
specific functions. The regional specialization is called tagmatizafion. In arthropods,,
body regions are called tagmata, tagmata are specialized for feeding and sensors
perception, locomotion and visceral functions.

7.9 Metamorphosis
The radical change in body form and physiology from immature larval stage to adult is
called metamorphosis. It is third most important characteristic of the arthropods.
Metamorphosis reduces competition between adult and immature stages. The evolution
of arthropods has created increasing divergence of body forms, behaviors and habitats
between immature and adult stages. For example adult crabs prawn the sandy bottoms of

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their marine habitats. They prowl for prey or decaying organic matter. One the other
hand, the larval crabs lives and feed on the plankton. Similarly, the caterpillar (insect
larva) feeds on leafy vegetation. It develops into a nectar-feeding adult butterfly or moth.
Thus adult and immature stages do not compete with each other for food or living space.
In some arthropod larvae also acts as dispersal stage.

7.10 External Structure and Locomotion

Nutrition and the Digestive System, Gas Exchange, Circulation and Temperature
Regulation, Nervous and Sensory Functionsclass Hexapoda (Insects)

Members of the class Hexapoda are the most successful land animals. They are called
insects. The arthropods with three pairs of wing, one pair of antennae and three pairs of
legs are called insects.

Figure 7.21: External structure of a generalized insect

7.10.1 External Structure and Locomotion

The body of an insect is divided into three tagmata: head, thorax, and abdomen.

1. Head: Head bears a single pair of antennae. Mouth parts, compound eve and zero,
two, or three ocelli.
2. Thorax: The thorax consists of three segments. These segments are; the prothurax,
mesothorax and metathorax. Following appendages are attached with the thorax:
a) Legs: One pair of legs attaches at the ventral margin of each thoracic
segment.
b) Wings: A pair of wings is attached at the dorsolateral margin of the
mesothorax and metathorax. Wings have thick hollow veins. These veins
increase the strength of the wings.
c) Spiracles: The thorax contains two pairs of spiracles. Spiracles are openings
of the tracheal system.

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3. Abdomen: Most insects have 10 or 11 abdominal segments. Following structures
are present on abdomen:
a) Lateral fold:Each abdominal segment has a lateral fold in the exoskeleton.
This folds allows the abdomen to expand when the insect is gorged. It also
expands the abdomen when abdomen is full of mature eggs.
b) Spiracles: Each abdominal segment has a pair of spiracles.
c) Genital Structures: Genital structures are used during copulation and egg
deposition.
d) Cerci: Cerci are sensory structures. They are present on each segment.
e) Gills: Gills are present on abdominal segments of some immature aquatic
insects.

7.10.2 Insect Flight

Flight is the most important form of insect locomotion. Insects were the first animals to
fly.

Evolution of Wing
There is a most popular hypothesis about the origin of wing. It suggests that the wing
have evolved from rigid, lateral outgrowths of the thorax. These outgrowths protected the
legs or spiracles. Later, insects started using these fixed lobes for gliding from the top of
tall plants to the forest floor. Later the w ing developed the ability to flap, tilt and fold
back over. Then the evolution of limited thermoregulation started in insects.
Thermoregulation is the ability to maintain body temperatures at a level different from
environmental temperatures. High body temperatures of 25° C or greater is needed or
flight muscles to contract rapidly.

Mechanism of Flight
There are two mechanisms of flight.

1. Direct or Synchronous Flight

In this case muscles move the wings directly. One group of muscles is present on
the bases oil the wings. They contract to produce a downward thrust. Second group
of muscles is attached dorsally and ventrally on the exoskeleton. These muscles
contract to produce an upward thrust. The synchrony of direct flight mechanisms
depends on the nerve impulse to the flight muscles. This nerve impulse must come
before each wing beat. Butterflies, dragonflies and grasshoppers are examples of
insects with a synchronous flight mechanism.
2. Indirect or Asynchronous Flight Mechanism
In this case a muscle does not move the wing directly. They change the shape of
the exoskeleton for both upward and downward wing strokes.

a) Upward Thrust: Dorsoventral muscles pullthe dorsal exoskeleton (tergum)

downward. It produces the upward wing thrust.

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Figure 7.22: lnsect Flight (a) Muscle arrangements for the direct or synchronous flight
mechanism. (b) Muscle arrangements for the indirect or asynchronous flightmechanism.

b) Downward Thrust: Then the longitudinal muscles contract. It pulls the

exoskeleton upward. Therefore, the exoskeleton forms an arch. It produces downward
thrust of wing. The exoskeleton has resilient properties. These properties enhance the
power and velocity of strokes. The thorax is deformed during a wing beat. It stores
energy for the exoskeleton. There is a critical point midway into the down stroke. This
stored energy reaches a maximum at the critical point. The resistance to wing movement
suddenly decreases at the same time. The wing uses the stored energy in the exoskeleton
and then complete the rest of the cycle. There is lack of one to one corn sequence
between nerve impulses and wing beats. It causes asynchrony of flight mechanism. A
single nerve impulse can cause approximately 50 cycles of the wing. The frequencies of
1,000 cycles per second are recorded in some midges. The asynchrony between wing beat
and nerve impulses is dependent on flight muscles. These muscles are stretched during

195
the “click” of the thorax. The stretching of longitudinal flight muscles during the upward
beat of the wing initiates the contraction of these muscles. Similarly, stretching during the
downward beat of the wing initiates contraction of dorsoventral fig muscles. Indirect
flight muscles are called fibular flight muscles. Flies and wasps are examples of insects
with an asynchronous flight mechanism.

Simple flapping of wings is not enough for flight. The tilt of the wing must be controlled.
This tilt provides lift and horizontal propulsion. The muscles that control wing tilting are
attached to sclerotization plates at the base of the wing.

Other Forms of Locomotion

lnsects can walk, run, and jump. Insects have three or more legs for walking. These legs
stay on the ground at all times. It keeps their position stable. They keep fewer legs in
contact with the ground during walking.
 A running cockroach reaches speeds of about 5 km/hour. But it seems much faster
when one person is trying to catch it. The apparent speed is the result of their small
size and ability to quickly change directions.
 Grasshoppers are jumping insects. They have long metathoracic legs. These legs
have larger muscles. These muscles produce large propulsive forces.
 The flea jump and elastic energy is stored in the exoskeleton. Muscles of legs
distort exoskeleton. A catch mechanism holds the legs in this cocked position.
Then special muscles release the catches. It allows the stored energy to quickly
ciond the legs. This action throws the Ilea for distance 100 times of its body length.
It is equal to two football field jump of a human.

7.10.3 Nutrition and the Digestive System

Mouth parts
Insects have following mouth parts:
a. Labrum: Labrum is an upper lip like structure. It is sensory in function. It is not
derived from segmental paired appendages.
b. Mandible: Mandibles are sclerotized chewing mouthparts.
c. Maxillae: The maxillae have cutting surfaces. They bear a sensory palp.
d. Labium: The labium is a sensory lower lip.

All of these mouth parts help in food handling. Some variations are present in these moot
parts for sucking or siphoning plant or animal fluids.

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Figure 7.23: Generalized internal structure of insect

Digestive system
The digestive tract consists of foregut, a midgut and a hindgut. There are gut
enlargements for storage food. Some diverticula are present for the secretion of digestive
enzymes.

7.10.4 Gas Exchange

Gas exchange requires a large surface area for the diffusion of gases. But water is lost
from these surfaces area in terrestrial environments. Invagination of respiratory surfaces
takes place in insects. It reduces respiratory water loss. Their respiratory surface is
composed of trachea. Trachea are highly branched systems of chitin-lined tubes. The
trachea opens outside through spiracles. The spiracles have closure device. It prevents the
excessive water loss. Spiracles lead to tracheal trunks. The tracheal trunk is bra cited and
rebranched and it gives rise to smaller branches called tracheoles. Tracheoles open into
intracellular spaces. Tracheoles are especially abundant in metabolically active tissues
like flight muscles. No cells are more than 2 or 3 pm from a tracheoles.

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Mechanism of respiration
Most insects have ventilating (exchange of gases) mechanisms. This mechanism move air
into and out of the tracheal system. The flight muscles alternatively compress and expand
the larger tracheal trunks. Therefore, they ventilate the tracheae.

Figure 7.24: Tracheal system of an insect, (a) Major tracheal trunks, (b) Tracheoles and
in cells

The carbon dioxide is dissolved in the haemocoel as bicarbonate ions (HCO3). Oxygen
diffuses from the tracheae to the body tissues. It is not replaced by carbon dioxide.
Therefore, a vacuum is created. This vacuum draws more air into the spiracles. This
process is called passive suction. Periodically, the dissolved bicarbonate ions are
converted back into carbon dioxide. It escapes through the tracheal system. Other insects
contract abdominal muscles in a lump like fashion. It moves air into and out of their
tracheal systems.

7.10.5 Circulation and Temperature Regulation

Blood circulation
The blood vessels are less well developed in insects. Blood distributes nutrients,
hormones and wastes and amoeboid cells. It participates. in body defense and repair
mechanism. Blood is not important in gas transport.

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Thermoregulation
1. Ectotherms: Thermoregulation is a necessary for flying insects. All insects warm
themselves by basking in the sun or resting on warm surfaces. They use external
heat sources in temperature regulation. Therefore, the insects are as ectotherms.
2. Heterotherms: Other insects (e.g., some moths, alpine bumblebees, and beetles)
can generate heat by rapid contraction of muscles. This process is called shivering
thermogenesis.Thermogenesis can raise the temperature of thoracic muscles from
near 0 to 30 oC. But some insects rely on a limited metabolic heat sources. Thus
they have a variable body temperature. They are called heterotherms.

Insects are also able to cool themselves by moving in cool moist habitats. Honeybees beat
their wings at the entrance of the hive. It circulates the cooler air outside through the hive
and cools their hive.

7.10.6 Nervous and Sensory Functions

Nervous System
The pattern of their nervous system is similar to other arthropods.
a. Supraesophageal Ganglion: It is associated with sensory structures of the head.
b. Sub-esophageal Ganglion: Connectives join the supraesophageal ganglion to the
sub-esophageal ganglion. This ganglion connects the mouth parts and salivary
glands. It has a general excitatory influence on other body parts.
c. Segmental Ganglia: The segmental ganglia of the thorax and abdomen fuse to
various degrees in different groups.
d. Visceral Nervous System: Insects also possess a well-developed visceral nervous
system. It connects the gut, reproductive organs and heart.

7.10.7 Learning in Insect

The insects are capable of some learning and have a memory. For example the bees
recognize flower like objects by their shape and ability to absorb ultraviolet light. If a bee
is given nectar and pollen, it learns the odor of the flower. Bees that feed once at
artificially scented feeders choose that odor in 90% of feeding trials. Odor is more
constant than color and shape. Therefore, it is a very reliable cue for bees.

Sense Organs
Sense organs of insects are similar to those to other arthropods. But they are usually
specialized for functioning on land.
i. Mechanoreceptors: Mechanoreeeptors detect physical displacement of the body of
body parts.
ii. Setae: Setae are distributed over the mouth parts antennae and legs. Touch, air
movements, and vibrations of the substrate can displace setae.
iii. Stretch Receptors: Stretch receptors at the joints, on other parts of the cuticle and
on muscles monitor posture and position.
iv. Hearing: Hearing is a mechanoreceptive sense. The air pressure waves displace
certain receptors. All insects can respond to pressure waves with distributed setae.
Some others have specialized receptors for hearing:

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a) Johnston’s Organs are present in the base of the antennae of most insects like
mosquitoes. Long setae vibrate when certain frequencies of sound strike them. It
vibrates the antennae of these Insects. Vibrating setae move the antenna in its
socket. It stimulates the sensory cells. Sound waves in the frequency range of 500
to 550 cycles per second attract for mating behavior in male mosquitoes. The
waves of this frequency are produced by the wings of females.
b) Tympanal (tympanic) Organs: Tympanal organs are present in the legs of
crickets and in the abdomen of grasshoppers and some moths and in the thorax of
other insects. Tympanal organs consist of a thin cuticular membrane. It covers a
large air sac. The air sac acts as–a resonating chamber. Sensory cells are present
just under the membrane of sac. They detect pressure waves. Grasshopper
tympanal organs can detect sounds Of 1,000 to 50,000 cps. Bilateral placement of
tympanal allows insects to differentiate between the direction and origin of sound.
c. Chemoreceptors: Insects use chemoreceptor in feeding, selection of egg sites mate
location and for social organization. Chemoreceptors are abundant on the
mouthparts, antennae, legs and ovipositors. They take the form of hairs, pegs, pits
and plates. They have one or more pores. These nerves enter into internal nerves.
Chemicals diffuse through these pores and bind to the nerve endings.
d. Compound Eyes: All insects can detect light. They use light in orientation,
navigation, feeding or other functions. Compound eyes are well developed in most
adult insects. They are similar in structure and function to arthropods. Although
zoologists debate their possiblehomology (common ancestry). Their eyes have
evolved from the eyes of crustaceans, horseshoe crabs and trilobites.

Structure of Compound Eye

Compound eyes consist of an up to 28,000 receptors called ommatidia. The ommatidia
are fused into a multifaceted eye. Ommatidia have following parts:

Lens: The outer surface of each ommatidium is a lens. This, lens form one facet of the
eye.

Crystalline Cone: Crystalline cone is present below the lens. Ihe lens and the crystalline
cone are light-gathering structures.

Rentinula Cells: These are special cells of the ommatidium. Retinula has a special light-
collecting area, called the rhabdom. The rhabdom converts light energy into nerve
impulses.

Pigment Cells: Pigment cells surround the crystalline cone and rhabdom. Pigment cells
prevent light from reflecting into an adjacent ommatidium.

Functions of Compound eye

Many insects form an image. But image has no real significance for most species. The
compound eye is used for detecting movement. Compound eye can detect movement of a
point o light less than 0.1. This light successively reflects the adjacent ommatidia. For

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this reason, bees are attracted to flowers blowing in the wind. Similarly, the predatory
insects select moving prey. Compound eyes detect wavelengths of light that the human
eye cannot detect. Compound eye can detect the ultraviolet end of the spectrum.
Compound eyes of some insects can also detect polarized light. Polarized light is used for
navigation and orientation.

Figure 7.25: (a) Compound eye: (b) Structure of an Ommatidium (c) Cross Section
through the Rhabdom

g. Ocelli
Ocelli consist of 500 to 1, 000 receptor cells. These cells are present beneath a
single cuticular lens. Ocelli are sensitive to changes in light intensity. Therefore,
the regulation of daily rhythms.

7.10.8 Excretion
Malpighian tubules and the rectum are primary excretory structures in insects.
Malpighian tubules end blindly in Hindgut the haemocoel. They open in to the gut at the
junction of mid gut and hindgut. Microvilli cover inner surface of their cells. Various
ions are actively transported into the tubules. Water moves in by diffusion. Uric acid is
secreted into the tubules. It is then transferred into the gut. Rectum reabsorbs water,
certain ions, and other materials. Finally uric acid is eliminated through anus.

7.10.9 Chemical Regulation in Insects

Hormones
The endocrine system controls many physiological functions of insects. These functions
are cuticular sclerotization, osmoregulation, egg maturation, cellular metabolism, gut
peristalsis and heart rate.

Control of Ecdysis by Hormones: Ecdysis is under neuroendocrine control. Two

endocrine glands are present in the subesophageal ganglion. Theseglands are the corpora

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allata and the prothoracic glands. These glands control different activities. Following
processes are involved in the secretion and regulation of ecdysone:
 Neurosecretory cells of the subesophageal ganglion synthesize ecdysiotropin.
 Ecdysiotropin hormone is then transferred to corpora cardiaca.
 The corpora cardiaca then releases thoracotropic hormone.
 Thoracotropic hormone stimulates the prothoracic gland to secrete ecdysone.
 Ecdysone initiates the reabsorption of the inner portions of the procuticle. It
initiates the formation of the new exoskeleton.

Other hormones are also involved in ecdysis. These hormones control the recycling of
materials absorbed from the procuticle. Changes in metabolic rates and pigment
deposition.

Role of hormones in metamorphosis: The corpora allata releases small amounts of

juvenile hormone in immature stages. The amount of juvenile hormone determines the
nature of the next molt. Large concentrations of juvenile hormone causes molt to second
immature stage. Intermediate concentrations causes molt to third immature stage. Low
concentrations causes molt to the adult stage. Thus there are decreases in the level of
circaulating juvenile hormone. Low level of this hormone also causes the degeneration of
the prothoracic gland. Therefore, molt is stopped in most of adult insects. But the level of
juvenile hormone increases again after the final molt. Now it promotes the development
of accessory sexual organs, yolk synthesis and the egg maturation.

Pheromones
The chemicals released by an animal that change the behavior or physiology of another
member of the same species is called pheromones. Many different insects use phermones.
Pheromones are much specific. An isomer of a pheromone is ineffective in initiating a
response. Wind or water carries pheromones several kilometers away. Phermones fall on a
chemoreceptor. A few pheromone molecules of another individual can produce enough
response.

Function of insect pheromones

 Sex pheromones: Sex pheromones excite or attract members of the opposite sex.
They accelerate or retard sexual maturation. Example. Female moths produce and
release pheromones that attract males.
 Caste-regulating pheromones: These are used by social insects to control the
development of individuals in a colony. Example: The female bee feed the larva
with royal jelly. The amount of royal jelly determines whether the larva will
become a worker or a queen.
 Aggregation pheromone: These are produced to attract individuals to feeding or
mating sites. Example: Certain bark beetles aggregate on pine trees during an
attack on a tree.
 Alarm pheromones: These are used to warn other individuals of danger. It causes
orientation toward the pheromone source and stimulates an insect to attack or
flight. Example: A sting from one bee alarms other bees in the area.

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 Trailing pheromones: It is released by foraging (21, 2.45″ a) insects. These
pheromones help other members of the colony to identify the location and quantity of
food. Example: Ants often moves on a pheromone path to and from a food source.

7.10.10 Reproduction and Development

Insects have high reproductive potential. It is one of the successes of insect. But
reproduction in terrestrial environments is much difficult. Temperature, moisture and
food supplies vary with the season. The gametes dry quickly. Therefore internal
fertilization requires active copulatory structures. A mechanism is also required to bring
males and females together at appropriate times.
 There are complex interactions between internal and external environmental
factors. These interactions regulate the sexual maturity.
 Internal regulation takes place by interactions between endocrine glands and
reproductive organs.
 External regulating factors are quantity and quality of food. For example, the eggs
of mosquitoes become mature after the female takes a meal of blood. The
production of number of eggs is proportional to the quantity of blood ingested.

The photoperiod indicates seasonal changes. Therefore, it controls the timing of

reproductive activities in many insects. Population density, temperature and humidity
also influence reproductive activities.

Fertilization: A few insects, including silverfish and springtails have indirect

fertilization. The male deposits a spermatophore. The female picks up later. Most
insects have complex mating behaviours. They can locate and recognize a potential mate
for copulation. Pheromones may be involved in the mating behavior. The visual signals
and auditory signals also play role in mating. These stimuli bring the male and female
near each other. Then the tactile stimuli from the antennae and other appendages adjust
the position of the insects for mating.

The insects have abdominal copulatory appendages. The male usually transfer sperm into
sperm receptacle of female. Sperm receptacle is an out pocket of the female reproductive
tract. Eggs are fertilized. The female laid the egg near the larval food supply. Some
females have an ovipositor to deposit eggs in or on some substrate.

7.10.11 Insect Development and Metamorphosis

Different insects have developed different mechanisms of development. Some insects are
produced in immature stages called larval instars. It is formed at the time of growth. Instar
stores food for the transition to adulthood. The adult stage is associated with reproduction and
dispersal. In these orders, insects spend a greater part of their lives in juvenial stages. The
developmental patterns of insects are classified into three or four categories.

Ametabolous Metamorphosis (a, without + metabolos, change):

In this case, primary differences between adults and larvae are body size and sexual
maturity. Both adults and larvae are wingless. The number of molts in the ametabolous

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development of a species varies. Molting continues after sexual maturity. Silverfish have
ametabolous metamorphosis.
i. Paurometabolous Metamorphosis (Or. pauros. small)
In this case, larvae undergo number of molts between egg and adult stages. The
number of molt is species-specific. The immature larva gradually changes into the
adult form. The external wings are developed. The adult body size is attained. The
genitalia develop during his time. Immature larva is called nymphs. Grasshoppers
and chinch bugs show paurco eiabolous metamorphosis.
ii. Hemimetabolous Metamorphosis (Or. hemi, halt)
Some insects have a series of gradual changes in their development. Therefore.
some zoologists use additional classification for insects, Their immature form is
much different from t e adult form due to the presence of gills. This kind of
development is called hemimetabolous metamorphosis. Their immatures are
aquatic. This immature is called naiads (naiad, water nymph).
iii. Holometabolous Metamorphosis (holos, whole)
Following stages are formed during homometabolous metamorphosis:
a) Larva: Their immature is different from the adult in body form. It has
different behavior and habitats. Therefore, the immature are called larvae.
The number of larval instars is species specific.
b) Pupa: The last larval molt forms the pupa. The pupa appears inactive. But it
is actually a time of radical cellular change. The characteristics of the adult
insect develop during this stage. A protective case encloses the pupal stage.
The last larval develops a cocoon. Cocoon is partially or entirely composed
of silk. The sails and puperium are the last larval exoskeletons. They are
retained through the pupal stage. Other insects like mosquitoes have pupae
unenclosed by a larval exoskeleton. Their pupa may be active. The final molt
occurs within the cocoon, chrysalis or puperium.
c) Adult: The adult open the cocoon with its mandibles and come out. This
final process is called emergence or eclosion.

Figure 7.26: Holometabolous development of Hosefly

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7.11 Insect Behaviour
Insects have many complex behavior patterns. Most of these are innate (inherited). For
example, a newly queen develops in a honeybee hive. It search out and try to destroy
other queen larvae and pupae in the hive. She was not taught to kill the previous queen.
Therefore, her behavior is innate. Similarly, she has no experience to differentiate
between the cell of queen and cells containing worker larvae and pupae. Some insects are
capable of learning and remembering. These abilities play important roles in insect
behavior.

7.12 Social Insects

Social behavior is present in many insects. It is particularly present in those insects that
live in colonies. Different members of the colony are specialized structurally and
behaviorally for performing different tasks. Social behavior is highly developed in the
bees, wasps and ants and in termites. Each kind of individual in an insect colony is called
a caste. Often three or four castes are present in a colony.
 Queens: Reproductive females are called queens.
 Workers: Workers may be sterile males and females (termites). Or they may be
sterile females. Workers support, protect and maintain the colony. Their
reproductive o gans are degenerated.
 Kings or Drones: Reproductive males are called kings or drones. They transfer
sperm to the queen.
 Soldiers: Soldiers are sterile. They possess large mandibles to defend the colony.

7.12.1 Social behavior in honeybees

Honey bees have three of these castes in their colonies.

Queen: A single queen lays all the eggs.

Workers: Workers are female. They produce wax and construct the comb from this wax.
They also gather nectar and pollen and led the queen and drones, They also gather nectar
and pollen and feed the queen and drones. They care for the larvae. They also guard and
clean the hive. These tasks are divided among workers according to age. Young workers
work in the hive. The older workers bring nectar and pollen. The workers live for about
one month.

Drones: Drones develop from unfertilized eggs. They do not work. They are fed by the
workers. They leave the hive and mate with queen. Queen releases a pheromone. This
pheromone controls the caste system. Workers lick and groom the queen and other
workers. Thus they pick up caste- regulating pheromones from queen and pass it to other
workers. This pheromone inhibits the workers from rearing new queens. The amount of
caste-regulating pheromone in the hive decreases with the death or aging of queen. Now
worker again feed the royal jelly to several female larvae. This food contains chemicals
that promote the development of queen characteristics. The larvae that receive royal jelly

205
develop into queens. The new queens start killing each her. Only one queen remains. This
queen goes on a mating flight and returns to the colony. She lives in the colony for
several years.

7.12.2 Evolution of social behavior

Many individuals leave no offspring in the evolution of social behavior. Thus these
individuals sacrifice for the survival of the colony. This behaviour has puzzled
evolutionists for many years. They explain it with the concepts of kin selection and
altruism.

7.13 Insects and Humans

Only about 0.5% of insect species adversely affect human health and welfare.

7.13.1 Beneficial insects

 Many insects provided valuable services and commercially valuable products.
These products are wax, honey, and silk.
 Insects cause pollination of 65% of all plant species. Insects and flowering plants
have coevolutionary relationships. It directly benefits humans. The annual value of
insect-pollinated crops is 19 billion dollar per year in the United States.
 Insects play role in biological control. For example: vedalia beetles are used for
control of cotton-cushion scale. The scale insect, lcerya purchasi, was introduced
into California in the 1860s. It destroyed the citrus industry in California in twenty
year. The vedalia beetle was brought to the United States in 1888 and 1889. They
cultured it on citrus tree. The scale insect was controlled in just a few years. Thus
the citrus industry recovered.
 Some insects live in soil. These insects play important roles in aeration, drainage,
and turnover of soil. They promote decay processes. Other insects play important
roles in food web.
 Insects are used in teaching and research. They are used in the study of genetics.
population ecology, and physiology. Insects also give pleasure to those who collect
them and enjoy their beauty.

7.13.2 Harmful insects

Some insects are parasites and vectors of disease.
 Parasitic Insects: Parasitic insects are:
a) Order Anoplura: Head body and pubic lice
b) Order Hemiptera, bedbugs
c) Order Siphonaptera, fleas.
 Vector of the Disease: Other insects transmit disease-causing microorganisms,
nematodes and flatworms. Insect transmit diseases like malaria, yellow fever,
bubonic plague encephalitis, leishmaniasis, and typhus.
 Pest of Domestic Animals and Plant: Some insects affect the health of domestic
animals and their quality. Insects feed on crops and transmit plant diseases. These
diseases are potato virus and asters yellow.

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Self Assessment Questions
Q.1 i. Which of the following is not a type of arthropod?
A. Insects B. Crustaceans C. Centipedes & Milipedes
D. Echinoderms E. Arachnids

ii. An insect is divided into how many a part?

A. One B. Two C. Three D. Four

iii. A scorpion is a member of the ___________ order.

A. Arachnid B. Insect C. Millipede & Centipede
D. Crustacean E. None of the above

iv. What are the characteristics of an arthropod?

A. Exoskeleton B. 6 legs C. Jointed legs
D. A and C E. A, C and a segmented body

v. A millipede is a(n) __________.

A. Carnivore B. Herbivore C. Omnivore
D. Crustacea

vi. These have a hard exoskeleton, two pairs of antennae, and a mouth for
crunching and grinding.
A. Centipedes and millipedes B. Insects
C. Crustaceans D. Arachnids E. Monkeys

vii. Give the sequence of complete metamorphosis in correct order.

A. Egg, adult, pupa, larva B. Pupa, egg, larva, adult
C. Egg, pupa, larva, adult D. Egg, larva pupa, adult
E. larva, egg, pupa, adult

Q.2 Answer ad directed:

i. Differentiate polyphyletic and monophyletic origin of Arthropods.
ii. Explain adaptations in insects.
iii. Write characterisrtics of Sub-phylum myriopoda.
iv. Write characteristics of class chilopoda.
v. Write salient features of class diplopoda.
vi. Identify common characteristics among class pauropoda and symphala.
viii. Classify subphylum crustacean and give only one example for each.
ix. Write a note on composition of Exoskeleton.
x. Differentiate between Metamerism and Tagmatization.
xi. Describe mechanism of flight in insects.
xii. Write mouth parts of insects.
xiii. Temperature regulation in bird is very important phenomenon. Explain
circulation helps in temperature regulations in insects.

207
xiv. Insects have memory and learning ability. Justify this statement with the help
of evidences.
xv. What do you know about Pheromones?
xvi. Write functions of insect pheromones.
xvii. Define metamorphosis. Describe different dimensions of metamorphosis in
insects.
xviii. Write an essay on social behavior of insects.
xix. In what ways insects are harmful.
xx. Elaborate importance of insects in our daily life.

208
References
 Atkins, M. D. 1978. Insects in perspective, vii + 513 pp. Macmillan, New York,
and Collier Macmillan, London.
 Borror, D. J., Triplehorn, C. A. and Johnson, N. F. 1989. An introduction to the
study of insects. Sixth edition, xiv + 875 pp. Saunders College Publishing,
Philadelphia.
 Brusca, R. C. and Brusca, G. J. 1990. Invertebrates, xviii + 922 pp. Sinauer
Associates, Sunderland, Massachusetts.
 Bücherl, W. and Buckley, E. E. (eds) 1971. Venomous animals and their venoms:
vol. 3, Venomous invertebrates, xxii + 537 pp. Academic Press, New York and
London.
 Chapman, R. F. 1982. The insects: structure and function. Third edition, xiv + 919
pp. Hodder & Stoughton, London.
 Clarke, K. U. 1988. Impact arthropodisation. Antenna 12: 49–54.
 Daly, H. V., Doyen, J. T. and Ehrlich, P. 1978. Introduction to insect biology and
diversity. x + 564 pp. McGraw-Hill, New York.
 Davies, R. G. 1988. Outlines of entomology. Seventh edition, vii + 408 pp.
Chapman & Hall, London and New York.
 Elzinga, R. J. 1981. Fundamentals of entomology. Second edition, x + 422 pp.
Prentice-Hall, Englewood Cliffs, New Jersey.
 Gillott, C. 1980. Entomology, xviii + 729 pp. Plenum Press, New York and
London.
 Goldsworthy, G. J. and Wheeler, C. H. (eds) 1989. Insect flight. 371 pp. CRC
Press, Boca Raton, Florida.
 Gupta, A. P. (ed.) 1979. Arthropod phylogeny. xx + 762 pp. Van Nostrand
Reinhold, New York.
 Hennig, W. 1981. Insect phylogeny. xxii + 514 pp. John Wiley, Chichester.
 Herreid, C. F. and Fourtner, C. R. (eds) 1981. Locomotion and energetics of
arthropods. viii + 546 pp. Plenum Press, New York and London.
 Hinton, H. E. 1981. Biology of insect eggs: vol. 1, ix + pp. 1–473. Pergamon Press,
Oxford.
 Hinton, H. E. 1981. Biology of insect eggs: vol. 2, xviii + pp. 475–778. Pergamon
Press, Oxford.

209
 Hinton, H. E. 1981. Biology of insect eggs: vol. 3, xvii + pp. 779–1125. Pergamon
Press, Oxford.
 Kerkut, G. A. and Gilbert, L. I. (eds) 1985. Comprehensive insect physiology,
biochemistry and pharmacology: vol. 1, Embryogenesis and reproduction, xvi +
487 pp. Pergamon Press, Oxford.
 Kerkut, G. A. and Gilbert, L. I. (eds) 1985. Comprehensive insect physiology,
 Kerkut, G. A. and Gilbert, L. I. (eds) 1985. Comprehensive insect physiology,
biochemistry and pharmacology: vol. 9, Behaviour, xvi + 735 pp. Pergamon Press,
Oxford.
 Kerkut, G. A. and Gilbert, L. I. (eds) 1985. Comprehensive insect physiology,
biochemistry and pharmacology: vol. 10, Biochemistry, xviii + 715 pp. Pergamon
Press, Oxford.
 Kerkut, G. A. and Gilbert, L. I. (eds) 1985. Comprehensive insect physiology,
biochemistry and pharmacology: vol. 11, Pharmacology, xiv + 740 pp. Pergamon
Press, Oxford.
 Kerkut, G. A. and Gilbert, L. I. (eds) 1985. Comprehensive insect physiology,
biochemistry and pharmacology: vol. 12, Insect control, xiv + 849 pp. Pergamon
Press, Oxford.
 Kerkut, G. A. and Gilbert, L. I. (eds) 1985. Comprehensive insect physiology,
biochemistry and pharmacology: vol. 13, Cumulative indexes, xxiv + 314 pp.
Pergamon Press, Oxford.
 King, R. C. and Akai, H. (eds) 1982. Insect ultrastructure: vol. 1, xxii + 485 pp.
Plenum Press, New York and London.
 King, R. C. and Akai, H. (eds) 1982. Insect ultrastructure: vol. 2, xxv + 624 pp.
Plenum Press, New York and London.
 Manton, S. M. 1973. Arthropod phylogeny — a modern synthesis. Journal of
Zoology 171: 111–130.
 Manton, S. M. 1977. The Arthropoda: habits, functional morphology, and
evolution, xx + 527 pp. Clarendon Press, Oxford.
 Matsuda, R. 1965. Morphology and evolution of the insect head. Memoirs of the
American Entomological Institute 4: 1–334.
 Matsuda, R. 1970. Morphology and evolution of the insect thorax. Memoirs of the
Entomological Society of Canada 76: 1–431.
 Matsuda, R. 1976. Morphology and evolution of the insect abdomen with special
reference to developmental patterns and their bearings upon systematics. viii + 534

210
 pp. Pergamon Press, Oxford. [Volume 56 of International Series in Pure and
Applied Biology (Zoology Division), ed. G. A. Kerkut.]
 Meglitsch, P. A. and Schram, F. R. 1991. Invertebrate zoology. Third edition, vi +
623 pp. Oxford University Press, New York and Oxford.
 Richards, O. W. and Davies, R. G. 1977. Imms’ general textbook of entomology.
Tenth edition. vol. 1, Structure, physiology and development, viii + pp. 1–418.
Chapman & Hall, London, and John Wiley, New York.
 Richards, O. W. and Davies, R. G. 1977. Imms’ general textbook of entomology.
Tenth edition vol. 2, Classification and biology, viii + pp. 421–1354. Chapman &
Hall, London, and John Wiley, New York.
 Sims, R. W. (ed.) 1980. Animal identification: a reference guide: vol. 2, Land and
freshwater animals (not insects), x + 120 pp. British Museum (Natural History),
London, and John Wiley, Chichester.
 Stehr, F. W. (ed.) 1987. Immature insects: vol. 1, xiv + 754 pp. Kendall/Hunt,
Dubuque, Iowa. [Coverage not stated on title page but work covers 27 orders,
including orthopteroid insects, lice, Trichoptera, Lepidoptera and Hymenoptera]
 Stehr, F. W. (ed.) 1991. Immature insects: vol. 2, xvi + 975 pp. Kendall/Hunt,
Dubuque, Iowa. [Coverage not stated on title page but work covers ten orders,
including hemipteroid and neuropteroid insects, Coleoptera, Siphonaptera and
Diptera
 Taylor, F. and Karban, R. (eds) 1986. The evolution of insect life cycles, x + 287
pp. Springer-Verlag, New York.
 Wigglesworth, V. B. 1964. The life of insects, xii + 360 pp. Weidenfeld &
Nicolson, London. [Easy and instructive reading at a semipopular level.
 Willmer, P. 1990. Invertebrate relationships: patterns in animal evolution, ix + 400
pp. Cambridge University Press, Cambridge.

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212
UNIT-8

ECHINODERMS

Written by: Dr. Tauseef Anwar

Reviewed by: Arshad Mehmood Qamar

213
 CONTENTS
Introduction ....................................................................................................... 215

Objectives ......................................................................................................... 215

8.1 Evolutionary Perspective: Relation to other Animals ........................... 216

8.2 Echinoderms Characteristics ................................................................. 217

8.3 Classification up to Class ...................................................................... 219

8.4 Maintenance functions and Regeneration ............................................. 223

8.5 Production and Development in Asteroidean ....................................... 229

8.6 Some Lesser-Known Invertebrates: The Lophophorates, Entoprocts,

Cycliophores and Chaetognaths ............................................................ 233

214
Introduction
All members of this phylum are called Echinoderms and are exclusively mrine. Starfish,
sea Urchins and Sea Cucumbers are some common examples of Echinoderms. This group
contains about 7000 species. This phylum contains the second largest members after
chordates on this earth. This phylim is considered every near to the phylum chordate. In
this Unit Evolutionar perspectives, characteristics of echinoderms of members upto class
level, maintenance functions, regeneration, maintenance functions and other phylogenetic
considerations are discussed in a very comprehensive way. It is tried to accumulate
maximum information regarding phylum Echinodermata.

Objectives
After completion of this Unit, you will be able to:
 identify organisms which belong to phylum Echnodermata on the basis of habitat
and other characteristics.
 describe evolution of Echinoderms.
 classify phylum upto class –level.
 explain maintenance functions like water vascular system and Canal system among
members of phylum Echinoderms.
 elaborate further phylogenetic considerations.
 define different terms use throughout the text.

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8.1 Evolutionary Perspective: Relation to Other Animals
Most zoologists believe that echinoderms evolved from bilaterally symmetrical ancestors.
Radial symmetry was evolved during the transition from active to more sedentary
lifestyles. The oldest echinoderm fossils are about 600 million years old. It does not give
direct evidence of this transition.

8.1.1 Relationship of Ancient Echinoderms and Crinoids

Ancient fossils give some evidence about the origin of the water-vascular system and the
calcareous endoskeleton. The crinoids are closely resembled the oldest fossils.

1. Crinoids use water vascular system for suspension feeding and filter feeding. They
do not use water vascular system for locomotion. Thus filter feeding was the
original function of the water vascular system.
2. The early echinoderms have a mouth-up position like crinoids. Therefore, they
attached aborally. They used arms and tube feet to capture food and move it to the
mouth
3. The calcium carbonate endoskeleton was evolved to support filtering arms. It also
protects these sessile animals.

8.1.2 Evolution of Modern Echinoderms

Many modern echinoderms are motile. They have secondarily derived following
characteristics:

1. Free -living lifestyle.

2. The mouth-down orientation. The mouth-down position has advantage for
predatory and scavenging lifestyles.
3. Changes occured in the water-vascular system. There was evolution of ampullae,
suction disks and feeding tentacles. These are adaptations for locomotion and
feeding in more mobile lifestyle.Some echinoderms like irregular echinoids and the
holothuroids have bilateral symmetry. This bilateral symmetry is derived from
pentaradial body. This observation also supports the idea that the free-living
lifestyle is a secondary adaptation in echinoderms.

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Figure 8.1: Evolutionary Perspective: Relation to other animals

Conclusion
The evolutionary relationships among the echinoderms are not clear. Numerous fossils of
echinoderms of the Cambrian period are present. But these fossils cannot develop definite
evolutionary relationships among living and extinct echinoderms. Most taxonomists
agree that the echinoids and holothuroids are closely related. But it is not clear whether
the ophiuroids are more closely related to the echinoid/holothuroid lineage or they are
related to asteroid lineage. The position of Concentricycloidea in Echinoderm is also not
clear.

8.2 Echinoderms Characteristics

Some of the general characters of Phylum Echinodermata are listed below:
1. Habitat: All existing echinoderms are marine. They generally live at sea bottom
borne are pelagic (free swimming in open water) and a few are sessile (attached to
the substratum).
2. Body Form: It varies considerably. The body is star-shaped, spherical or cylindri-
cal. It is un-segmented. The body lacks head.

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3. Spines and Pedicellariae:Many echinoderms bear spines and pincer-like
pedicellariae. The spines are protective in function. The pedicellariae keep the body
surface clear of debris and minute organisms.
4. Symmetry: The symmetry is bilateral in larvae but pentamerous radial in adults i.
e., body parts are arranged in fives or multiples of five.
5. Body Wall: Epidermis is single layered and ciliated. In many echinoderms there is
endoskeleton of calcareous plates in the dermis which are mesodermal in origin.
6. Body Cavity: There is a true enterocoelic coelom.
7. Ambulacral System (= Water Vascular System): Presence of ambulacral system
is the characteristic feature of phylum echinodermata. A perforated plate called
madreporite is present in this system. The pores of the madreporite allow water into
the system Tube feet of this system help in locomotion, capture of food and
respiration. Water vascular system is of coelomic origin.
8. Digestive Tract: It is usually complete. Brittle stars have incomplete digestive
tract.
9. Haemal and Perihaemal Systems: Instead of blood vascular system, there are
present haemal and perihaemal systems which are of coelomeongin. Thus the so
called circulatory system is open type and includes haemal and perihaemal systems.
The so blood is often without a respiratory pigment. There is no heart.
10. Respiratory Organs: Gaseous exchange occurs by dermal branchae or papulae in star
fishes peristominal gills in sea urchins, genital bursae in brittle stars, and cloac
respiratory ‘trees in holothnrians. Exchange of gases also takes place through tnbe feet.
11. Excretory Organs: Specialized excretory organs are absent. Nitrogenous wastes
are diffused out via gills. Ammonia is chief excretory matter.
12. Nervous System: It consists of a nerve ring and radial nerve cords. Brain as such is
absent.
13. Sense Organs: They are poorly developed.
14. Sexes and Fertilization: Except a few individuals, the sexes are separate. There is
no sexual dimorphism. Fertilization is usually external.
15. Asexual Reproduction: Some forms reproduce asexually by self-division.
16. Autotomy and Regeneration: Phenomena of autotomy and regeneration are often
well marked in echinoderms.
17. Development: The development is indirect and includes a ciliated, bilaterally sym-
metrical larva that undergoes metamorphosis to change into the radially
symmetrical adult. Different larval forms are found which are mentioned in the
classes of Echinodermata.

8.2.1 Unique Features of Echinoderms

i) Presence of spines and pedicellariae.
ii) Ambulacral system (water vascular system),
iii) Haemal system,
iv) Mesodermal endoskeleton of calcareous plates,
v) Bilateral symmetry in the larva and radial symmetry in the adult.

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Degenerate Characters:
i) Lack of head,
ii) Simple sense organs,
iii) Incomplete digestive tract in some forms,
iv) Reduced circulatory system,
v) Absence of excretory system.

Resemblance with Chordates:

i) Radial and indeterminate cleavage,
ii) Gastrulation by invagination,
iii) Mouth derived as an ectodermal invagination,
iv) Adult anus derived from embryonic blastopore,
v) Mesodermal endoskeleton.
vi) Enterocoelous coelom.
vii) Both are deuterostomes.

From these resemblances, it is clearly proved that the Echinoderms are nearer to the
Chordates than any other group. It also indicates that the chordates have been evolved
from Echinoderm-like ancestors.

8.3 Classification Up to Class

Phylum Echinodermata is divided into the following five classes:
1. Asteroidea
2. Ophiuroidea
3. Echinoidea
4. Holothuroidea
5. Crinoidea

8.3.1 Class-1: Asteroidea

i. They are commonly known as starfish or sea stars.
ii. This class contains about 15,00 species which are found within 6000 m depth
ranges.
iii. They have a radial symmetrical, flattened, star-like body with five arms.
iv. The mouth is pentagonal which is located centrally on the ventral side (oral
surface) while the anus is on the dorsal side (aboral surface).
v. They have flexible and calcareous endoskeleton and the surface of the body may be
smooth, spiny or granular and covered with calcareous overlapping plates. They are
carnivorous and mostly feed on benthic invertebrates.
vi. They have a calcareous opening or madreporite on the dorsal side to filter the water
into the water vascular system.

L The radial canal contains tube feet with suckers which aid in locomotion and
capturing food.

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Figure 8.2: Asteriasrubens (Common Starfish)

vii. They bear wrench or claw-shaped structure which is known as pedicellaria for
cleaning and capturing tiny prey.
viii. LRespiration takes place through papulae.
ix. Indirect development occurs with different larval forms (Bipinnaria or
Brachiolaria).
Examples: Asteriasrubens (Common Starfish), Astropecten articulates (Royal starfish)

8.3.2 Class-2: Ophiuroidea

i. They are commonly known as Brittle stars or ophiuroids or serpent stars.
ii. They have pentamerous disc-shaped flat body with five slender, long, and whip-
like arms that reach up to 60 cm in length.
iii. The water vascular system contains one madreporite.
iv. The tube feet do not contain suckers and ampullae.
v. Respiration and excretion take place through cilia-lined sacs or bursae.

Figure 8.3: Ophiothrixfragilis (Brittle stars)

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vi. The body does not have an anus, ambulacral groove, and intestine. In this case,
egestion and ingestion occur through the mouth.
vii. They are scavengers or detrivores but sometimes they take small crustaceans or
worms.
viii. Their lifespan ranges up to five years.
ix. In most species, sexes are separate but few are hermaphroditic or protandric.
x. Indirect development occurs with the distinct larval stage (Pluteus larva).

Example: Ophiothrixfragilis (Brittle stars), Ophiodermalongicauda (smooth brittle star)

8.3.3 Class-3: Echinoidea

i. They are also known as sea urchins and sand dollars which inhabit all oceans
within 5000 meters depth ranges.
ii. This class contains about 950 species. Among them, the most notable species are
sea urchins, sand dollars, heart urchins, etc.
iii. They have a round (sea urchin), oval or heart-shaped (heart urchin), or flattened
(sand dollar) body.
iv. The mouth is located on the ventral side while the anus is on the dorsal surface of
the body.
v. The body of the echinoids has a rigid skeleton or test which is made up of calcium
carbonate or ‘stereom’ which makes the interlocking plates of the skeleton.
vi. The body is covered with spines and the body color may be purple, brown or tan.

Figure 8.4: Echinus esculentus (Common sea urchin)

vii. They have water-filled tube feet with suckers which aid in locomotion.
viii. The echinoids are very popular among the shell collectors. In some areas, sea
urchins, eggs are eaten by many people.
ix. They have chewing apparatus which is known as Aristotle’s Lantern.
x. Indirect development occurs with the distinct larval stage (Echinopliteus larva).

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Examples: Echinus esculentus (Common sea urchin), Echinarachniusparma (the
Common Sand Dollar)

8.3.4 Class-4: Holothuroidea

i. Holothurians are also known as sea cucumbers which are found in all oceans with
about 1700 described species.
ii. They inhabit in shallow water areas to deep ocean floors.
iii. They have leathery skin and an elongated body with black, brown, or olive green in
colors.
iv. The length of the body ranges from three cm to one meter with a diameter of 24
cm.
v. The mouth and anus are situated on the opposite side of the body.
vi. They have five rows of sucking type tube feet which run from the mouth to the
anus along the cylindrical body.
vii. The body does not contain arms, spines, and pedicellariae.
viii. 10-30 tentacles are present around the mouth.
ix. They have ring canal around the gut with 1-50 pollian vesicles for hydraulic
function.

Figure 8.5: Holothurialeucospilota (the black sea cucumber)

x. Five radial canal arises from the ring canal with rows of ampullae.
xi. A short stone canal is present which arises from the ring canal that follows the
madreporite.
xii. Respiration occurs through the cloacal respiratory trees which branch out near the
rectum for gas exchange.
xiii. Holothurians are dioecious and their fertilization is external.
xiv. Development is indirect with different types of planktotrophic or lecithotrophic
larval stages (Auricularia, doliolaria, pentactula larva).

Examples: Cucumariaminiata (Orange sea cucumber), Holothurialeucospilota (the black

sea cucumber)

8.3.5 Class-5: Crinoidea

i. They are commonly known as sea lilies and feather stars.
ii. This class contains about 700 living species which inhabit deep waters.
iii. They have a cup-shaped body with five or more flexible and active arms.

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iv. At the edge of the arms, feathery projections are present which are known as
pinnules that bear the reproductive organs with numerous tube feet without
suckers.

Figure 8.6: Antedonmediterranea (Mediterranean feather star)

v. On the dorsal (oral) surface mouth and anus are present.

vi. They are passive suspension feeders which feed on plankton and small particles of
debris.
vii. They do not contain madreporite but spines and pedicellariae are present.
viii. They bear water vascular system which controls hydraulic pressure in the tube feet.
ix. They are dioecious and development is indirect with the distinct larval stage
(Vitellaria larva).

Examples: Antedonmediterranea (Mediterranean feather star), Rhizocrinuslofotensis

8.4 Maintenance Functions and Regeneration

Echinoderms have a distinct endoskeleton, an unusual organ system called the water
vascular system, and a number of other less unique features.

8.4.1 Endoskeleton
The echinoderm endoskeleton consists of a meshwork of plates and spines connected by
mesodermal tissue. The plates are called ossicles. The organization of the ossicles that
compose the endoskeleton of a sea star is shown in Figure 8.7. The ossicles are made up
of microscopic networks of calcium carbonate crystals that form a unique structure
referred to as the stereom. The ossicles may be tightly packed together, as they are in sea
urchins, or they may be more loosely connected, as they are in sea stars. The same is true
for the spines. Sea urchin spines are often quite loosely attached (despite the tight

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packing of the ossicles), and it is not uncommon to find a sea urchin alive and well but
lacking most of its spines.

Figure 8.7: An X-ray of a sea star, showing the endoskeleton. Notice the many tiny
plates (ossicles) that are arranged in a clear pattern throughout the body of the organism.

There are several specialized aspects of the echinoderm endoskeleton that vary between
species. One example is the presence of pedicellariae on species of sea stars and sea
urchins. Pedicellariae, shown in Figure 8.8, are spines modified as pincer-like structures
that can be used to thwart predators. Another example of a modified endoskeletal feature
is a feeding organ called Aristotle’s lantern (discussed in the Echinoderms:
Classification (Advanced) concept), which are found in sea urchin species.

Figure 8.8: The pedicellariae found on sea stars and sea urchins. Pedicellariae are
modified spines that have a pincer-like structure at one end, as shown in this drawing.

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They may be used for self-defense against predators. The pedicellariae is shown both
closed (left) and open (right).

8.4.2 Water Vascular System

The water vascular system of echinoderms is essentially a system of fluid-filled canals
that extend along each of the body regions and have many external projections called
tube feet. There are several functions of the system, one of which is to use water pressure
to mediate movement and assist in feeding. It also carries out respiratory, excretory, and
some circulatory functions within

Figure 8.9: The components of the Water Vascular System of Echinoderms. Water enters
the system through the madreporite, flows through the stone canal, and enters the ring
canal. A number of bulbous structures, called polian vesicles, that branch off of the ring
canal serve as reservoirs to maintain extra water stores. From the ring canal, water enters
the radial canals that extend into each of the arms, or rays. The lateral canals branch from
the radial canal and bring the water into the ampullae and podia of the tube feet.

The animal. The individual components of the water vascular system are the following:
 Madreporite.
 Stone canal.
 Circular ring canal.
 Radial canals.

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 Lateral canals.
 Tube feet.

What does each of these components do, and how do they contribute to the functioning of
the water vascular system? The madreporite is a sieve-like, calcified plate that connects
the system to the aquatic environment. Water enters through the madreporite and flows
through a tube, called the stone canal that connects to the circular ring canal surrounding
the mouth. From there, water can flow into five radial canals that branch off the circular
ring. The radial canals each extend into a different ray, or arm, of the organism along a
groove called the ambulacral groove. Numerous lateral canals, leading to rows of tube
feet on either side of the ambulacral groove, branch from each side of the radial canals.

Figure 8.10: A diagram of a cross-sectioned sea star arm/ray and central disk.

The tube feet extend between the endoskeletal plates to reach outside of the organism.
They are basically thin-walled cylinders with muscular bulb-shaped structures called
ampullae on the internal end and suckered structures called podia on the external ends.
The tubular canals that make up the water vascular system are lined with cilia. Cilia are
small hair-like, cellular projections that beat back and forth repeatedly to help maintain
water flow through the canals.

So how do tube feet work to move the animal? When the muscles of the ampullae
contract, water is forced into the suckered podia, which then extend outward. As the
podia stretch, they can use their suckers to attach to a location farther away from their
previous point of attachment. This results in a slow but powerful form of movement.
Extended tube feet can also be used to generate small waves of water current toward the
mouth region to assist in food collection.

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Figure 8.11: The tube feet of a sea star. If you look closely, you can see the suckered podia at
the end of the tube feet protruding from the bottom (or oral) surface of the animal.

In addition to the mechanical work it conducts, the water vascular system also provides
respiratory and excretory functions. Most echinoderms do not have respiratory or
excretory organs, so the thin walls of the tube feet serve this purpose by allowing oxygen
to diffuse in and waste to diffuse out.

When discussing the bilaterally symmetrical phyla, we have been able to use terms such
as anterior, posterior, dorsal, and ventral to refer to the front, back, top, and bottom of the
animal. With radially symmetrical animals, such as echinoderms, it is more difficult to
specify which region of the animal you are talking about. Generally, the surface of the
animal with the mouth opening is called the oral surface, and the one without the mouth
opening is called the aboral surface. The madreporite opens into the aboral surface.
Because the madreporite is one of the few non-repeating structures of echinoderms and is
not directly in the center of the animal, it is used as an orientation point to distinguish
between the various radial projections (arms/rays) of echinoderms. For example,
echinoderm rays are named A through E in a counter-clockwise direction, and A is
defined as the ray opposite the position of the madreporite.

Figure 8.12: Describing the body parts of an echinoderm. The rays, or units of
pentameral symmetry, of an echinoderm are labeled A through E. The ray located

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opposite the madreporite is called A, and subsequent rays are labeled B, C, D, and E in a
counterclockwise direction.

8.4.3 Other Organ Systems

Almost all echinoderms have a simple but complete digestive tract. Although most
species lack respiratory and excretory organs, echinoderms do have a circulatory system.
They generally have an open circulatory system but lack a distinct heart. In an open
circulatory system, circulating blood is not entirely contained within blood vessels. The
nervous system is not centralized and usually consists of a network of interconnected
nerve cells distributed throughout the body. There is a nerve ring surrounding the mouth
in the central region and nerves that extend from the ring into each arm. Echinoderms are
able to sense chemical and physical stimulation using sensory cells located on the surface
of their bodies. Some of these cells have chemoreceptors that bind to molecules called
pheromones. Individuals within a species secrete pheromones that bind to the receptors
found on other individuals as a means of communication. One example of this is the
signaling of metamorphosis. Pheromones secreted by a cluster of adult echinoderms may
indicate to larvae swimming nearby that they have found a good place to settle. The
larvae sense the pheromone and respond by settling and initiating the process of
metamorphosis (changing from a free-swimming larval form to a more sedentary adult
form). In the next section we will examine the development of echinoderms and the
nature of their metamorphic changes.

Reproduction and Development

Most echinoderms reproduce by sexual reproduction through the fusion of sperm and
eggs. They generally have separate sexes, and fertilization is usually external. Sea
cucumbers, for example, release sperm and eggs into the open ocean where they may
come in contact and undergo fertilization. The majority of echinoderm species undergo
indirect development with a free-swimming bilaterally symmetrical larval stage. The
bilateral symmetry of the larvae is strikingly different from the radial symmetry of the
adult stage. Larvae undergo a process called metamorphosis, in which the organization of
the body shifts from bilateral to radial symmetry. Some species, however, undergo direct
development, where the young are simply smaller, sexually immature versions of their
parents. There are also a few species, particularly among those living in extreme
environments such as polar regions, that exhibit parental care behavior. They nurture the
young until they are old enough to fend for themselves. Some species of sea stars and
brittle stars are also capable of asexual reproduction by fission. In this process, the animal
splits itself into two parts and each part regenerates the missing regions to produce a
complete individual.

Echinoderms have the ability to regenerate a missing arm, or ray. The limb may be lost
as the result of a bite from a predator, or in some cases it is purposely released by the
echinoderm in a process called self-amputation, or autotomy. This is usually done to
escape a predator that has a hold of that particular limb. Most echinoderms can only
undergo regeneration if a certain portion of the animal remains intact, usually including

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the central disk region. However, sea stars have quite extensive powers of regeneration,
and they can often produce an entire organism from a small segment such as an arm.

Figure 8.13: A sea star in the process of regenerating a missing limb. Echinoderms, such
as this sea star, have the amazing ability to regenerate lost limbs, and, in some cases,
almost the entire body can be regenerated from one limb.

Self Assessment Questions

1. How do echinoderms use their water vascular system to move?
2. What kind of functions does the water vascular system have other than movement?
3. Describe the nervous system of an echinoderm.
4. What role do pheromones play in the metamorphosis of certain echinoderms?
5. How well can echinoderms regenerate?

8.5 Reproduction and Development in Asteroidean, Ophiuroidea,

Echinoidea, Holothuroidea, Crinoidea, Further Phylogenetic
Consideration
8.5.1. Reproduction and Development in Asteroidean
Sexual reproduction in most species of starfish are gonochorous, there being separate
male and female individuals. Some species are simultaneous hermaphrodites, producing
eggs and sperm at the same time and in a few of these, the same gonad, called an
ovotestis, produces both eggs and sperm. When these grow large enough they change
back into females. In others, the eggs may be stuck to the undersides of rocks. In certain
species of starfish, the females brood their eggs – either by simply enveloping them or by
holding them in specialized structures. Brooding may be done in pockets on the starfish's

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aboral surface, inside the pyloric stomach (Leptasteriastenera) or even in the interior of
the gonads themselves.

Those starfish that brood their eggs by "sitting" on them usually assume a humped
posture with their discs raised off the substrate. Pterastermilitaris broods a few of its
young and disperses the remaining eggs, that are too numerous to fit into its pouch. In
these brooding species, the eggs are relatively large, and supplied with yolk, and they
generally develop directly into miniature starfish without an intervening larval stage. An
intragonadal brooder, the young starfish obtain nutrients by eating other eggs and
embryos in the brood pouch. Brooding is especially common in polar and deep-sea
species that live in environments unfavorable for larval development and in smaller
species that produce just a few eggs. Spawning takes place at any time of year, each
species having its own characteristic breeding season.

The first individual of a species to spawn may release a pheromone that serves to attract
other starfish to aggregate and to release their gametes synchronously. In other species, a
male and female may come together and form a pair. This behavior is called pseudo
copulation and the male climbs on top, placing his arms between those of the female.
When she releases eggs into the water, he is induced to spawn. Starfish may use
environmental signals to coordinate the time of spawning (day length to indicate the
correct time of the year, dawn or dusk to indicate the correct time of day), and chemical
signals to indicate their readiness to breed. In some species, mature females produce
chemicals to attract sperm in the sea water. Asexual reproduction some species of starfish
are able to reproduce asexually as adults either by fission of their central discs or by
autonomy of one or more of their arms. Which of these processes occurs depends on the
genus? Among starfish that are able to regenerate their whole body from a single arm,
some can do so even from fragments just 1 cm (0.4 in) long. Single arms that regenerate a
whole individual are called comet forms.

The division of the starfish, either across its disc or at the base of the arm, is usually
accompanied by a weakness in the structure that provides a fracture zone. The larvae of
several species of starfish can reproduce asexually before they reach maturity. They do this
by autotomizing some parts of their bodies or by budding. When such a larva senses that food
is plentiful, it takes the path of asexual reproduction rather than normal development. Though
this costs it time and energy and delays maturity, it allows a single larva to give rise to
multiple adults when the conditions are appropriate. Some species of starfish have the ability
to regenerate lost arms and can regrow an entire new limb given time. A few can regrow a
complete new disc from a single arm, while others need at least part of the central disc to be
attached to the detached part. Regrowth can take several months or years and starfish are
vulnerable to infections during the early stages after the loss of an arm.

A separated limb lives off stored nutrients until it regrows a disc and mouth, and is able
to feed again. Other than fragmentation carried out for the purpose of reproduction, the
division of the body may happen inadvertently due to part being detached by a predator,
or part may be actively shed by the starfish in an escape response. The loss of parts of the

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body is achieved by the rapid softening of a special type of connective tissue in response
to nervous signals. This type of tissue is called catch connective tissue and is found in
most echinoderms. An autonomy-promoting factor has been identified which, when
injected into another starfish, causes rapid shedding of arms.

Larval development and maturation Most starfish embryos hatch at the blastula stage.
The original ball of cells develops a lateral pouch, the archenteron. The entrance to this is
known as the blastopore and it will later develop into the anus. Another invagination of
the surface will fuse with the tip of the archenteron as the mouth while the interior section
will become the gut. At the same time, a band of cilia develops on the exterior. This
enlarges and extends around the surface and eventually onto two developing arm-like
outgrowths.

At this stage the larva is known as a bipinnaria. The cilia are used for locomotion and
feeding, their rhythmic beat wafting phytoplankton towards the mouth. The lifecycle of a
starfish varies considerably between species, generally being longer in larger forms and
in those with planktonic larvae. For example, Leptasteriasbexactis broods a small number
of large-yolked eggs. It has an adult weight of 20g, reaches sexual maturity in two years
and lives for about ten years. Pisasterochraceus releases a large number of eggs into the
sea each year and has an adult weight of 80g. It reaches maturity in five years and has a
maximum recorded lifespan of 34 years.

Figure 8.14: Asterias. Development and life history

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8.5.2 Reproduction and Development in Ophiuroidea

a. Regeneration and Asexual reproduction

Ophiuroids can regenerate lost arms. If a brittle star is grasped by an arm, the
contraction of certain muscles separate the arm form the body. Hence they are
named as brittle star. This process is called autotomy (Or autos, self + winos to
cut). Brittle star use autotomy to escape from prey. It later regenerates the arm.
Some Species have a fission line across their central disk.Ophiuroids split into
halves along this line and two ophiuroids are formed by regeneration.
b. Sexual Repeoduction
Ophiuroids are dioecious. Males are smaller than females. Females carry the males.
The gonads are attached with each bursa. The gametes are released into the bursa.
Eggs may be shed to the outside. Or they may be retained in the bursa. They are
fertilized and stay there during early development. Embryos are protected in the
bursa. They are sometimes nourished by the parents. Embryo change into a larva
called an ophiopluteus. It is panktonic (floating). Its long arms have ciliary bands.
These cilia are used to feed on plankton. This larva undergoes metamorphosis and
sinks into substrate.

8.5.3 Reproduction and Development in Echinoidea

Echinoids are dioecious. Gonads are present in internal body wall of the interambulacral
plates. They nearly fill the coelom during breeding season. One gonopore is present in
each of five ossicles called genital plates. The genital plates are present at the aboral end
of the echinoid. The sand dollars have only four gonads and gonophores. Gametes are
shed into the water. Fertilization is external. Development starts and pluteus larva is
formed. Pluteus larva spends several months in the plankton. It undergoes metamorphosis
and becomes adult.

8.5.4 Reproduction and Development in Holothuroidea

Sea cucumbers are dioecious. They possess a single gonad. Gonad is located anteriorly in
the coelom. It has single gonopore near the base of the tentacles. Fertilization is external.
The embryos develop in to planktonic larvae. Metamorphosis take place and adult animal
settle to the substrate. In some species, the female’s tentacles trap eggs. The eggs are
transferred to the body surface alter fertilization. They are brooded there. In some
species, coelomic brooding also occurs. Eggs are released into the cavity. Fertilization
take place and early development occur. The young come out through a rupture in the
body wall. Sea Cucumbers can also reproduce by transverse fission and regeneration of
lost parts.

8.5.5 Reproduction and Development in Crinoidea

Crinoids are dioecious. Gametes arc formed by germinal epithelium in the coelom. They
are released by the rupturing the walls of the arms. Some species spawn in seawater.
Fertilization and development occur in water. Some species brood embryos on the outer
surface of the arms. The larvae are attached to the substrate and metamorphosis occurs.
Crinoids can regenerate lost parts.

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8.6 Some Lesser-Known Invertebrates: The Lophophorates,
Entoprocts, Cycliophores, and Chaetognaths
8.6.1 Lophophorates
Phoronida are wormlike marine forms that live in secreted tubes in sand or mud or
attached to rocks or shells. Ectoprocta are minute forms, mostly colonial, whose
protective cases often form encrusting masses on rocks, shells, or plants. Brachiopoda are
bottom-dwelling marine forms that superficially resemble molluscs because of their
bivalved shells. One might wonder why these three apparently different types of animals
are lumped together in a group called lophophorates. Actually they have more in common
than first appears. They are all coelomate; they have some deuterostome and some
protostome characteristics; and none has a distinct head. But other phyla share these
characteristics. What really sets this group apart from other phyla is the common
possession of a ciliary feeding device called a lophophore (Gr. lophos, crest or tuft, +
phorein, to bear). A lophophore is a unique arrangement of ciliated tentacles borne on a
ridge (a fold of the body wall), which surrounds the mouth but not the anus. The
lophophore with its crown of tentacles contains within it an extension of the coelom, and
the thin, ciliated walls of the tentacles are not only an efficient feeding device but also
serve as a respiratory surface for exchange of gases between the environmental water and
the coelomic fluid. The lophophore can usually be extended for feeding or withdrawn for
protection. In addition, all three phyla have a U-shaped alimentary canal, with the anus
placed near the mouth but outside the lophophore. The coelom is primitively divided into
three compartments, protocoel, mesocoel and metacoel, and the mesocoel extends into
the hollow tentacles of the lophophore. The protocoel, where present, forms a cavity in a
flap over the mouth, the epistome. The portion of the body containing the mesocoel is
known as the mesosome, and that containing the metacoel is the metasome. Members of
all three phyla have a freeswimming larval stage but are sessile as adults.

Figure 8.15: Lophophore

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8.6.2 Entoprocta
Entoprocta (en´to-prok´ta) (Gr. entos, within, + proktos, anus) is a small phylum of about
150 species of tiny, sessile animals that superficially resemble hydroid cnidarians but
have ciliated tentacles that tend to roll inward. . Most entoprocts are microscopic, and
none is more than 5 mm long. They are all stalked and sessile forms; some are colonial,
and some are solitary. All are ciliary feeders. With the exception of the genus Urnatella
(L. urna, urn, + ellus, dim. suffix), all entoprocts are marine forms that have a wide
distribution from the polar regions to the tropics. Most marine species are restricted to
coastal and brackish waters and often grow on shells and algae. Some are commensals on
marine annelid worms. Freshwater entoprocts occur on the underside of rocks in running
water. U. gracilis is the only common freshwater species in North America.

Form and Function

The body, or calyx, of an entoproct is cup shaped, bears a crown, or circle, of ciliated
tentacles, and may be attached to a substratum by a single stalk and an attachment disc
with adhesive glands, as in the solitary Loxosoma and Loxosomella (Gr. loxos, crooked, +
soma, body) or by two or more stalks in colonial forms. Both tentacles and stalk are
continuations of the body wall. The 8 to 30 tentacles making up the crown are ciliated on
their lateral and inner surfaces, and each can move individually. Tentacles can roll inward
to cover and protect the mouth and anus but cannot be retracted into the calyx. Movement
is usually restricted in entoprocts, but Loxosoma, which lives in the tubes of marine
annelids, is quite active, moving over the annelid and its tube freely. The gut is U-shaped
and ciliated, and both the mouth and the anus open within the circle of tentacles.
Entoprocts are ciliary filter feeders. Long cilia on the sides of the tentacles keep a
current of water containing protozoa, diatoms and particles of detritus moving in between
the tentacles. Short cilia on the inner surfaces of the tentacles capture the food and direct
it downward toward the mouth. The body wall consists of a cuticle, cellular epidermis
and longitudinal muscles. The pseudocoel is largely filled with a gelatinous parenchyma
in which is embedded a pair of protonephridia and their ducts, which unite and empty
near the mouth. There is a well-developed nerve ganglion on the ventral side of the
stomach, and the body surface bears sensory bristles and pits. Circulatory and respiratory
organs are absent. Exchange of gases occurs through the body surface, probably much of
it through the tentacles.

Some species are monoecious, some dioecious, and some appear to be protandrous; that
is, the gonad at first produces sperm and later eggs. The gonoducts open within the circle
of tentacles.

Fertilized eggs develop in a depression, or brood pouch, between the gonopore and the
anus. Entoprocts have a modified spiral cleavage pattern with mosaic blastomeres. The
embryo gastrulates by invagination. The trochophore-like larva is ciliated and free
swimming. It has an apical tuft of cilia at the anterior end and a ciliated girdle around the
ventral margin of the body. Eventually the larva settles to the substratum and inverts to
form the adult.

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Figure 8.16: A, Urnatella, a freshwater entoproct, forms small colonies of two or three
stalks from a basal plate. B, Loxosomella, a solitary entoproct. Both solitary and colonial
entoprocts can reproduce asexually by budding, as well as sexually.

8.6.3 Cycliophora
The phylum Cycliophora, consists of at least three species of acoelomate, bilaterally
symmetrical organisms that are obligate commensalists on the mouthparts of lobsters.
There are two formally described species in the phylum, Symbionpandora and
Symbionamericanus, with at least one additional, undescribed species known.
Symbionpandora was first discovered on the mouthparts of Norway lobsters
(Nephropsnorvegicus) in Scandinavian waters, and Symbionamericanus was described
from American lobsters (Homarusamericanus) in North American waters. The third,
undescribed, species may be found on European lobsters (Homarusgammarus), in
European waters. These organisms are among the smallest known free-living metazoans,
with females measuring around 350 µm in length, and males only reaching lengths of 30
to 40 µm, and containing (often significantly) fewer than 200 cells in their entire body.
These organisms filter feed on bacteria and food particles that escape from their host's
mouthparts, and despite their small size, exhibit a complex and distinctive life cycle with
multiple stages, including asexual feeding stages that may reproduce by budding, as well
as free-swimming male (Prometheus), female (Pandora), and internally brooded chordoid
larvae and sessile dwarf males that live attached to the body wall of females.

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Figure 8.17: A cycliophoran

Habitat
Cycliophorans live on the mouthparts of their lobster hosts. Feeding individuals and
chordoid cysts may be found on all six feeding mouthparts, most often on individuals
with a carapace length greater than 35 mm. The numbers of individuals living on a host
increases with size; over a thousand feeding cycliophorans and nearly 200 chordoid cysts
have been found on larger lobsters. Sessile larvae may settle near their female
progenitors, or disperse and colonize a new host. They have been found from the
intertidal zone to depths of 720 m.

Physical Description
Cycliophorans have an anterior buccal funnel; oval-shaped trunk; and posterior, acellular
stalk, with an adhesive disc they use to attach themselves to their hosts’ mouthparts.
Sessile stage females are approximately 350 µm long and 100 µm wide. The trunk and
adhesive disc are covered in a layered cuticle (the disc itself may also be comprised of
cuticle). Cycliophorans are acoelomate, with the area between their guts and body walls
filled with mesenchyme. They have a feeding ring around the buccal funnel that is
densely packed with cilia and contractile cells, which form a pair of sphincters capable of
closing the oral area. Two muscle fibers extend dorsally from the base to the ventral side
of the trunk, and are likely used to move the buccal tube during feeding. The gut, which
is entirely ciliated, is U-shaped. A curved esophagus connects the buccal funnel to a
stomach with large gland cells and a narrow lumen. The intestine leads to a dorsal rectum
and anus, located near the buccal funnel.

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Cycliophorans exhibit sexual dimorphism. Males only measure 30 to 40 µm in length
with heavily ciliated bodies and a ventral-posterior penis and associated pouch. They are
typically found free swimming or on the body of a female. Their bodies may be round or
more rectangular in shape. Once thought to have greatly reduced body structures, recent
research has shown that males do possess complex musculature, a large cerebral ganglion
and nerve cords, fully developed gonads and mating structures, and sensory organs, as do
females.

8.6.4 Chaetognatha
The phylum Chaetognatha, also known as arrow worms, contains nearly 200 species of
mostly planktonic, bilaterally symmetrical, coelomate, worm-like organisms. The phylum
contains two orders, Phragmophora and Aphragmophora. The main difference between
the two is the presence of ventral transverse muscle bands in Phragmophora, which are
absent in Aphragmophora. Chaetognaths may be found in marine and some estuarine
environments throughout the world. About a fifth of the total species are benthic, some
living just above the deep ocean floor; these are often attached to the substrate by
adhesive secretions. Chaetognaths may range from 1 mm to 12 cm in length and are
typically transparent, although some deep-water species may be orange in color, and
phragmophorids may be opaque, due to their musculature. The common name, arrow
worms is derived from their streamlined appearance, with paired lateral fins and a single
caudal (tail) fin, while their scientific name comes from the hooked set of jaws that
protrude lateral to the mouth. These structures are used in prey capture, with
chaetognaths feeding on a number of crustacean (mainly copepods) and fish (mainly
larvae) species, which they track through daily vertical migrations in the water column
(these migrations may also protect them from predators). Chaetognaths are
hermaphroditic, and may undergo reciprocal, nonreciprocal or self-fertilization.

Geographic Range
Chaetognaths may be found in marine and some estuarine environments throughout the
world, including polar and tropical regions

Habitat
Chaetognaths are mainly planktonic organisms in marine and estuarine environments.
About a fifth of the total species are benthic, some living just above the deep ocean floor.
They are often found in great numbers, particularly in mid-water and neritic waters, and
may be found in rock pools or associated with certain oceanic currents.

Physical Description
Chaetognaths range from 1 mm to 12 cm in length and are typically transparent, although
some deep-water species may be orange in color (carotenoid pigmentation), and
phragmophoridsmay be opaque, due to their musculature. They are bilaterally
symmetrical and have long, streamlined bodies, which may be divided into head, trunk
and tail regions. They have paired lateral fins and a single tail fin. The mouth is located
ventrally on the head, and is set into a vestibule; this structure is typically associated with
grasping spines or hooks, located laterally to the mouth, as well as teeth, which are

237
located at the front of the mouth. Some species have serrated hooks and/or cuspidate
teeth. A hood (anterolateral body wall fold) may be pulled over the head to enclose the
vestibule.

Chaetognaths are covered in a thin, flexible cuticle on top of the epidermis. Epidermal
cells are mainly squamous and have interlocking margins; they may be stratified.
Epidermal cells covering the fins are elongated and the cells lining the vestibule are
columnar rather than squamous. The cuticle is not continuous and, where it is not present,
there are many secretory cells in the epidermis. There is a basement membrane present
between the epidermis and body wall; the body wall is made up of four quadrants of
dorsolateral and ventrolateral longitudinal muscles. The body cavities are most likely
derived from enterocoelic cavities, which form during development. The body cavity has
a tripartite arrangement, with a head cavity (protocoel, reduced in space by the cephalic
musculature), and paired trunk and tail coeloms with dorsal and ventral longitudinal
mesenteries, which correspond to the mesocoel and matcoel, respectively. Transverse
septa separate the body regions. The body fluid has a variety of cells and cell-like
structures, although their functions are largely unknown. The fluid-filled coeloms, body
wall, basement membrane, and cuticle all provide support to the body. They do not have
circulatory, respiratory, or excretory organs; gases are diffused across the body wall and
fluid transport is via cilial action within the body cavities. A few species of deep sea
chaetognaths,including Eukrohniafowleri and Caecosagittamacrocephala,are
bioluminescent.

Figure 8.18: Chaetognatha

238
Development
Chaetognaths are hermaphroditic. Cross-fertilization is most common, although some
species will self-fertilize. Fertilization is typically internal and eggs may be released into
the water, deposited on the sea floor or other substrate, or brooded in pouches near the
tail. Cleavage is radial, holoblastic, and equal, leading to a coeloblastula. Development is
direct and accomplished quickly, typically from zygote to juvenile within 48 hours.

Reproduction
Chaetognaths may undergo reciprocal, nonreciprocal, or self-fertilization. Some benthic
species have been documented performing a mating “dance,” with an individual
depositing balls of sperm onto a mate.

Mating Systempolygynandrous (promiscuous)

Chaetognaths have paired ovaries located in their trunks and paired testes located in their
tails. Sperm mature before eggs (which makes self-fertilization less likely), and are stored
in coelomic cavities within the tail until they are released in clusters outside the body via
a pair of seminal vesicles. Ovaries have oviducts, which lead to genital pores located near
the trunk-tail junction. In populations of at least a few species, breeding occurs twice a
year, and hatching occurs from April to June and late September to December (typically
fewer hatchlings).

Summary/Key Points
1. (Echinos: Spines; derma: Skin)
2. Kingdom: Animalia
3. Habitat: These are exclusively marine
4. Grade of organization: organ system grade
5. Germ layer: triploblastic
6. Symmetry: Adults are radially symmetrical while the larvae are bilaterally
symmetrical.
7. Coelom: present ( coelomate)
8. Body without segmentation
9. The shape of the body is flat, star like, spherical or elongated.
10. Head is absent
11. Presence of tube feet
12. Presence of water vascular system
13. Mouth is present on ventral side while anus is present on dorsal side
14. Respiration: by papule, gills or clocal respiratory tree
15. Nervous system: absent, they are brainless organism.
16. Circulatory system: is reduced, heart is absent
17. Blood has no pigment.
18. Digestive system: complete
19. Excretory system: absent
20. Sexes: mostly dioecious, rarely monocious

239
21. Reproduction:
 Sexual: by gamatic fusion
 Asexual: regeneration
1. Fertilization: external
2. Development: indirect with characteristic larvae

Self Assessment Questions

Q.1 Multiple Choice Questions MCQs
1. The origin of ossicle is:
(a) Ectoderm (b) Mesoderm (c) Endoderm (d) None
2. Hamel system is derived from:
(a) Ectoderm (b) Mesoderm (c) Endoderm (d) Coelom
3. Echinoderms are:
(a) Deutrostome (b) Protostome (c) Schizocoelous (d) None
4. Echinoderms are:
(a) Enterocoelous (b) Protostome (c) Schizocoelous (d) None
5. The sieve like plate is:
(a) Stone canal (b) Madreporite (c) Pollan vesicles (d) Tiedemann body
6. The ring canal usually opens outside through:
(a) Stone canal (b) Madreporite (c) Polian vesicles (d) Tiedemann body
7. Sites of production of phagocytes are:
(a) Tiedemann body (b) Madreporite (c) Polian vesicles (d) Stone canal
8. Canals present in the arm are:
(a) Stone canal (b) Radial canal (c) Lateral canal (d) Tiedemann body
9. The canal end in tube feet are:
(a) Stone canal (b) Radial canal (c) Polian vesicles (d) Lateral canal
10. Brittle star belongs to:
(a) Echinoidea (b) Asteroidea (c) Ophiuroidea (d) Crinoidea
11. Sea star belongs to:
(a) Echinoidea (b) Asteroidea (c) Ophiuroidea (d) Crinoidea
13. Globular disc shaped echinoderms are:
(a) Echinoidea (b) Ophiuroidea (c)Crinoidea (d) Asteroidea
14. The echinoderms without rays are:
(a) Echinoidea (b) Asteroidea (c) Holothuroidea (d) Crinoidea
15. The number of species of Asteroidea are:
(a) 500 (b) 1500 (c) 1000 (d) 2000
16. Class Ophiuroidea includes:
(a) Sea star (b) Brittle star (c) Sea cucumber (d) sea anemone
17. The larva present in the Ophiuroidea is:
(a) Hpinnaria (b) Brichiolaria (c) plueus larva (d) ophiopluteus
18. Aristotle’s lantern is present in:
(a) Sea star (b) Brittle star (c) Sea cucumber (d) sea urchin
19. Number of plates in the test of sea urchin are:
(a) 5 (b) 10 (c) 15 (d) 21)

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 20. Larva present in the Echinoidea
(a) Bipinnaria (b) Brachiolaria (c) pluteus larva (d) ophiopluteus
21. The number of species of Holothuroidea are:
(a) 500 (b) 1500 (c) 1000 (d)2000
Ans
1. (h) 2. (d) 3. (a) 4. (a) 5. (b) 6. (a) 7. (a) S. (b) 9. (d) 10. (c)
11. (b) 12. (a) 13. (e) 14. (1) 15. (I) 16. (d) 17. (d) 18. (h) 19. (c) 20. (h)

Q.2 Fill in the blanks

1. The 400-million years-old Paleozoic rocks have almost all the fossils of the
animal, up to…………………………
2. Modern echinoderms ………………… symmetry.
3. The skeleton of echinoderms consists of calcium carbonate plates called
…………………
4. The tube feet end in a bulblike muscular.
5. The pyloric stomach gives rise to a duct. This duct connects to the
pyloric……………………….
6. The dermal branchiae and pedicellariae are absent in ………………………
7. ……………..help in the distribution of nutrients and the removal of wastes.
8. Mouth has a chewing apparatus, called Aristotle’s……………………………

Ans
1. Echinoderms 2. Pentaradial 3. Ossicle 4. ampulla
5. cecea 6. Ophiuroids 7. Coelomocytes 8. lantern

Q.3 True/False
1. The 400-million ncar-old Paleozoic rocks have almost all the fossils of the
animals up to arthropods.
2. The skeleton of echinoderms consists of calcium carbonate plates called
ossicle.
3. The tube feet end in a bulblike muscular bulb.
4. The pyloric stomach gives rise to a duct. This duct connects to the pyloric
CCeile.
5. Coelomocytes help in the distribution of nutrients and the removal of wastes.
6. Mouth has a chewing apparatus. called Aristotle’s lantern.
7. Some sea cucumbers can bring out Cuverian tubules through the anus.

Ans:
1. F: echinoderms 2. T 3. F: ampulla 4. T 5. T
6. T 7. T

241
Suggested Readings
 Stöhr, Sabine (2014). "Echinodermata". WoRMS. World Register of Marine
Species. Retrieved 23 February 2014.
 Jump up to:a b "echinoderm". Online Etymology Dictionary.
 "Sea Lily". Science Encyclopedia. Retrieved 5 September 2014.
 "Animal Diversity Web - Echinodermata". University of Michigan Museum of
Zoology. Retrieved 26 August 2012.
 "Computer simulations reveal feeding in early animal".
 ^ Jump up to:a b Dorit, R. L.; Walker, W. F.; Barnes, R. D. (1991). Zoology.
Saunders College Publishing. pp. 777–779. ISBN 978-0-03-030504-7.
 Richard Fox. "Asterias forbesi". Invertebrate Anatomy OnLine. Lander University.
Retrieved 19 May 2012.
 ^ Jump up to:a b Wray, Gregory A. (1999). "Echinodermata: Spiny-skinned
animals: sea urchins, starfish, and their allies". Tree of Life web project. Retrieved
19 October 2012.
 ^ Telford, M. J.; Lowe, C. J.; Cameron, C. B.; Ortega-Martinez, O.; Aronowicz, J.;
Oliveri, P.; Copley, R. R. (2014). "Phylogenomic analysis of echinoderm class
relationships supports Asterozoa". Proceedings of the Royal Society B: Biological
Sciences. 281 (1786): 20140479. doi:10.1098/rspb.2014.0479. PMC 4046411.
PMID 24850925.
 ^ Jump up to:a b c d Uthicke, Sven; Schaffelke, Britta; Byrne, Maria (1 January
2009). "A boom–bust phylum? Ecological and evolutionary consequences of
density variations in echinoderms". Ecological Monographs. 79: 3–24.
doi:10.1890/07-2136.1.
 ^ Siera104. "Echinodermata". Retrieved 15 March 2008.
 ^ Jump up to:a b c Waggoner, Ben (16 January 1995). "Echinodermata: Fossil
Record". Introduction to the Echinodermata. Museum of Paleontology: University
of California at Berkeley. Retrieved 14 March 2013.
 ^ Smith, Dave (28 September 2005). "Vendian Animals: Arkarua". Retrieved 14
March2013.
 ^ Jump up to:a b Dorit, Walker & Barnes (1991) p. 792–793
 ^ UCMP Berkeley, edu. "Echinodermata: Morphology". University of California
Museum of Paleontology. Retrieved 21 March 2011.
 ^ Jump up to:a b Ruppert, Fox & Barnes (2004) p. 873

242
 ^ Australian Echinoderms: Biology, Ecology and Evolution
 ^ Messing, Charles. "Crown and calyx". Charles Messing's Crinoid Pages.
Archived from the original on 29 October 2013. Retrieved 29 July 2012.
 ^ Behrens, Peter; Bäuerlein, Edmund (2007). Handbook of Biomineralization:
Biomimetic and bioinspired chemistry'. Wiley-VCH. p. 393. ISBN 978-3-527-
31805-6.
 ^ Davies, A. Morley (1925). An Introduction to Palaeontology. Thomas Murby. pp.
240–241.
 ^ Weber, W.; Dambach, M. (April 1974). "Light-sensitivity of isolated pigment
cells of the sea urchin Centrostephanus longispinus". Cell and Tissue Research.
148 (3): 437–440. doi:10.1007/BF00224270. PMID 4831958.
 ^ Motokawa, Tatsuo (May 1984). "Connective tissue catch in echinoderms".
Biological Reviews. 59 (2): 255–270. doi:10.1111/j.1469-185X.1984.tb00409.x.
 ^ Jump up to:a b Dorit, Walker & Barnes (1991) pp. 780–791
 ^ Dorit, Walker & Barnes (1991) p. 784–785
 ^ Dorit, Walker & Barnes (1991) p. 788–789
 ^ Dorit, Walker & Barnes (1991) p. 780–783
 ^ Ruppert, Fox & Barnes (2004) p. 905
 ^ Echinoderm Nutrition
 ^ Rigby, P. Robin; Iken, Katrin; Shirayama, Yoshihisa (1 December 2007).
Sampling Biodiversity in Coastal Communities: NaGISA Protocols for Seagrass
and Macroalgal Habitats. NUS Press. p. 44. ISBN 9789971693688.
 ^ Ruppert, Fox & Barnes (2004) p. 885
 ^ Ruppert, Fox & Barnes (2004) p. 891
 ^ Ruppert, Fox & Barnes (2004) pp. 902–904
 ^ Ruppert, Fox & Barnes (2004) p. 912
 ^ Ruppert, Fox & Barnes (2004) p. 920
 ^ J. Moore. An Introduction to the Invertebrates, Cambridge Univ. Press, 2nd ed.,
2006, p. 245.
 ^ C. Hickman Jr., L. Roberts, A. Larson. Animal Diversity. Granite Hill Publishers,
2003. p. 271.
 ^ "Macrobenthos of the North Sea - Echinodermata > Introduction".
etibioinformatics.nl.

243
 ^ Nielsen, Claus. Animal Evolution: Interrelationships of the Living Phyla. 3rd ed.
Oxford, UK: Oxford University Press, 2012, p. 78
 ^ Jump up to:a b Ramirez-Gomez, Fransisco (27 September 2010). "Echinoderm
Immunity". Invertebrate Survival Journal.
 ^ Smith, Courtney (January 2010). "Echinoderm Immunity". Advances in
Experimental Medicine and Biology. 708: 260–301. doi:10.1007/978-1-4419-
8059-5_14. ISBN 978-1-4419-8058-8. PMID 21528703.
 ^ Jump up to:a b Ruppert, Fox & Barnes (2004) pp. 872–929
 ^ Edmondson, C.H (1935). "Autotomy and regeneration of Hawaiian starfishes"
(PDF). Bishop Museum Occasional Papers. 11 (8): 3–20.
 ^ Jump up to:a b c d McAlary, Florence A (1993). Population Structure and
Reproduction of the Fissiparous Seastar, Linckia columbiae Gray, on Santa
Catalina Island, California. 3rd California Islands Symposium. National Park
Service. Retrieved 15 April 2012.
 ^ Jump up to:a b c d See last paragraph in review above AnalysisHotchkiss,
Frederick H. C. (1 June 2000). "On the Number of Rays in Starfish". American
Zoologist. 40 (3): 340–354. doi:10.1093/icb/40.3.340. Retrieved 14 July 2011.
 ^ Jump up to:a b c d Fisher, W. K. (1 March 1925). "Asexual Reproduction in the
Starfish,Sclerasterias" (PDF). Biological Bulletin. 48 (3): 171–175.
doi:10.2307/1536659. ISSN 0006-3185. JSTOR 1536659. Retrieved 15 July 2011.
 ^ Dobson, W. E.; S. E. Stancyk; L. A. Clements; R. M. Showman (1 February
1991). "Nutrient Translocation during Early Disc Regeneration in the Brittlestar
Microphiopholis gracillima (Stimpson) (Echinodermata: Ophiuroidea)". Biol Bull.
180 (1): 167–184. doi:10.2307/1542439. JSTOR 1542439. PMID 29303627.
Retrieved 14 July 2011.
 ^ Mashanov, Vladimir S.; Igor Yu. Dolmatov; Thomas Heinzeller (1 December
2005). "Transdifferentiation in Holothurian Gut Regeneration". Biol Bull. 209 (3):
184–193. doi:10.2307/3593108. JSTOR 3593108. PMID 16382166. Retrieved 15
July 2011.
 ^ Hart, M. W. (January–February 2002). "Life history evolution and comparative
developmental biology of echinoderms". Evolution and Development. 4 (1): 62–71.
doi:10.1046/j.1525-142x.2002.01052.x. PMID 11868659.
 ^ Young, Craig M.; Eckelbarger, Kevin J. (1994). Reproduction, Larval Biology,
and Recruitment of the Deep-Sea Benthos. Columbia University Press. pp. 179–
194. ISBN 0231080042.

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UNIT-9

CHORDATES

Written by: Dr. Sobia Mushtaq

Reviewed by: Arshad Mehmood Qamar

245
 CONTENTS
Introduction ....................................................................................................... 248

Objectives ......................................................................................................... 248

9.1 General Characteristics of Chordates.................................................... 249

9.2 General Characteristics ......................................................................... 255

9.3 Survey of Fishes .................................................................................... 257

9.4 Cartilaginous (Chondrichthyes) ............................................................ 261

9.5 Nutrition and the Digestive System ...................................................... 267

9.6 Circulation and Gas Exchange .............................................................. 268

9.7 Swim Bladders and Lungs .................................................................... 270

9.8 Nervous and Sensory Functions............................................................ 272

9.9 Excretion and Osmoregulation ............................................................. 274

9.10 Reproduction and Development ........................................................... 276

9.11 General Characteristics of Amphibians and their Trends for land habitat;
Amphibians as Unsuccessful Land Vertebrate ..................................... 278

9.12 Survey of Amphibians .......................................................................... 279

9.13 Evolutionary Pressures in Amphibians ................................................. 282

9.14 External Structure and Locomotion ...................................................... 282

9.15 Nutrition and the Digestive System ...................................................... 285

9.16 Circulation............................................................................................. 286

9.17 Gas Exchange........................................................................................ 286

9.18 Nervous and Sensory Functions............................................................ 288

9.19 Excretion and Osmoregulation ............................................................. 291

246
9.20 Further Phylogenetic Considerations in Amphibians ........................... 295

9.21 Reptiles; Why Reptiles are Considered as Successful land Vertebrates.

Reptiles: The First Amniotes ................................................................ 297

9.22 Adaptive Radiations in Reptiles............................................................ 298

9.23 Survey of the Reptiles ........................................................................... 299

9.24 Evolutionary Pressures in Reptiles ....................................................... 304

9.25 External Structure and Locomotion ...................................................... 304

9.26 Further Phylogenyetic Considerations in Reptiles ................................ 313

9.27 Birds and Adaptations for Aerial mode of Life (Flight Adaptations);
Flightless and flying .............................................................................. 314

9.28 External Structure and Locomotion ...................................................... 317

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Introduction
The phylum Chordata consists of animals with a flexible rod supporting their dorsal or
back sides. ... Examples of vertebrate chordates include fishes, amphibians, reptiles,
birds, and mammals. A modern human—one species of mammal—is a familiar example
of a chordate. Phylum Chordata has about 45,000. Its members are very successful at
adapting in aquatic and terrestrial environments. Characteristics of the phylum Chordata
are: They are bilaterally symmetrical deuterostomes animals.Four unique characteristics
present at some stage in development. These are: noto hord, pharyngeal slits or pouches,
dorsal tubular nerve cord, and postanl tail an endostyle or thyroid gland is present in
them. They have complete digestive tract. The ventral contractile blood vessel (heart).
This unit contains the descriptions of all five classes, their general characteristics and
classification. Evolutionary perspectives of chordates are also discussed. This Unit
contains huge information about all five classes’ of chordates.

Objectives
After completion of this unit, you will be able to:
 Identify the class of organisms from the natural world.
 Write general characteristics of fishes, upto sublass level.
 Explore the adaptations that favoured the survival of fishes uring periodic drought
pre-adapted vertebrates to life on land. There were two lineage of ancient
amphibians, one gave rise to modern amphibians and the other lineage resulted in
amniote vertebraes.
 Explain how adaptive radiation resulted in the large variety of fishes present today.
Evolution of some fishes led to the terrestrial vertebrates.
 Elaborate General Characteristics of all members of all classes and sub classes of
phylum Chordata.
 Describe how reptiles have adaptations that allow them to spend most of their lives
apart from standing or flowing water. These include adaptations for support and
movement, feeding, gas exchange, temperature regulation, excretion,
osmoregulation and reproduction.
 Give evidences and examples of migration and navigation allow birds to live, feed
and reproduce in environments favorable to the survival of adults and young.

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9.1 General Characteristics of Chordates
a) Notochord (noton, the back + chorda, cord)
This phylum is named after the notochord. It is a supportive rod. It extends most of the
length of the animal into the tail. It is dorsal to the body cavity. It consists of a
connective-tissue sheath. This sheath encloses cells. Each cell contains a large fluid-filled
vacuole. This arrangement gives the notochord some turgidity. This turgidity compresses
the antero-posterior axis. At the same time, the notochord is flexible. It allows lateral
bending like lateral undulations of a fish during swimming. Cartilage or bone partly or
entirely replaces the notochord in most adult vertebrates.

b) Pharyngeal Gill Slits

Pharyngeal slits are a series of openings. These openings are present in pharyngeal region
between the digestive tract and the outside of the body. In some chordates, diverticula
from the gut are present in the pharyngeal region. These diverticula do allow an open
passageway to the outside. These diverticula are called pharyngeal pouches. The earliest
chordates used the slits for filter feeding. Some living chordates still use them for
feeding. Other chordates have developed gills in the pharyngeal pouches for gas
exchange. Incomplete pharyngeal slits are developed in terrestrial vertebrates.

c) Tubular Nerve Chord

The tubular nerve cord and its associated structure are responsible for chordate success.
The nerve cod runs the longitudinal axis of the body. It is present just dorsal to the
notochord. It also expands anteriorly as a brain. This central nervous system is associated
with the development of complex systems. These systems are used for sensory
perception, integration, and motor responses.

d) Postanal Tail
It is the fourth chordate characteristic. A postanal tail extends posteriorly beyond the anal
opening. The notochord or vertebral column supports the tail (Figure 9.1).

Figure 9.1: Chordate body plan (a) Lateral view (b) Cross section

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9.1.1 Subphylum Urochordata (TunIcates) (Uro, tail + chorda, cord)

Habitat
Members of the sub-phylum Urochordata are the tunicates or sea squirts. The ascidia is
the largest class of tunicates. Their adults are sessile, solitary or colonial. The adult
apendicularians and thaliaceans are planktonic. In some localities, tunicates occur in large
numbers and become dominant animals.

Body form
Sessile Urochordata have saclike bodies. These attached to rocks, pilings, shi hulls, and
other solid substrates. The unattached end of urochordates contains two siphons. These
siphons allow sea water to circulate through the body.

a) Oral siphon
One siphon is the oral siphon. It is the inlet for circulating water through the body. It is
present opposite to the attached end of the ascidia. It also acts as mouth opening.

b) Atrial siphon
The second siphon the atrial siphon. It is the opening for excurrent water.

Tunic: The body wall of most tunicates is covered by tunic. Tunic is connective-tissues
like covering. It appears gel like. But it is quite tough. It is secreted by epidermis. It is
composed of proteins, salts and cellulose. It also has some mesodermally derived tissues
like blood vessels and blood cells. Root like extensions of tunic are called stolons.
Stolons help to anchor tunicate to the substrate. It also connects individuals of a colony
(Figure 9.2).

Figure 9.2: Internal Structure of a Tunicate (a) Longitudinal section. Black arrows show
the path of water. (b) Cross section at the level of the atrial siphon. Small black arrows
show movement of food trapped in mucus that the endostyle produces.

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Maintenance Functions

Locomotion
Longitudinal and circular muscles are present below the body wall epithelium. They help
to change the shape of the adult tunicate. They act against the elasticity of the tunic and
the hydrostatic skeleton. Hydroskeleton is produced by sea water confined to internal
chambers.

Nervous system
The nervous system of tunicates is present in the body wall. It forms a nerve plexus. It
has single ganglion located on the wall of the pharynx between the oral and atrial
openings. This ganglion is not vital for coordinating bodily functions. Tunicates are
sensitive to many kinds of mechanical and chemical stimuli. The receptors for these
sense are distributed over the body wall. They are also present around the siphons. There
are no complex sensory organs.

Nutrition
Pharynx and atrium
The urochordates have a very large pharynx and a cavity, called the atrium. Atrium
surrounds the pharynx laterally and dorsally. The pharynx of tunicates starts to the oral
siphon. It is continuous with the remainder of the digestive tract. The oral margin of the
pharynx has tentacles. These tentacles prevent large objects from entering the pharynx.
Numerous pharyngeal slits called stigmas perforate the pharynx. Cilia associated with the
stigmas. These cilia circulate water into the pharynx through the stigmas and into the
surrounding atrium. Water leaves the tunicate through the atrial siphon.

Digestive tract
The pharynx opens into digestive tract of adult tunicates. Digestive tract ends at the anus
near the atrial siphon. Ventral ciliated groove is present in pharynx. It is called the
endostyle. It forms a mucous sheath during feeding. Cilia move the mucous sheet
dorsally across the pharynx. Food particles are brought into the oral siphon with incurrent
water. They are trapped in the mucous sheet and passed dorsally. Food is incorporated
into a string of mucus. The ciliary action moves this string into the next region of the gut
tract. Digestive enzymes are secreted in the stomach. Most absorption occurs across the
walls of the Intestine. Excurrent water carries digestive wastes from the anus out of the
atrial siphon.

Respiration
The pharynx also functions in gas exchange. Water circulates through the tunicate and
gases are exchanged.

Blood Vascular system

The tunicate heart is present at the base of the pharynx. One vessel from the heart runs
anteriorly under the endostyle. Another vessel runs posteriorly to the digestive organs and
gonads. Blood flow through the heart is not unidirectional. Peristaltic contractions of the

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heart propel blood in one direction. Then the direction is reversed. The significance of
this reversal is not understood. Tunicate blood plasma is colorless. It contains various
kinds of amoeboid cells.

Excretion
Ammonia diffuses into water. This water passes through the pharynx and is excreted.
Additionally, amoeboid cells of the circulatory system accumulate uric acid. These cells
transport uric acid into the intestinal loop. Pyloric glands on the outside of the intestine
also have excretory functions.

Reproduction and Development

Reproduction
Urochordates are monoecious. Gonads are located near the loop of the intestine. The
genital ducts open near the atrial siphon. Gametes are shed through the atrial siphon for
external fertilization. In some case, eggs are retained in the atrium for fertilization and
early development. Self-fertilization occurs in some species. But mostly cross-
fertilization occurs.

Development and metamorphosis

Tadpole like larva is developed during development. It has all four chordate
characteristics. Metamorphosis begins after a brief free-swimming larval period. Larva
does not feed during metamorphosis. The larva settles to a firm substrate. It attaches by
adhesive papillae. These papillae are located below the mouth. The outer epidermis
shrinks during metamorphosis. It pulls the notochord and other tail structures internally.
They are organized into adult tissues. The internal structures rotate at 180o. It brings the
oral siphon opposite the adhesive papillae. It also bends the digestive tract into a U shape
(Figure 9.3).

Figure 9.3: Tunicate metamorphosis.

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9.1.2 Subphylum Cephalochordata (kepholo, head + chorda, cord)

Habitat
Members of the subphylum Cephalochordata are called lancelets. Lancelets have four
chordate characteristics. Therefore, they are often studied in introductory zoology
courses. The cephalochordates consist of two genera. These are Branchiostoma
(amphioxus) and Asymntetron. Lancelets have about 45 species. They are distributed
throughout the oceans of world. They live in shallow waters that have clean sand
substrates (Figure 9.4).

External features
Cephalochordates are small (up to 5 cm long). They are tadpole like animals. They are
elongated and laterally flattened. They are nearly transparent. They have streamlined
shape. The cephalochordates are relatively weak swimmers. They spend most of their
time in a litter feeding position. They remained buried in this position and their anterior
end sticking out of the sand.

Notochord and locomotion

The notochord of cephalochordates extends from the tail to the head. So they are name
cephalochordates. Most of the cells of their notochord are muscle cells. It makes the
notochord contractile. Both these characteristics are adaptations for burrowing.
Contraction of the muscle cells compresses the fluids within. It increases the rigidity of
the notochord. It gives additional support when pushing into sandy substrates. Relaxation
of these muscle cells increases flexibility for swimming.

Muscle cells on the side of the notochord cause undulations. It propels the
cephalochordate through the water. Longitudinal, ventro-lateral folds of the body wall
help to stabilize cephalochordates during swimming. A median dorsal fin and a caudal fin
also help in swimming.

Oral hood, pharynx and atrium

a) Oral Hood
An oral hood projects from the anterior end of cephalochordates. Ciliated fingerlike
projections hang from the ventral side of the oral hood. These are called cirri. Cirri are
used in feeding.

b) Pharynx and Slits

The posterior wall of the oral hood have mouth opening. Mouth opens into a large
pharynx. Numerous pairs of pharyngeal slits are present in pharynx. These are supported
by cartilaginous gill bars.

c) Atrium
Large folds of the body wall extend ventrally around the pharynx. These folds fuse at the
ventral midline of the body. It forms atrium. Atrium is a chamber. It surrounds the

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pharyngeal region o the body. Atrium protects the delicate, filtering surfaces of the
pharynx from bottom sediments. The opening from the atrium to the outside is called the
atriopore.

Figure 9.4: Sub-phylum Cephalochordata. Internal structure of Branchiostoma

(amphioxus)

Maintenance Functions
Nutrition

1. Filter feeding
Cephalochordates are filter feeders. They bury in sandy substrates during feeding. Their
pointed end is placed upward. Cilia are present on the lateral surfaces of gill bars. They
sweep water into the mouth. Water passes from the phyarynx through pharyngeal slits to
the atrium. It moves out of the body through atriopore. Larger materials catch on cilia of
the cirri. Smaller, edible parts are pulled into the mouth with water. These are elected by
cilia the gill bars in mucus. This mucous is secreted by the endostyle. Endostyle is a
ciliated groove. It extends longitudinally along the midventral side of pharynx.

2. Digestion in gut
Cilia move food and mucus dorsally. It forms a food cord to the gut. A ring of cilia
rotates food cord and dislodge food. Digestion is both extracellular an intracellular. A
diverticulum of the gut is called midgut caecum. It extends an riorly. It ends blindly along
the right side of the pharynx and secretes digestive enzymes. An anus is present at the left
side of the ventral fin.

1. Bloodvascular and excretory system

Cephalochordates do not possess a true heart. Contraction waves in the walls of major
vessel propel blood. Blood contains amoeboid cells. It bathes tissues in open spaces.
Excretory tubules are modified coelomic cells. These cells are closely associated with

254
blood vessels. Thus there is an active transport of materials between the blood and
excretory tubules.

2. Coelom
The coelom of cephalochordates is reduced. It is restricted to canals near the gill bars,
endostyle and the gonads.

3. Reproduction and Development

Cephalochordates are dioecious. Gonads bulge into the atrium from the lateral body wall.
Gametes are shed into the atrium. They leave the body through the atriopore. External
fertilization takes place. Bilaterally symmetrical larva is formed during development.
Larvae are free-swimming. It settles to the substrate before metamorphosis and become
adults.

Self Assessment Questions

Q: Fill in the blanks.
i. Phylum chordate is named after the ………………. (Notochord).
ii. Numerous pharyngeal slits called perforate the pharynx (Stigmas)
iii. Members of the subphylum Cephalochordata are called
……………….(Lancelets)
iv. An oral hood projects from the anterior end of ………………….
(Cephalochordates)
v. Cephalochordates are ……………… (Dioecious)

Q: Answer the following.

i. What is metamorphosis?
ii. What do you know about cirri?
iii. What is known about internal structure of tunicate?
iv. What are pharyngeal pouches?
v. What are stolons?

9.2 General Characteristics (including aquatic adaptations) of

Super-Class Pisces and its Sub-classes
9.2.1 The Fishes Vertebrate Success in Water
1. Evolutionary Perspective
Water is a buoyant medium. It resists rapid fluctuations in temperature. Water covers
over 70% of the earth’s surface. Life began in water. The living tissues of the organism
are mostly made up of water. Therefore, life is impossible without water.
2. Adaptations in Fishes
The fishes are adapted to aquatic. No other animal is adapted to aquatic environment like
fishes. A variety of beautiful fishes is present every where. A variety of evidence of
adaptive radiation in fishes. The adaptive radiations in fishes started more than 500

255
million years ago. These radiations are still continuing. Fishes dominant many watery
environments. They are also ancestors of all other members of the subphylum Vertebrata.

9.2.2 Phylogenetic Relationships

Fishes are members of the chordate subphylum Vertebrata. They have vertebrae. These
vertebrates surround the spinal cord. Vertebral column provides the primary axial
support. Fish also have a skull. It protects the brain. Zoologists do not know about the
first vertebrate. Molecular evidences are gathered by comparing gene of
cephalochordates.

These evidences suggest that the vertebrate lineage goes back to 750 million years. This
date cannot be confirmed by fossil evidence. Cladistic analysis indicates that hag fishes
are the most primitive vertebrates. Two key vertebrate characteristics develop connection
between this lineage and other vertebrates. These characteristics are brain and bone.

1. Evolution of Brain
Chinese researchers have discovered the oldest vertebrate fossils. It was 530 million year-
old animal. It is a small lancelet shaped animal. Its characteristics suggest that these
animals have active predatory lifestyle. A brain is present in them. It processed sensory
information from the pair of eyes. Muscle blocks were present along the body wall. It
suggests that they were active swimming existence. These evidences show that these
animals locate prey by sight and then follow it in prehistoric seas.
2. Evolution of Bones
The origin of bone in vertebrates is also not clearly known. There are two hypotheses
about the origin of bone:
i) Origin from Dentine
The fossils of the group of ancient animal conodonts were discovered. Conodonts
were eel-like animals. They were present 510 million years ago. They were placed
in different phyla. But now zoologists have accepted them as full- fledged
vertebrates due to fossil evidences. They have two large eyes and a mouth. The
mouth is filled with root like structures made of dentine. Dentine is found in the
vertebrate skeleton. These structures show the presence of bone in old vertebrates.
ii) Origin from Denticles
Other hypotheses suggest that the bone was arised from the denticles in the skin.
Denticles were used for mineral storage like calcium phosphate. Bone was well
developed by 500 million years ago. Bones were present in the bony armor fishes
called ostracoderms. Ostracoderms were inactive filter feeders. It lived on the
bottom of prehistoric lakes and seas. They lack jaws and paired appendages. Then
evolution of fishes took place from ostracoderms. The fishes developed jaws,
paired appendages and many other structures.

9.2.3 Evolution of Fishes in Marine or Fresh Water

It is difficult to know that whether the first vertebrate was marine or fresh water. The
ancient deutrostome phyla were all marine. Therefore, the first vertebrates were also
marine. However, vertebrates were adapted to freshwater very early. Much of the

256
evolution of fishes took place there. There was back and forth movement between marine
and freshwater environments during early vertebrates. Majority of the evolutionary
history of some fishes took place in ancient seas. Most of the evolutionary history of
other fishes occurred in freshwater. Freshwater habitat is only a small percentage
(0.0093% by volume) of the earth’s water resources. But 41% of all fish species are
freshwater. It shows the importance of fresh water in the evolution of fishes (Figure 9.5).

Figure 9.5: Interpretation of the phylogeny of craniata with emphasis on fishes.

9.3 Survey of Fishes

Debate on taxonomy of fishes is going on for many years. Modern cladistic analysis has
revised he taxonomy of this group of vertebrates.

a) Traditional classification
The fishes are traditionally divided into super classes. This classification is based
on the presence or absence of jaws and paired appendages.
b) Cladistic analysis
It shows that some members of the agnathans (lampreys and some ostracoderms)
are more closely related to jawed fishes (gnathostomes) than to other agnathans.

These traditional groupings are paraphyletic. It has been rejected by most researchers.
But terms like agnathans and gnathosomes are common in zoological literature. They are
used as convenient non taxonomic grouping in this text book.

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9.3.1 Agnathans
They have following characteristics:
1. They lack jaws and paired appendages.
2. They have cartilaginous skeleton.
3. They have persistent notochord.
4. Two semicircular canals are present in them. Hagfishes have one semicircular
canal. It is formed by the fusion of two canals.

Ostracoderms
Ostracoderms are extinct agnathans. They are belonged to several classes. the fossils of
predatory water scorpions are found with fossils of ostracoderms. Ostracoderms were
sluggish animals. They have bony armor used for defense. Ostracoderms were bottom
dwellers. They were about 15 cm long. Most were filter feeders. They filter suspended
organic matter from the water or they extract annelids and other animals from muddy
sediments. Some ostracoderms used bony plates around the mouth as a jaw. They used
these bones to crack gastropod shells or the exoskeletons of arthropods. Agnathans have
two classes:

9.3.2 Class Myxini (mysa, slime)

1. Their mouth has four pairs of tentacles.
2. The olfactory sacs open to mouth cavity.
3. They have 5 to 15 pairs of pharyngeal slits
Example: Hagfishes

Hagfishes
Hagfishes are the members of the class Myxini. Hagfishes remain buried in the sand and
mud of marine environment. They feed on soft-bodied invertebrates and eat dead and
dying fish. Hagfishes enter the fish through the mouth. It eats the contents of its body.
They leave only skin and bones of victim fish. Anglers must be careful about hagfishes.
Hagfishes bite at a baited hook. Hagfishes swallow the hook deeply. The hook is lodged
near the anus. Hag fishes have slimy bodies. Therefore, fishermen cannot capture it. Al
last he has to cut his lines and tie on a new hook. Most zoologists now consider the
hagfishes as the most primitive group of vertebrates (Figure 9.6).

Figure 9.6: Class Myxini. Hagfish external structure.

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9.3.3 Class Cephalaspidomorphi (cephak, head + aspidos, shield + morphe, form)
1. They have sucking mouth with teeth and rasping tongue.
2. They have seven pairs of pharyngeal slits.
3. They have blind olfactory sacs.
Example: Lampreys

Lamprey
Habit and Habitat
Lampreys are belonged to class Cephalaspidomorphi. They live in marine and freshwater
environments in temperate regions (Figure 9.7).

Figure 9.7: Lamprey (a) Mouth (b) Adult fish

Nutrition
Most adult lampreys prey on other fishes. Their larvae are filter feeders. The mouth of an
adult of an adult is sucker like. It is surrounded by lips. They lips have sensory and
attachment functions. Numerous epidermal teeth line the mouth. They cover a movable
tongue like structure. Adult lamprey is attached to prey with their lips and teeth. It uses
its tongues to rasp away scales. Lampreys have salivary glands. These glands produce
anticoaglant secretions. Lampreys feed mainly on the blood of their prey. Some
Lampreys are not predatory.

Members of the genus Lampreys are called brook lampreys. The larval stages of brook
lamprey last for three years. They adults do not feed or leave their stream. They
reproduce. They die soon after metamorphosis.

Life cycle
1. Reproduction
Adult sea lampreys live in the ocean or the Great lakes. They migrate near the end
of their lives to a spawning bed in a freshwater stream. Lamprey makes their nest
small depressions in the substrate. A female attaches itself to a stone with her
mouth. A male uses his mouth to attach to the female’s head. It wraps his body
around the female. Eggs are shed in small groups over a period of several hours.
Fertilization is external. After that the adult died.
2. Development
The sticky eggs are then covered with sand. Eggs are hatched in three weeks and
ammocoete larvae are formed. The larvae move down to softer themselves in sand

259
 and liter. The larvae feed like amphioxus. Ammocoete larvae grow from 7cm to
about 17cm in three years. Finally, the larvae are metamorphosed in the adult in
several months. Its mouth becomes sucker like. It develops teeth, tongue and
feeding musculature. Now lamprey leaves the mud permanently. It goes back to sea
and becomes predator (Figure 9.8).

Figure 9.8: External structure and life history of Sea Lamprey.

9.3.4 Gnathostomes
1. They have hinged jaws and paired appendages.
2. Vertebral column replaccs the notochord.

Two major adaptations take place in the evolution vertebrate. These adaptations were the
appearance of jaws and paired appendages.
a) Jaws are used in feeding. It is responsible for a transition to more active and
predator lifestyles. Pectoral and pelvic fins of fishes are paired appendages in
fishes.
b) Pectoral fins are present just behind the head. The pelvic tins are located ventrally
and posteriorly. Both paired fins are used for steering mechanism. It increases
agility in fishes (Figure 9.9).

There are four classes of gnathostomcs.

1. Class Chondrichthyes: These are cartilaginous fishes.
2. Class Osteichthyes: These are bony fishes. Both these classes have living fishes.
3. Placoderms: They are the armored fishes. They contained the earliest jawed fishes.
These are now extinct. They left no descendants.
4. Acanthodians: It is a fourth group of ancient extinct fishes. They are more closely
related to the bony fishes.

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Figure 9.9: Paired pectoral and pelvic appendages of gnathostomes.

Q: Fill in the blanks.

i. Then evolution of fishes took place from ………………. (Ostracoderms)
ii. Members of the genus Lamprey are called …………….. (Brook lampreys)
iii. …………… are belonged to class Cephalaspidomorphi (Lampreys)
iv. …………… are the members of the class Myxini (Hagfishes)
v. Agnathans have …………. Classes (Two)

Q: Answer the following.

i. How origin of fish bones takes place?
ii. Write characteristics of agnathans.
iii. Write about life cycle of lamprey.
iv. What are armored fishes?
v. Write about ostracoderms.

9.4 Cartilaginous (chondrichthyes) and bony Fishes

(osteichthyes); Some Familiar Edible Fishes in Pakistan
9.4.1 Class Chondrichithyes (chondros, cartilage + ichthyos, fish)
These are cartilaginous fishes. Class Chondrichthyes include the sharks, skates, rays, and
rat fishes.
1. Most chondrichthians are carnivores or scavengers. Most are marine.
2. They have biting mouth parts.
3. They have paired appendages.
4. They possess dermal placoid scales and a cartilaginous endoskeleton

A. Subclass Elasmobranchii (elasmos, plate metal + branchia, gills)

1. Their cartilaginous skeleton is partially ossified (bony).
2. Placoid scales are present or they have no scales.

Examples: Sharks, skates, rays

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Sharks

1. Evolution of sharks
Sharks arose from early jawed fishes in the Devonian period, about 375 million years
ago. Certain features of bony fishes are absent in elasmobranchs. These features are swim
bladder, regulation of buoyancy, a gill cover and a bony skeleton. It is believed that these
are primitive features. This is wrong interpretation. These characteristics are lost due to
different adaptations in the two groups.
2. Body wall
Sharks have tough skin. Dermal placoid scales over the skin. These scales project
posteriorly. They give the skin tough sandpaper like texture. Posteriorly pointed scales
also reduce friction in water during swimming (Figure 9.10).

Figure 9.10: External features of shark.

3. Teeth in sharks
The teeth of shark are modified placoid scales. There is a row of teeth on the outer edge.
Another row of teeth is present behind the first row. The teeth of second row, are attached
to a band of ligament.

This band covers the jaw cartilage inside the mouth. Sharks shed the old and useless teeth
of outer row. New teeth move into their position from inside the jaw and replace them.
This replacement is rapid in young sharks. New row of teeth is developed after every
seven or eight days in them. Crowns of teeth are adapted for tearing prey or for crushing
the shells of molluscs.

Figure 9.11: Teeth of sharks.

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4. Size and habits
Sharks range in size from less than 1m to greater than 10m (e.g., basking sharks and
whale sharks). The largest sharks are not predatory. They are filter feeders. They have
modifications in pharyngeal-arch. These arches capture planktons. The great white shark
and the mako are most feared sharks. Extinct specimens have reached the lengths of 15m
in or more.

Skates and Rays

Skates and rays are specialized for life on the ocean floor. They live in shallow water.
They use their blunt teeth to feed on invertebrates. They have lateral expansion of the
pectoral tins into wing like appendages. It is a modification for life on the ocean floor.
Locomotion takes place by dorso-ventral muscular waves. This wave passes posteriorly
along the fins. They have elaborate color patterns on the dorsal surface. It provides
effective camouflage. The tail of sting ray is modified into a defensive lash. The dorsal
fin forms a venomous spine. The electric rays (Narcine and Torpedo) and manta rays
(Mania) also include in this group (Figure 9.12).

Figure 9.12: Rays and skates.

B. Sub class Holocephali (Or. holos, whole + kephalidos head)

1. Operculum covers pharyngeal slits
2. They lack scales.
3. Their teeth are modified into crushing plates.
4. Tat rid line receptors are present in an open groove.
5. It c mains about 30 species.

Example: Chimaeras (Ratfishes)

Evolution of holocephalans
Holocephalans evolved from the Chondrichthyes nearly 300 million years ago. Since then
many specializations took place in them. These specializations are: Formation of a gill
cover called an opereulum. Teeth are modified in to large plates for crushing the shells of
molluscs.

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Chimaera
It has a large head with a small mouth. Mouth is surrounded by a large lip. It has a
narrow tapering tail. Therefore, they are named as radish (Figure 9.13).

Figure 9.13: Chimaera

9.4.2 Class Osteichtfites (Or. osteon, bone + icthyus, fish)

1. Most fishes have bony skeleton.
2. Operculum covers single gill opening.
3. They have pneumatic sacs which ftmction as lungs or swim bladders.
4. They have 20.000 species. Therefore, it is a successful group of fishes.

Example: Bony fishes.

The First fossils of bony fishes were found from late Silurian deposits (approximately
405 million years old). The two subclasses of Osteichthyes evolved in Devonian period
(350 million years ago).

Sub class Sarcopterygii (Lobe finned fishes) (sark, flesh + pteryx, fin)
1. They have paired fins with muscular lobes.
2. The pneumatic sacs function as lungs.

Example: Lungfishes and coelacanths (lobe-finned fishes).

1. Lung Fishes
The air sac in these fish is changed into lungs. Therefore, they are called lung fishes.
Only three genera survive today. They all are present in the regions where seasonal
droughts are common. These fishes use lungs to breathe in stagnate and dry freshwater
lakes and rivers. The species are:

i) Neoceratodus: It lives in fresh waters of Queensland, Australia. It survives in

stagnant water by breathing in air. But they normally use gills. They can not
withstand total dry condition.
ii) Protopterus: These are found in freshwater rivers and lakes in tropical Africa.
iii) Lepidosiren: These are found in tropical South America. They can survive when
rivers or lakes are dry. They form burrow into the mud. They keep an air pathway.
They open this pathway by bubbling air to the surface. Small opening in the earth
are produced after the substrate dries. These opening are the only evidence of the

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 sense of lung fish, Lung fishes may remain in aestivation for six months or more.
Aestivation is a dormant state that helps an animal to withstand hot, dry periods.
The rain again fills the lake or river bed. The lung fishes emerge from their burrows
to feed and reproduce.

2. Coelacanths
It is a second group of sarcopterygians. The most recent coelacanth fossils are over 70
million yeaars old. However in 1938, coelacanth was found from the deep water of the
coast of South Africa. After that numerous other specimens have been caught in deep
water around the Comoro Islands of Madagascar. Then an important fish was discovered.
It is called Latimeria chalumnae. It is the closest living fish relative of terrestrial verte
brates.

Latimaria is a large fish. Its weight is up to 80 kg. It has heavy scales on its body wall.
Ancient coelacanths lived in freshwater lakes and rivers. Thus the ancestors of Latimaria
must have moved from freshwater habitats.

3. Osteolepiforms
A third group of sarcoplerygians is called Osteolepiforms. It became extinct in the
Paleozoic period. They are taken as ancestors of ancient amphibians.

Subclass Actinopterygii (Ray tinned fishes) (aktis, ray + pteryx, fin)

1. Their paired fins are supported by dermal rays.
2. The basal portions of paired fins are not muscular.
3. They have homocercal tail. In this case, the tail fin has equal upper and lower
lobes.
4. They have blind olfactory sacs.

These fishes are commonly called the ray- finned fishes. Their fins lack muscular lobes.
They usually possess swim bladders. Swim bladders are gas-filled sacs. These are present
along the dorsal wall of the body cavity. Swim bladder regulates the buoyancy.
Zoologists now realize that there are many points of divergence in the evolution of the
Actinopterygii. One modern classification system divides the Actinopterygii into two
infra classes.

a) Infra Class Chondrosteans

It contains many species. These species lived during the Permian, Triassic, and Jurassic
periods. But only 25 species remain today. Ancestral Chondrosteans had a bony skeleton.
But living members like sturgeons and paddlefishes have cartilaginous skeletons.
Chondrosteans have a tail with a large upper lobe.

Sturgeons
Most sturgeons live in the sea. They migrate into rivers to breed. Some sturgeons live in
freshwater. But they maintain the migratory habits of their marine relatives. They are
large fishes. Bony plates cover the anterior portion of the body. Heavy scales cover the

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tail. The sturgeon mouth is small and jaws are weak. Sturgeons feed on invertebrates.
Sturgeons are important for their caviar. Therefore, they are severely over-fished (Figure
9.14).

Figure 9.14: Sturgeon

Paddle fishes
Paddle fishes are large, freshwater chondresteans. They have a large paddle like rostrum.
Rostrum contains many sensory organs. These sense organs can detect weak electrical
fields. They swim through the water with their mouths open. They filter crustaceans and
small fishes. They are found mainly in lakes and large rivers of the Mississippi River
basin. They are also present in western North America and China.

b) Actinopterygians
They flourished in the Jurassic period. They succeeded most chondrosteans. Two very
primitive genera occur in temperate to warm freshwaters of North America.

1. Lepisosteus (garpike)
It has thick scales. It has lone jaws. It uses these jaws to catch fishes.
2. Amia (teleosts or modern bony fishes)
These are commonly called dogfish or bowfin. Most living fishes are members of
this group. They are called as teleosts or modern bony fishes. They have a
symmetrical caudal fin and a swim bladder.The swim bladder has lost its
connection to the digestive tract. The teleosts have diverged from ancient marine
actinopterygians in the late Triassic period. After that remarkable evolutionary
diversification take place in them. Teleosts have adapted to nearly available aquatic
habitat. The number of teleosts species are 20,000.

Evolutionary Pressures
The aquatic environments have physical characteristics. These characteristics are
important selective forces for aquatic animals. Fishes have following adaptive
characteristics:

Locomotion
Water is dense. It makes movement through it difficult. It makes movement through it
difficult. However, fishes uses less energy for swimming than running from a terrestrial
organism. The fish have stream lined. Its mucoid secretions lubricate its body surface.

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It reduces friction between the fish and the water. The buoyant properties of water also
increase the efficiency of movement fish through the water. A fish spends little energy in
support against the pull of gravity. Fishes move through the water with the help of fins.
They use the body wall to push against incompressible surrounding water. The muscle
bundles of most fishes are arranged in a special pattern. These muscles extend posteriorly
and anteriorly in a zigzag fashion. The contraction of each muscle bundle affects a large
portion of the body wall. The vertical caudal (tail) fin supplements body movements in
very efficient like fast-swimming fishes like tuna and mackerel. Their caudal fin is tall
and forked. The forked shape of the caudal fin reduces surface area.

9.5 Nutrition and the Digestive system

Evolution of Nutrition
The earliest fishes were filter feeders and scavengers . They move through the mud of
ancient sea floors. These eat decaying organic matter, annelids, molluscs or other bottom
dwelling invertebrates. The evolution of jaws transformed early fishes into efficient
predators. Therefore, fish nutrition has changed now.

Types of Food
Most modern fishes are predators. They spend much of their life searching for food. They
have different types of preys. Some fishes feed on invertebrate animals. These animals
float or swim or live in or on the substrate. Many feed on other vertebrates. Similarly, the
fishes eat different kinds of food during different period of life. Fish feed on plankton as
a larva. Adult fishes eat large prey like annelids or smaller fish.

Ingestion
Fishes swallow prey as a whole. Teeth capture and hold prey. Some fishes have teeth
modified for crushing the shells of molluscs or the exoskeletons of arthropods. Some
fishes use the suction to capture prey. The fishes close the opercula and rapidly open the
mouth. It creates negative pressure. This pressure sweeps water and prey inside the mouth

Herring, paddlefishes and whale sharks are filter feeders. They have long gill processes
called gill rakers. These gill rakers trap plankton during swimming with Open mouth. A
few fishes, such as carp, feed on different plants and small animals. A few fishes like
lamprey are external parasites for some part of lives. A few are herbivores. They feed on
plants.

Digestive tract
The digestive tract of fish is similar to other vertebrates. They have a large stomach. It
stores large meals. The enzymes are secreted in small intestine. It is the primary site for
food digestion. Sharks and other elasmobranchs have a spiral valve in their intestine. The
bony fishes possess pyloric coeca. It is pockets of the intestine. It increases absorptive
and secretary surfaces.

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9.6 Circulation and Gas exchange
All vertebrates have a closed circulatory system. Heart pumps blood. It contains red
blood cells containing hemoglobin. Blood passes through a series of arteries, capillaries
and veins. The evolution of lungs and circulatory systems take at the same time. These
changes are the loss of gills, delivery of blood to the lungs and separation of oxygenated
and deoxygenated blood in the heart (Figure 9.15).

Figure 9.15: Circulatory system of fishes (a) Bony fishes (b) Lung fishes

Blood Circulation in Fishes

Four embryological enlargements or a ventral aorta take place during development of
heart of vertebrates. ‘These enlargements are sinus venosus, atrium, ventricle and conus
arteriosus.
1. Blood flows from the venous system of fishes.
2. It passes through atrium, ventricle, and colitis arteriosus.
3. Then it enters into the ventral aorta.
4. Five afferent vessels carry blood to the gills. These vessels branch into capillaries
in gills.
5. Oxygenation takes place and blood is collected by efferent vessels.
6. It is passed to dorsal aorta. Dorsal aorta distributed it to the body.

Circulation of Blood in Lung Fishes

The lungs have altered the circulatory pattern. Circulation to gills continues. But a branch
of aortic arch VI forms pulmonary artery. This artery supply blood to lungs. Blood
returns to heart through pulmonary veins from the lungs. It enters into the left side of the
heart. The atrium and ventricle of the lung fish heart are partially divided. These partial
divisions keep deoxygenated blood separate from the oxygenated blood from the lungs. A

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spiral valve is present in the coin’s arteriosus. It directs blood from the right side of the
heart to the pulmonary artery. It directs the blood from the left side of the heart to the
remaining aortic arches. Thus distinction between a pulmonary circuit and a systemic
circuit is present in lung fishes.

Gas Exchange
Water contains 2.5 % of the oxygen present in air. Therefore, less amount of oxygen is
available to fishes in water. Fishes must pass large quantities of water across gill surfaces.
They extract a small amount of oxygen from this water. It maintains adequate levels of
oxygen in their blood stream. There are following mechanisms of inspiration and
expiration in fishes:

1. Pumping Mechanism
Most fishes have a muscular pumping mechanism. It moves the water into the
mouth and pharynx and over the gills. It also moves water out of the fish through
gill opening.
2. Ram Ventilation
This mechanism is present in some elasmobranchs and open-ocean bony fishes like
tuna. These fishes keep their mouths open during swimming. It maintains water
flow in the pharynx. This method is called ram ventilation. Elasmobranchi do not
have opercula for pumping water. Therefore, some sharks must keep moving to
survive.
3. Some elasmobranchs have gill bars with external flaps. These flaps are closed and
form an opercular cavity like other fishes. Spiracles are modified pharyngeal slits.
They open just behind the eyes of elasmobranchs. These spiracles are used as an
alternate route for water entering the pharynx.

Mechanism
Gas exchange through gills is very efficient. Gill arctics support gills. Gill filaments
extend from each gill arch. Gill filament is composed of pharyngeal lamellae. These
lamellae are vascular folds of epithelium. Branchial arteries carry blood to the gills and
into gill filaments. The arteries break into capillary in pharyngeal lamellae. Blood and
water move in opposite directions on lamellar epithelium and exchange of gases take
place. This opposite flow is called countercurrent mechanism. It maintains a
concentration gradient between the blood and the water over the entire length of the
capillary. Therefore, exchange of gases takes place efficiently (Figure 9.16).

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Figure 9.16: Gas Exchange at the Pharyngeal Lamellae. (a) The gill arches under the
operculum (b) Electron micrograph tip of a trout gill filament showing numerous
lamellae. (c,d) A comparison of an inrercurrent and parallel exchanges. (d).Oxygen
diffuse from water

9.7 Swim Bladders and Lungs

The Indian climbing perch spend its life completely on land. These fishes have gas
chambers called pneumatic sacs.

a) In some fishes, a pneumatic duct connects the pneumatic sacs with the esophagus
or another part or the digestive tract. These fishes are nonteleost and some teleosts.
Swallowed air enters these sacs. Exchange of gas occurs through its vascular
surfaces. Thus, pneumatic sacs function as lungs in the Indian climbing perch. lung
fishes, and ancient rhipidistians.
b) In other bony fishes, pneumatic sacs act as swim bladders (Figure 9.17).

270
Figure 9.17: Possible Sequence in the Evolution of Pneumatic Sacs. (a) Pneumatic
sacs may have originally developed from ventral outgrowths of the esophagus. (b)
Primitive lungs developed further during the evolution of vertebrates. (c) In most bony
fishes, pneumatic sacs are called swim bladders; and these arc modified for buoyancy
regulation.

Most zoologists believe that lungs are more primitive than swim bladder. The evolution
of curl bony fishes took place in warm, fresh water lakes and streams during Devonian
period. These rivers and streams frequently became stagnant and dried. Only those fishes
survive in this condition which had lung. The later evolution of modem bony fishes takes
places in marine and freshwater environment. Stagnation was not a problem there. In
these environments, they use pneumatic sacs in buoyancy regulation.

Buoyancy Regulation
Fishes maintain their vertical position in a column of water by four adaptations:

1. First Adaptation
The fishes incorporate low-density compounds into their tissues. Fish s are
saturated with buoyant oils.
2. Second Adaptation
The fishes use fins to provide lilt. I he pectoral fins of’ a shark arc ;limning
devices. It creates lift as the shark moves through the water. The caudal fins of
sharks have large upper lobe. It.provides upward thrust for the posterior end of the
body.
3. Third Adaptation
There is reduction in the heavy tissues of fishes. The bones of the fishes are less
dense than the terrestrial vertebrates. The development of cartilaginous skeleton in
elasmobranchs is the adaptive features. The cartilage is only slightly heavier than
water.

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4. Fourth Adaptation
The fourth adaptation is the swim bladder. A fish regulates buoyancy controlling
the volume of gas in its swim bladder. There are two adaptations in swim bladders:

a) The pneumatic duct connects the swim bladders to esophagus or another part
of the digestive tract. It occurs in garpike, sturgeons and other primitive bony
fishes. These fishes gulp air and force air into their swim bladders.
b) The swim bladders of most teleosts have lost a connection to the digestive
tract. The blood secretes gases (various mixtures of nitrogen and oxygen)
into the swim bladder. Gases are reabsorbed into the blood at the posterior
end of the bladder.

9.8 Nervous and Sensory Functions

The central nervous system of fishes consists of a brain and a spinal cord. Sensory
receptors widely distributed over the both. The receptors for touch and temperature are
distributed on the body of fishes. Fishes also possess specialized receptors for olfaction,
vision, hearing, equilibrium and balance, and for detecting water movements.

1. Olfaction Receptors
The snouts of fishes open outside by external nares. Snout has olfactory receptors.
Most fishes have blind-ending olfactory sacs. In a few fishes, the external nares
open in to nasal passages and the mouth cavity. Some fishes rely heavily on their
sense of smell. For example, salmon and lampreys return to spawn in the streams in
which they hatched years earlier. They cover distances of hundreds of kilometers
during their migrations. The characteristic odors of their spawning stream guide
them.
2. Eyes
The eyes of fishes are similar to other vertebrates. However, they are lidless. Their
lenses are rounded. They move the lens forward and backward during focusing.
3. Ear
Receptors for equilibrium, balance and hearing are present in the inner ears of
fishes. Their functions are similar to other vertebrates.
a) Equilibrium
Semicircular canals detect rotational movements. Other sensory patches
detect the direction of the gravitational pull for equilibrium and balance.
Fishes lack the outer or middle ear. Outer and middle ears conducts sound
win es to the inner ear in other vertebrates.
b) Hearing
Most fishes can hear. Vibrations pass from the water to the middle ear
through the bones of the skull. A few fishes have chains of bony ossicles.
These ossicles connect the swim bladder to the hack of the skull. Swim
bladders can amplify the vibrations. The ossicles then send them to skull.

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4. Lateral Line System
Most fishes have lateral-line system. It runs along each side and branching over the head
of fishes. The lateral line system consists of sensory pits. These pits are present in the
epidermis of the skin. These pits are connected to canals. This canal runs just below the
epidermis. The pits have receptors. These receptors are stimulated by water moving
against them. Lateral lines are used to detect water currents. They are also used to detect
a predator or a prey. Fishes can also detect low frequency sounds with these receptors.

Electric Fishes
The activities of nerves and muscles produce weak electrical fields in all organisms.
Electroreception is the detection of electrical fields that the fish or another organism
generates in the environment. Electroreception and electrogeneration has been discovered
in live hundred species of fishes in seven families of chondriehthyes and Osteichthyes.
These fishes use their electroreceptive sense for detecting prey. They also used it for
orienting towards or away from objects in the environment.

Electroreception in Sharks
The sense of prey detection is better developed in rays and sharks. Spin dogfish sharks
locate prey by electroreception. A shark can lind and eat a flounder that is buried in sand.
It will try to find and eat electrodes that are creating electrical signals similar to those that
the flounder. But a shark cannot find a dead flounder buried in the sand or a live flounder
covered by an insulating polyvinyl sheet.

Electroreception and Electrogeneration in Electric Fish

Some fishes are capable of electroreception. These can also generate electrical currents.
An electric fish (Gymnarchus niloticus) lives in freshwater in Africa. Muscles near its
caudal fin are modified into electrical discharge organ. Its current spreads between the
tail and the head. Pore like perforations are present near the head. These pores contain
electroreception. The electrical waves circulate between the tail and the head. If any
object comes between tail and head, it distorts its electric field. This distortion is detected
by changing patterns of receptor stimulation. Gymnarchus live in murky fresh water.
Thus it has limited use of eyes. Therefore, it uses electrical sense to locate prey.

Electric eel and Electric Rays

Electric eel (a bony fish) and electric ray (an elasmobranch) produce strong electrical
currents. The electric eel live in river of the Amazon Basin in South America. Electrical
currents producing organs is present in the trunk of the electric eel. It can deliver shocks
of 500 volts. The electric ray has electric organs n its fins. It can produce pulses of 50
amperes at about 50 volts. These shocks can stun or kill prey. It discourages large
predators to come close to it (Figure 9.18).

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Figure 9.18: Electric organ in electric ray.

9.9 Excretion and Osmoregulation

The maintenance of water and salt balance in the body is called osmoregulation. Fishes
must maintain a proper balance of electrolytes (ions) and water in their tissues.

Kidney
The osmoregulation is a major function of the kidneys and gills of fishes. Kidneys are
located near the midline of the body. These are present dorsal to peritoneal membrane.
This peritoneal membrane lines the body cavity. The excretory structures in the kidneys
are called nephrons. Nephrons filter nitrogenous wastes, ions, water, and small organic
compounds through glomeruli. The filtrate then passes through a tubule system. These
tubules can reabsorb essential components. The remaining filtrate in the tubule system is
then excreted.

Osmoregulation in Fresh Water Fishes

Freshwater contains few dissolved substances. Therefore, osmotic uptake of water across
gill, oral and intestinal surfaces take place. Thus excretion and defecation lose essential
ions. These fishes have following adaptations:
1. The freshwater fishes never drink water to control excess water and ion loss. They
take water in only during feeding.
2. The nephrons of freshwater fishes possess large glomeruli and short tubule
systems. Reabsorption of some ions and organic compounds takes place after
filtration. Their nephrons have short tubule system. Thus little water is reabsorbed.
Therefore, freshwater fishes produce large quantity of very dilute urine.

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3. Ions can be lost through the urine. These are also lost by diffusion across gill and
oral surfaces. The gills of these fishes can absorb ions by active transport. It
compensates this ion loss. Fresh water fishes also get some salts through their food
(Figure 9.19 a).

Osmoregulation in Marine Fishes

Marine fishes face the opposite problems. Their environment contains 3.5% ions. But
their tissues contain 0.65% ion. Therefore, marine fishes face the problem of water loss
and accumulation of excess ions. They drink water to compensate the loss of water. They
eliminate excess ions by excretion, defecation and active transport through gill. The
nephrons of marine fishes possess small glomeruli and long tubule systems. Therefore,
less blood is filtered than fresh water fishes. Water is efficiently reabsorbed from the
nephron (Figure 9.19 b).

Figure 9.19: Osmoregulation. (a) Fresh water (b) Marine fishes

Osmoregulation in Elasmobranchs
Flasmobranchs have a unique osmoregulatory mechanism. They convert some of their
nitrogenous wastes into urea in the liver. But most other fishes excrete ammonia. Urea is
distributed in tissues all over the body. Thus enough urea is stored in the body. It makes
body tissues isosmotic with sea water. Therefore, elasmobranchs do not loss water to
their environment. Thus they save the energy used in water conservation. Urea can
disrupt important enzyme systems in the tissues. Therefore, this adaptation requires the
development of tolerance to high levels of urea.

Despite this unique adaptation, elasmobranchs still regulate the ion concentrations in their
tissues. They have ion-absorbing and secreting tissues in their gills and kidneys. The
elasmobranchs possess a rectal gland. It removes excess sodium chloride from the blood
into the cloaca.

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Osmoregulation in Diadromous Fishes
The fishes which migrate between freshwater and marine environments are called
diadromous. Salmon and marine lampreys migrate from the sea to freshwater to spawn.
The freshwater eel (Anguilla) migrates from freshwater to marine environments to
spawn. The gills of the diadromous fishes can balance the ions in the body. But
osmoregulatory power may not develop during all life-history stages. For example, young
salmon cannot enter the sea until certain cells on the gills develop ion-secreting powers.

Exertion in Fishes
Fishes do not face much problem in removing the nitrogenous wastes. These nitrogenous
wastes are byproducts of protein metabolism. 90% of nitrogenous wastes are eliminated
as ammonia through gill by diffusion. Ammonia is a toxic substance. But the aquatic
organisms can diffuse ammonia easily in the surrounding water. The remaining 10% of
nitrogenous wastes are excreted as urea creatine or creatinine. These wastes are produced
in the liver. They are excreted through the kidneys.

9.10 Reproduction and Development

Fertilization
Fishes produces a large number of eggs. It increases the chance of fertilization for the
survival of the fish. There are other adaptations to increase the chance of fertilization.
Some fishes mating behavior. It ensures fertilization. Some fishes show nesting behavior.
Nest protects eggs from predation, sedimentation and fouling.

Mating may occur in large groups. One individual releases eggs or sperm. It often release
spawning pheromone. This pheromone induces many other adults to spawn. Huge lasses
of eggs and sperm are released into the open ocean. It ensures the fertilization of many
eggs.

Some fishes have specialized structures for transfer of sperms. Male elasmobranchs have
modified pelvic fins called claspers. The male inserts clasper into the cloaca of a female
during copulation. Sperm travel along grooves of the clasper. Fertilization occurs in the
female reproductive tract. A large number of eggs are fertilized than in external
fertilization. Thus, fishes with internal fertilization produce fewer eggs.

Development
The fishes may be:
1. Oviparous: The majority of fishes are oviparous. Their eggs develop outside the
femalefrom stored yolk.
2. Ovoviviparous: Some elasmobranchs are ovoviviparous. Their embryos develop
in a modified oviduct of the female. Nutrients are supplied from yolk stored in the
egg.
3. Viviparous: Other elasmobranchs like gray reef sharks and hammerheads are
viviparous. Their oviduct is modified in to a placenta like outgrowth. Placenta
transfers nutrients from the female to the yolk sacs of developing embryos. Internal

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 development of viviparous bony fishes occurs in ovarian follicles. The eggs are
retained in the ovary in guppies (Lebistes). Fertilization and early development take
place there. Embrvos are then released into a cavity within the ovary. The
development starts. Nourishments are supplied by yolk and ovarian secretions.

Parental Care in Fishes

1. Care before hatching

In many fishes, care of the embryos is limited or absent. However, some fishes
construct and tend nests. Some fishes carry embryos during development. Clusters
of embryos are brooded in special pouches. These pouches are attached to some
part of the body. Embrvo may also he brooded in the mouth. Some best-known
brooders are the seahorses (Hippocampus) and pipefishes (Syngnathus). Males of
these fishes carry embryos in ventral pouches. Development takes place there. The
male Brazilian catfish broods embryos in an enlarged lower lip.
2. Care after hatching
Most fishes do not care for young after hatching. However, sunfishes and
sticklebacks provide short-term care after hatching of young.
i) Male sticklebacks collect fresh plant material into a mass. The youngs live in
this mass of leaves. Sometimes, young moves away from the nest. The male
bring it in its mouth and spits it back into the nest. Sunfish males also show
similar behavior.
ii) The Cichlidae shows longer-term care. The young are brooded in mouth in
some species. In others the species tend young in a nest. After hatching, the
young come out from the parent’s mouth or nest. The parent signals danger
with a flicking of the pelvic fins. So young return quickly to nest or mouth.

Q: Fill in the blanks.

i. Most ………………. are carnivores (Chondrichthians)
ii. Sharks arose from early …………….. in the Devonian period (Jawed fishes)
iii. Skates and …………. are specialized for life on the ocean floor (Rays)
iv. …………….. evolved from the Chondrichthyes nearly 300 million years ago
(Holocephalans)
v. A third group of sarcoplerygians is called ……………. (Osteolepiforms)

Q: Answer the following.

i. What are placoid scales?
ii. What is chimaera?
iii. What do you know about lung fishes?
iv. What are sturgeons?
v. What is electroreception?

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9.11 General Characteristics of Amphibians and Their Trends for
Land Habitat; Amphibians As Unsuccessful Land Vertebrates
9.11.1 Evolutionary Perspective (amphibia, living a double life)
Class Amphibia includes frogs, toads, salamanders and caecilians. Class name shows that
amphibians move between water and land. They spend one stage or their life in water and
another on land. Amphibians are tetrapods. Term tetrapod is a non-taxonomic It is used
for vertebrates other than fishes. Most tetrapods are adapted on land.

9.11.2 Phylogenetic Relationships

Evolution of Land Vertebrates
Many adaptive radiations produced in vertebrates during first 250 million years of
vertebrate history. Thus the vertebrates filled most aquatic habitats Prehistoric waters
contained many active, powerful predators however vertebrates sere absent on land. Only
some arthropods lived there as predators. Therefore, there was no vertebrate predator on
land to capture animals that moved near the edge. Some vertebrates developed lungs for
breathing and muscular fins for moving. These animals found ample food on land in the
form of arthropods. The arthropods are the major component of the diet of most modern
amphibians. Thus evolution of amphibians took place. Then adaptive radiations were
produced in amphibians. It produced greater variety of amphibians than exists today.
Later convergent and parallel evolution took place in amphibians. Then widespread
extinction took place (Figure 9.20).

First Amphibian
No one knows what the first amphibian was. The structure of limbs, skulls and teeth of
lchthyostega are similar to the earliest amphibians. Two lineages of amphibians are
formed in the late Devonian and early Carboniferous periods. These lineage can be
differentiated by the structure of roof and the attachment of posterior portion of the skull
to each other.

1. Amniotic Lineage: One lineage of amphibians became extinct in the late

Carboniferous period. An amniotic egg evolved in this group. It resisted dryness.
This lineage is called the amniotic lineage. This lineage formed reptiles, birds; and
mammals.
2. Nonamniote lineage: A second lineage flourished in the Jurassic period. Most of
animals of lineage have become extinct. But some of them gave rise to the three
orders of living amphibians. This lineage is called the non-amniote lineage.

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Figure 9.20: Evolutionary relationship.

9.12 Survey of Amphibians

Amphibians occur on all continents except Antarctica. There are three orders of
amphibians: Caudata, (salamanders), Anura, (frogs and toads) and Gymnophiona
(caecilians).

9.12.1 Order Caudata (cauda, tail + Or. ara, to bear)

The characteristics are:
1. They possess a tail throughout life.
2. Both pairs of legs are present in them.
3. They lack middle ear.
4. They have 150 species.

Example: Salamander, newts (Figure 9.21)

Figure 9.21: Salamander

Salamanders
Habit and Habitat
115 species of salamanders live in North America. Most terrestrial salamanders live in
moist forest-floor with litter. They produce aquatic larvae. A number of families live in

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caves. These caves have constant temperature and moisture conditions. It is an ideal
environment for salamander. Salamanders have two families:

1. Plethodontidae: They are fully terrestrial salamander. They lay their eggs on land.
Their voting are similar to the adult.
2. Salamandridac: They are commonly called newts. They spend most of their lives
in water. They retain caudal fins.

The length of salamanders is from a few centimeters to 1.5 m. The largest North
American salamander is the hellbender (Crysobranchus alleganiensis). Its lengths reach
up to 65c m.

Life cycle
Fertilization: Most salamanders have internal fertilization. Males produce a pyramidal,
gelatinous spermatophore. Spermatophore has a cape of sperms. It deposits
spermatophore on the substrate. Females pick up the sperm cap with the cloaca. It stores
the sperm in a special pouch called spermatheca.

Development: Eggs are fertilized as they pass through the cloaca. They are deposited
singly or in clumps or in strings. Larvae are similar to adults but smaller. Larvae possess
external gills a tail fin larval dentition and a rudimentary tongue. The aquatic larva
undergoes metamorphoses and forms a terrestrial adult. Many other salamanders undergo
incomplete metamorphosis. They are paedomorphic. A phenomenon in which larva
ecome sexually mature while still showing larval characteristics is called
paedomorphosis.

Order Gymnophiona (gymnon, naked + ophineos, like a snake)

1. They have elongate body.
2. They are limbless.
3. They are segmented by annular grooves.
4. They are specialized for burrowing.
5. Their tail is short and pointed.
6. They have rudimentary left lung.
7. They have about 160 species.
Example: Members of the order Gymnophiona are the caecilians.

Caecilians
They live in tropical regions. Caecilians are worm like burrowers. They feed on worms
and other invertebrates in the soil. Folds in the skin separate the muscle bundles.
Therefore, the caecilians appear segmented. A retract le tentacle is present between their
eyes and nostrils. It transports chemicals from the environment into the olfactory the roof
of the mouth. Skin covers the eyes. Thus caecilians are nearly blind (Figure 9.22).

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Figure 9.22: Caecilians

Fertilization is internal in caecilians. Larval stages are passed within the oviducts. Larva
stages the inner lining of the oviducts with fetal teeth for feeding. The young come out
from the female. It is similar to adults. Other caecilians lay eggs. These eggs develop into
aquatic larvae or embryos. Embryos develop on land.

9.12.2 Order Anura (Salientia) (a, without + oura, tail)

1. Anurans live in most moist environments. A few even occur in very dry deserts.
2. They are tailless amphibians.
3. The limbs are modified tor jumping and swimming. Hind limbs are long and
muscular and end in webbed feet.
4. They have five to nine presacral vertebrae with transverse processes.
5. The postsacral vertebrae are fused into a rod like urostyle.
6. Tympanum and larynx are well developed.
7. They have about 3.500 species of frogs and toads.
Example: Frogs, Toads (Figure 9.23)

Life Cycle of Anurans

Anurans have diverse life histories. Fertilization is external. The eggs and larvae are
aquatic. Their larva is called tadpoles. It has well-developed tail. Larval bodies lack
limbs. Larva develops limbs near the end of their larval stages. The larvae are herbivores.
They possess a proteinaceous beak like structure. It is used in feeding. Anuran larvae
undergo rapid metamorphosis and adult body is formed. There are not many differences
between toad and frogs, Toads have dry and warty skin. They are mostly live on land.
True toads belong to the family Bufonidae.

Figure 9.23: Difference between Frog and Toad

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 Frog Toad
1 They have large sizes. i They have small sizes.
They spend most of their life in They spend most of their life
2 water or near water. ii on land.
All their eggs are equal in
3 They have longer hind legs. iii size.
Their skin is transparent and
4 less warty. iv They have dry and warty skin

9.13 Evolutionary Pressures in Amphibians

The lives of the most amphibians is divided into freshwater and land. Thus they show
adaptations to both environments. The amphibians are supported by the buoyant of the
water. They exchange gases with the water. They face the same osmoregulatory problems
as freshwater fishes. On the other hand amphibian support themselves again gravity on
land. They exchange gases with the air.

9.14 External Structure and Locomotion

Skin
Function: Vertebrate skin protects them from microorganisms, ultraviolet Light,
dessication and mechanical injury. The skin of amphibians is used in gas exchange,
temperature regulation, absorption and storage of water.

Skin Glands: Amphibian skin lacks scales, feathers or hairs. However, it is highly
glandular. It secretions protect the body. These glands keep the skin moist and prevent it
from drying. They also produce sticky secretions. These secretions help male to attach
with female during mating. It also produces toxic chemicals that discourage the predators.
The skin of many amphibians is smooth. But some epidermal thickenings produce warts
and claws. It makes the skin sandpapery. The deposition of keratin or the formation of
hard, bony areas produces these warts.

Coloration: Chromatophores are specialized cells in the epidermis and dermis of the
skin. They are responsible for skin color and color changes. Cryptic coloration (warning
color), aposematic coloration (matching with the habitat) and mimicry are common in
amphibians.

Support and Movement

Water buoys and supports aquatic animals. Their skeletons protect the internal organs and
attach the muscles. It also prevents the body from collapsing during movement. However,
there are different adaptations in terrestrial vertebrates. Their skeleton is modified to

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provide support against gravity. It is strong and it supports the powerful muscles (Figure
9.24).

Axial skeleton

1. Skull
The amphibian skull is flattened. It is relatively smaller. It has lesser bony elements
than the skull of fishes. These changes lighten the skull. Thus body can support it
easily. There are certain changes in jaw structure and musculature. Therefore, the
terrestrial vertebrates can crush prey in the mouth.
2. Vertebral Column
The vertebral column of amphibians provides support and flexibility on land. It
supports the weight of the body between anterior and posterior paired appendage.
a) Every vertebra has a supportive process called zygapophyses. It prevents the
vertebral column from twisting.
b) The amphibians have a neck. The first vertebra is cervical vertebra. It
moves against the back of the skull. It allows the head to nod vertically.
c) The last trunk vertebra is a sacral vertebra. This vertebra attaches the pelvic
girdle with the vertebral column.
d) Sternum is present in the anterior trunk region. It supports the forelimbs and
protects internal organs. It is reduced or absent in the Anura.

Figure 9.24: Skeletons of Amphibians. (a) The salamander skeleton is divided into four
regions: cervical, trunk, sacral and caudal. (b) Interlocking processes, called
zygapophyses. (c) A frog skeleton shows adaptations for jumping

Appendicular Skeleton
The exact origin of the bones of vertebrate appendages is not known. However,
similarities are present in the structure of the bone. of the amphibian appendages and the
bones of the fins of ancient sarcopterygians fishes. It suggests homologies between these
two. Joints are present at the shoulder, hip, elbow, knee, wrist and ankle. These joints
allow freedom of movement. They also develop better contact between the body and the
substrate.

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The pelvic girdle of amphibians consists of three bones. These are ilium. ischium and
pubis. These bones firmly attach pelvic appendages with the vertebral column. These
bones are important for support on land. Tetrapods depend on appendages for
locomotion. They do not depend on body wall for locomotion. Therefore, the body wall
musculature is reduced and appendicular musculature has become strong (Figure 9.25).

Figure 9.25: Origin of tetrapod appendages. A comparison of (a) the fin bones of
rhiphidistian (b) the limb bones of tetrapod.

Mode of Locomotion
1. Salamanders: They have unspecialized form of locomotion. It is like undulatory
waves in fish. Terrestrial salamanders also move with the help of limb and body
movements. They show alternate movement of appendages and muscle
contractions. It bends the body into a curve. This curve moves the limb forward
(Figure 9.26).
2. Caecilians: They show an accordion (musical instrument)-like movement. In this
case, adjacent body parts push or pull forward at the same time.
3. Anurans: The long hind limbs and the pelvic girdle of anurans are modified for
jumping. The dorsal bone of the pelvis (the ilium) extends anterior. It is attached to the
vertebral column. Their urostyle extends posterior and attaches to the pelvis. These
skeletal modifications stiffen the posterior half of the anrans. Long hind limbs and
powerful muscles are used for jumping efficiently. Pectoral girdle is attached to the
skull and vertebral column by elastic connective tissues and muscles. These connective
tissues are used as shock absorbers for landing on the forelimbs.

Figure 9.26: Salamander Locomotion

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9.15 Nutrition and the Digestive System
Types of Food
Most adult amphibians are carnivores. They feed on different invertebrates. Some
anurans have more diverse diet. For example, a bull frog eats small mammals, birds and
other anurans. The prey size and availability determine the type of diet. Most larvae are
herbivorous. They feed on algae and other plant matter. Most amphibians locate their
prey by sight. They simply wait for prey to pass by it. Olfaction plays an important role in
prey detection in aquatic salamanders and caecilians.

Mechanism of Feeding
Many salamanders are unspecialized in their feeding methods. They use their jaws to
capture prey. Anurans and plethodontid salamanders use their tongue and jaws in flip and
grab feeding mechanism. A true tongue is first seen in amphibians. The amphibian
tongue is attached at the anterior margin of the jaw. It folds back over the floor of the
mouth. Mucous and buccal glands are present on the tip of the tongue. They release
sticky secretions. When prey comes within range, an amphibian flicks out its tongue. The
tongue turns over, and the lower jaw is depressed. The head tilts on its cervical vertebra.
The tip of the tongue traps the prey. Then tongue and prey are licked back inside the
mouth. All of this happens in 0.05 to 0.15 second. The amphibian holds the prey by
pressing it against teeth on the roof of the mouth. The tongue and other muscles of the
mouth push food towards the esophagus. The eyes sink downward during swallowing.
They also push the food towards the esophagus (Figure 9.27).

Figure 9.27: Flip and Grab feeding in toad

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9.16 Circulation
Gas exchange and temperature regulation
Circulatory System
The circulatory system of amphibians is adapted for both aquatic and terrestrial habitats.
The separation of pulmonary and systemic circuits is less efficient in amphibians than
lung fishes. The atrium is partially divided in urodeles. It is completely divided in
anurans. The ventricle has no septum. A spiral valve is present in the conus arteriosus. It
directs the blood into pulmonary and systemic circuits.

Blood Circulation to Lungs: The exchange of gases takes place through the skin and
lungs in amphibians. Therefore, blood entering the right side of the heart is also
oxygenated. All gas exchange occurs through the skin and other moist surfaces when
amphibians are in water. Therefore, blood in right atrium has a higher oxygen
concentration than left atrium. Left atrium receives blood from the lungs. Therefore,
blood vessels leading to the lungs constrict. It reduces blood flow to the lungs for
conserving energy. The hibernating frogs and salamanders use this mechanism during
their hibernation in winter.

Aortic Arches: Adult amphibians have lesser aortic arches than fishes. The corms
arteriosus give rises to three blood vessels:
1. Carotid Artery (aortic arch III): It supplies blood to the head.
2. Systemic Artery (aortic arch IV): It supplies blood the body.
3. Pulmonary Artery (aortic arch VI): It carries blood to lungs.

Lymphatic System: The amphibians have a well developed lymphatic system. It is

composed of blind ending vessels. It filters fluids, proteins and ion from capillaries in
tissue spaces and returns them to the circulatory system. The lymphatic snstem also
transports water absorbed across the skin. The amphibians have contractile vessels called
lymphatic hearts. These hearts pump fluid through tile lymphatic system. Lymphatic
system between the body wall muscles and the skin transport and store water. This water
is absorbed through the skin.

9.17 Gas Exchange

Air contains 20 times more oxygen than water. Therefore, terrestrial animals spend less
energy for gas-exchange than aquatic animals. But the exchanges of gases require moist
surfaces. Thus terrestrial animal loss water during exposure of respiratory surfaces to air.
There are three types of respirations in amphibians (Figure 9.28).

1. Cutaneous respiration: The skin of amphibians is kept moist. Amphibian skin is

richly supplied with capillaries. Thus their skin functions as a respiratory organ.
Gas exchange through the skin is called cutaneous respiration. Cutaneous
respiration can take place both in water and on land. This ability allows a frog to

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 hibernate in winter. In salamanders, 30 to 90% of gas exchange occurs through the
skin.

Figure 9.28: Amphibian lung

2. Buccopharyngeal respiration: Gas exchange can also take place through moist
surfaces of the mouth and pharynx. It is called buccopharyngeaI respiration. It is
only 1 to 7% of total gas exchange (Figure 9.29).

Figure 9.29: Buccal pumps and buccopharyngeal ventilation.

3. Pulmonary respiration: Most amphibians possess lungs. Lungs are absent in

plethodontid salamanders. The lungs of salamanders are relatively simple sacs. The
lungs of salamanders are subdivided into chambers. It increases the surface area for
gas exchange. Pulmonary ventilation occurs by a buccal pump mechanism.
Muscles of the mouth and pharynx create a positive pressure. This pressure forces
air into the lungs.

Ratios of Different Methods of Gas Exchanges:

Cutaneous and buccopharyngeal respiration have a disadvantage. Their percentage in the
respirations is very small. The quantity of gas exchanged across these surfaces cannot be
increased with increase in metabolic rate. However, lungs compensate this shortcoming.
More gas exchange takes place through lungs with the increase of environmental
temperature and activity. At 50 C, 70% of gas exchange occurs through the skin and
mouth of a frog. At 25° C, the gas exchanged through skin and mouth remains the same.

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But pulmonary respiration increases. Now die exchange through skin and mouth is only
about 30% of total oxygen exchange.

Gill: Amphibian larvae and some adults respire by external gills. Cartilaginous rods are
present between embryonic pharyngeal slits. These rods support three pairs of gills. The
gills are reabsorbed and pharyngeal slits are closed during metamorphosis and lungs
become functional.

Temperature Regulation
Amphibians are ectothermic. They depend on external heat sources to maintain body
temperature. Water has powerful heat-absorbing properties. Therefore, it quickly absorbs
heat from aquatic amphibians. Thus their temperature becomes equal to the temperature
of water. But their body temperatures can differ from the environment on land.

Temperature regulation is mainly behavioral. Amphibians have different adaptations for

regulation of temperature:
1. They cool their body by evaporative heat loss.
2. Many amphibians are nocturnal.
3. They remain in cooler burrows or under moist leaf litter during the hottest part of
the day.
4. Amphibians warm themselves by basking in the sun or on warm surfaces. Body
temperatures may rise 100 degree centigrade above the air temperature. Metabolic
reactions are increased with the increase in body temperature. Thus heat also -
reases the functions of digestive system. Therefore, basking after a meal is mmon.
It increases the growth. and the fat deposition Fat deposition is necessary r periods
of dormancy.

The daily and the seasonal environmental temperatures of amphibians fluctuate widely.
Therefore, amphibians have wide range of tolerance of temperature. Critical temperature
for salamander lie between 2 and 27oC. Critical temperature for some anurans is between
3 and 41° C.

9.18 Nervous and Sensory Functions

Brain
Nervous system of amphibians is similar to that of other vertebrates. The brain of adult
vertebrates develops from three embryological subdivisions. The brain of amphibians is
divided into three parts:
1. Forebrain: It contains olfactory centers. It also has regions that control color
change visceral functions.
2. Midbrain: It contains a region called the optic tectum. Optic tectum collects
sensory i 111wmation and initiates motor responses. The midbrain also processes
visual sensory information.
3. Hindbrain: It functions in motor coordination. It regulates heart rate and the
respiration.

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Serve Organs
i) Nerve Endings
Sensory receptors are distributed over the skin in many amphibians. Some of these
are bare nerve endings. These nerve endings respond to heat, cold, and pain.
ii) Lateral–line System:
They also have lateral-line system like fishes. A lateral line system is present in all
aquatic larvae, aquatic adult salamanders and some adult anurans. Lateral-line
organs are present singly or in small groups. They are distributed in the lateral and
dorsolateral surfaces of the body and on head. These receptors respond to low
frequency vibrations in the water. Lateral line receptors are less important on land.
iii) Chemoreceptors
Chemoreception is an important sense in many amphibians. Chemoreceptors are
present in the nasal epithelium, in the mouth, on the tongue, and over the skin.
Olfaction is used in late recognition. It can detect toxic chemicals.
iv) Eyes
The amphibians are primarily sight feeders. Therefore, vision is the most important
sense them. There are number of adaptations in the eyes of amphibians for
terrestrial environments.
a) The eyes of some amphibians are on the front of the head. It forms the
binocular vision and well-developed image. This image is necessary for
capturing prey.
b) Other amphibians like some salamanders have smaller lateral eyes. They do
not form binocular vision.

Structure of Eye
The lower eyelid is movable in amphibians. It cleans and protects the eye. Its transparent
part is called the nictitating membrane. The eyeball retracts into the orbit of the skull and
the nictitating membrane covers the cornea. Amphibians also have orbital glands. These
glands lubricate and wash the eye. Eyelid and glands keep the eye tree from dust. The
lens is large and rounded. It is present in the back of cornea. A fold of epithelium called
the iris surrounds the lens. The iris can dilate or constrict and control the size of the pupil
(Figure 9.30).

Figure 9.30: Structure of eye

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Accommodation of Eye
The bending (refracting) of light rays at a focal point on the retina is called focusing or
accommodation. Light waves moves from air into the cornea. These waves are refracted
due to change in density between the two media. The lens increase refraction. The eye of
amphibian can focus on distant objects at rest. But the protractor lentis muscle move the
lens forward for focusing near objects. Receptors called rods and cones are present in the
retina. Cones detect co lours. Thus amphibians can distinguish between some
wavelengths of light with the help of cones. Amphibians have complex neuronal
interconnections in the retina. Therefore, amphibian can distinguish between flying insect
prey, shadow of predator and background movements.

4. Ears
The auditory system of amphibians is adapted for life on land. It transmits both substrate
borne vibrations and airborne vibrations. The ears of anurans consist of a tympanic
membrane, a middle ear and an inner car. :
i) Tympanic Membrane: The tympanic membrane is a piece of integument. It
stretches over a cartilaginous ring. This ring receives airborne vibrations. It
transmits vibrations to the middle ear.
ii) Middle Ear: It is a chamber beneath the tympanic membrane. A bone of middle
ear called the stapes (columella) touch the tympanic membrane. Stapes transmits
the vibrations of the tympanic membrane into the inner ear. Ear receives two types
of vibrations:
a) High- frequency (1,000 to 5,000 Hz): These are air borne vibrations. These
are transmitted to the inner ear though tympanic membrane.
b) Low-frequency (100 to 1,000 Hz): These are substrate borne vibrations.
These are transmitted through the front appendages and the pectoral girdle.
These waves finally enter into the inner ear through operculum.

Control of Sound Frequency: The anuran can lock operculum and stapes with the help
of muscles. Thus they can screen out high or low-frequency sounds. The anurans use low
and high frequency sounds in different situations. For example, mating calls are high-
frequency sounds. Thus it is used only during breeding season. The low-frequency
sounds are used for warning of predators.

Ear in Salamander: Salamanders lack a tympanic membrane and middle ear. They live
in streams, ponds, caves, and beneath leaf litter. They have no mating calls. They hear
only low-frequency vibrations. These vibrations are transmitted through the substrate and
skull to the stapes and inner ear.

Equilibrium: The inner ear of amphibians has semicircular canals. These canals detect
rotational movements.

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9.19 Excretion and Osmoregulation
Excretion
The kidneys of amphibians are present on the sides attached to the dorsal wall of the body
cavity. A duct opens in cloaca. The cloaca has ventral outgrowth called urinary bladder.
Urinary bladder stores urine. There are following adaptations in the amphibians for
excretions:
1. Aquatic Amphibians: The nitrogenous wastes of amphibians are ammonia or
urea. The freshwater amphibians excrete ammonia. It is the immediate end product
of protein metabolism. Therefore, they do not spend energy on converting
ammonia into other compound. The ammonia diffuses into the surrounding water.
Therefore, it does not produced toxic effect.
2. Terrestrial Amphibians: Amphibians that spend more time on land excrete urea.
Urea is produced from ammonia in liver. Urea is less toxic than ammonia. But it
still requires large quantities of water for its excretion. Urea can be stored in the
urinary bladder. Some amphibians excrete ammonia in water and urea on land.

Osmoregulation
Osmoregulation is a biggest problem of the amphibians. They must remove excess water
and conserve essential ions. Amphibian kidneys produce large quantities of hypotonic
urine. Their skin and walls of the urinary bladder transport Na, Cl and other ions into the
blood.

The amphibians conserve water on land. Adult amphibians do not drink water. Their skin
is also not impermeable like other tetrapods. Their kidneys are unable to produce
hypertonic urine. Instead, amphibians loss water by their behavior. They show following
type of behaviours:

1. Nocturnal Amphibians: They do not come out in desiccating conditions. Many

terrestrial amphibians are nocturnal. The go to high humidity area during day
times. These areas are present under stones, in logs , leaf mulch or burrows. They
come out at night and absorb the lost water through skin.
2. Diurnal Amphibians: They live in areas of high humidity. They rehydrate
themselves by entering the water.
3. Reduction in Aurface Area: Many amphibians reduce exposed surface area of
body to air. It reduces loss of water by evaporation. They curl their bodies and tails
into tight coils. They bring their limbs close to their bodies. Many individuals come
close to each other in groups. It reduces overall surface area.
4. Protective Covering: Some amphibians have protective coverings. It reduces the
water loss. Their skin has some hardened regions. These regions are resistant to
water loss. They close the mouth of burrows with these hardened regions of skin. It
maintains high humidity in the burrow.
5. Cocoon Formation: Some amphibians form cocoons. It covers the body during
long periods of dormancy and reduces the loss of water. Cocoons are made from
outer rs of the skin. This layer of skin detaches and become parchment like. These

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 cocoons open only at the flares or the mouth. Cocoon can reduce water loss by 20
to 50%.
6. Rehydration: The skin is also most important structure for rehydration. The
amphibian flattens its body on moist surfaces. It absorbs water. The permeability,
vascularization, and epidermal layering of skin promote water reabsorption. Minute
channels increase surface area. These areas spread water over surfaces which are
not directly exposed to water.
7. Storage of Water: Amphibians can also temporarily store water. Water is stored in
urinary bladder and lymph sacs. This water is absorbed to replace the loss of water
by evaporation. Amphibians living in very dry environments can store water equal
to 35% of their total body weight.
9. Reproduction, Development and Metamorphosis
Amphibians are dioeious. Their ovaries and testes are located near the dorsal body
wall. Fertilization is external. The developing eggs lack any resistant coverings.
Therefore, development takes place in moist habitats in water. A few anurans have
terrestrial nests.

These nests are covered with foam. This foam reduces the loss of water. Sometimes these
nests are placed near water. In a few species, larval stages are passed in the egg
membranes. The immature hatch into an adult like body.

Fertilization and Development

External fertilization is less common in salamanders. Only 10% salamanders have
external fertilization. Remaining salamanders develop spermatophore and fertilization is
internal. Eggs are deposited in soil or water. Or they may be retained in the oviduct
during development.

All caecilians have internal fertilization. 75% caecilians have internal development.
Tadpole larvae are formed during development of amphibians. Amphibian tadpole larva
is different from the adults. It has different mode of respiration, form of locomotion and
diet. These differences reduce competition between adults and larvae.

Breeding Behaviour
Amphibian: Internal factors like hormones and external factors determine the timing of
reproductive activities. Temperature is most important environmental factor in temperate
region. It induces physiological changes in the amphibians. These changes control the
breeding and breeding periods. Breeding occurs in spring and summer. In tropical
regions, rainy seasons induce breeding in amphibians. The individuals locate breeding
sites and identify potential mates by courtship behavior. It also prepares the individuals
for reproduction. It also ensures that eggs are fertilized and deposited at proper locations.

Salamander: Salamanders use olfactory and visual signs for courtship and mating. The
anurans use male vocalizations and tactile signs. Many species congregate in one location
during breeding activity. Male vocalizations are species specific. These are used for
initial attraction and contact between mates. Then tactile cues become more important.

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The male mounts on the female. The male grasps the female with the help of his
forelimbs around her waist. The male is dorsal to the female. This positioning is called
amplexus. It lasts from I to 24 hours. The male releases sperm and the female releases
eggs during amplexus.

Caecilian: Little is known about the breeding behavior in caecilian. Males have an
intermittent organ (copulatory organ). It is a modification of the cloaca! wall. Therefore,
fertilization is internal.

Vocalization
Sound production has reproductive function in male anurans (Figure 9.31).mFrogs
produces different calls.

1. Advertisement Calls: It is used to attract females to breeding areas. It is also an

announcement for other males the given territory is occupied. Advertisement calls
are species specific. These calls induce psychological and physiological changes in
female. As a result the female get ready to breed.

Figure 9.31: Vocalization of frog

2. Receptive Calls: If female get ready for breeding then it produces receptive calls.
3. Release Call: Release call informs the partner that it is incapable of reproducing.
Some male try in amplexus unresponsive females. Such females also give release
calls. Sometimes, a male mistakenly identified other male as female. Thus otlit r
male also produce release call.
4. Distress Calls: These calls are not associated with reproduction. These calls are
produced in response to pain or danger of a predator. The calls are much aloud. It
frightens the predator to release the frog. The distress call of the South American
jungle frog is as loud as a cat in distress.

Sound Producing Apparatus

The sound production apparatus of frogs consists of the larynx and its vocal cords. It is
called laryngeal apparatus. It is well developed in males. They possess a vocal sac.
Vocal sac is a diverticulum from the lining of the buccal cavity. Lungs force air in vocal
cords and cartilages of the larynx. This air produces vibrations in them. Muscles control

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the tension of the vocal cords and regulate the frequency of sound. Vocal sacs act as
resonating structures. They increase the volume of the sound.

Advantages of vocalization
1. The amphibians live in widely dispersed habitats. Therefore, it is useful for them to
attract mates from distant places.
2. Many species of frogs collected at the same pond for breeding. It becomes difficult
for them to find their proper mate. Vocalizations help to attract their mate of same
species.

Parental Care
Parental care increases the chance of development of an egg. But it requires large amount
of amount from the parents. Mostly both parents care for the egg clutches. It is most
common form of parental care in amphibians. It may be:

1. Maternal Care: It takes place in species with internal fertilization, e.g.

salamanders an caecilians.
2. Parernal Care: It takes place in species with external fertilization, e.g. anurans. It
involves aeration of aquatic eggs, cleaning and moistening of terrestrial eggs,
protection of eggs from predators, or removal of dead and infected eggs.

Eggs transported during development on land. Females of the genus Pipa carry eggs on
their backs. Two species of Rheobatrachus were discovered in Australia. Rheobatrachus
females brooded tadpoles in their stomachs. The young come out through mouths the
female. But it is not known whether the female swallow egg or tadpole larvae. The
stomachs of female expanded and till most of her body cavity during brooding. Thus the
stomach stops producing digestive secretions. Viviparity and ovo-viviparity occur in
salamanders and caecilians.

Metamorphosis
Metamorphosis is a series of structural, physiological and behavioral changes that
transform a larva into an adult. A number of environmental conditions influence the
time required for metamorphosis. These conditions are collections and availability of
food. Metamorphosis is directly controlled of neurosecretions of hypothalamus,
hormones of the anterior lobe of the pituitary gland and the thyroid gland.
1. Minor morphological changes take place during metamorphosis of caecilians and
slamanders. Reproductive structures develop, gills are lost, and a caudal fin is lost.
2. Major changes take place during metamorphosis of tadpole into the small frog in the
anuran. Limbs and lungs are developed. The tail is reabsorbed and the skin thickens.

Many changes take place in the head and digestive tract.

Paedomorphosis
A phenomenon in which larva becomes sexually mature while still showing larval
characteristics is called paedomorphosis. The mechanism of metamorphosis explains

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paedomorphosis in amphibians. Paedomorphosis mostly takes place in some families of
salamander. In other families, the occurrence of paedomorphosis is variable. It is
influenced by environmental conditions. Two conditions promote paedomorphosis:

1. Some salamanders do not respond to thyroid hormones. Therefore, it becomes

paedomorphic.
2. Some larvae do not produce the hormone necessary tbr metamorphosis. Therefore,
they become paedomorphic.

Amphibians in Peril
Frogs and salandam are disappearing rapidly. We do not know the exact reasons of their
disappearance. Some reasons are:

1. Local Factors: Some local factors influence the amphibian populations. These
factors are cutting of forests. It allows sunlight to reach at the forest floors. It dries
the moist habitats that amphibians require. Mining, drilling, industrial and
agriculture and urbanization also destroy habitat. However, amphibian populations
are also disappearing in the regions where local damage has not occurred.
2. Acid Deposition: Another factor partly responsible for the decline of amphibian
populations is acid deposition. Amphibian embryos are sensitive to changes in the
pH of their watery environment. A pH I of 5 or less will kill most embryos. In the
northern hemisphere the environments has become 100 times more acidic than they
were before the industrial Revolution.
3. Ultraviolet Radiations: Ultraviolet radiation, especially in the 280 to 320 nm
range (UV-B) also kills amphibian eggs and embryos.
4. Pollutants: Some pollutants are increasing acid deposition. It depletes the ozone
shield of the earth. Therefore, these pollutants are also responsible for the
reductions in amphibian populations.

These explanations do not completely explain the problems with amphibian populations.
None, of these problems exist in remote tropical regions, where, amphibian populations
are experiencing similar declines. The causes are unknown. It is necessary that the
governments and funding agencies give funds to help scientists. So that they can discover
the causes of extinction of amphibians and stop their extinction.

9.20 Further Phylogenetic Considerations in Amphibians

There is a problem in amphibian phylogeny. This problem is the relationship among the
three orders of modern amphibians. There are two hypotheses about the origin of
amphihians:

Monophyletic lineage
Some zoologists place anurans, urodeles, and caecilians into a single sub class
Lissamphibia. It shows a common ancestry or modem amphibians. It suggests that they

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are more closely related to each other than to any other group. Supporters or this
classification give common characteristics between these groups. These characters are:
1. They have complex.stapes and operculum.
2. They use skin in gas exchange.
3. They have similar structures of the skull and teeth.

Paraphyletic lineage
Other Zoologists think that modern amphibians were derived from two nonamniotic
lineages. They noted difference in structure of vertebral column of these two lineages.
They suggests that the three orders have separate origins. It shows that Amphibia is a
paraphyletic group. It should be divided into multiple monophyletic taxa. This
controversy is still going on.

Three sets of evolutionary changes occurred in amphibian lineages for movement onto
land. Two of these changes occurred much earlier. Therefore, these changes are present
in all amphibians.

1. One change was adaptation in the skeleton and muscles of these amphibians. These
changes allowed greater mobility on land.
2. A second change occurred in jaw mechanism and moveable head. These changes
helped the amphibians to uses insect efficiently as food resources on land. A jaw-
muscle arrangement was derived from fishes. This method was adaptive by early
tetrapods for feedings on insects in terrestrial environments.
3. The third set of changes occurred in the amniote lineage. It is the development of on
egg. This egg is resistant to drying. The aminiotic egg is not completely
independent of water. But the extraembroynic membranes protect the embryo from
desiccation. These membranes store wastes and promote gas exchange. This egg
also has leathery or calcified shell for protection. It is porous and allows gas
exchange with the environment. These evolutionary changes gave rise to the
remaining three vertebrate group: reptiles, birds and mammals.

Q: Fill in the blanks.

i. Amphibians are ………… (Tetrapods)
ii. Most terrestrial ……………….. live in moist forest-floor with litter
(Salamanders)
iii. Caecilians are …………. like burrowers (Worm)
iv. …………….. are specialized cells in the epidermis and dermis of the skin
(Chromatophores)
v. Every vertebra has a supportive process called …………………
(Zygapophyses)

Q: Answer the following.

i. What do you know about non-amniote lineage?
ii. Differentiate frog and toad.
iii. Write about skeleton of amphibians.

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 iv. How salamanders locomotion takes place.
v. What are aortic arches in amphibians?

9.21 Reptiles; Why Reptiles Are Considered As Successful Land

Vertebrates
Reptiles: The First Amniotes
9.21.1 Evolutionary perspective (reptos, to creep)
Reptiles were the first vertebrates that possess amniotic eggs. Amniotic eggs have extra
embryonic membranes. These membranes performs the following functions:

1. They protect the embryo from dessication.

2. These support the embryo like cushion.
3. These membranes promote gas transfer.
4. These membranes remove the waste materials.

The amniotic eggs of reptiles and birds also have following parts (Figure 9.32).

1. Shells: Birds and reptiles have hard or leathery shell. This shell protects the
developing embryo.
2. Albumin: The albumen cushions the embryo. It also provides moisture and
nutrients to the embno.
3. Yolk: The yolk supplies food to the embryo.

Figure 9.32: Reptilian egg

All these features are adaptations for development on land. The amniotic egg is the major
synapomorphy (distinguishing characteristic). It distinguishes the reptiles, birds, and
mammals from non-aminote vertebrates. The amniotic egg has played an important role
in success of vertebrate in terrestrial habitats. Thus the members of this class flourished
on land. Living reptiles are turtles. Lizards, snakes, worm lizards, crocodilians and the
tuatara.

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9.22 Adaptive Radiations in Reptiles
Fossil records of mans reptiles are abundant. But much is not known about the origin of
reptilian. The ancestral amniote has not yet been discovered. The adaptive radiation of
the early amniotes began in the late Carboniferous and early Permian periods. The
adaptive radiation of insects also occurred at the same time. Insect were the major prey of
early amniotes. There are many lineages of reptiles. These lineages can he distinguished
with the help of skull structure and modifications in jaw, muscle attachment.

1. Sub Class Anapsida (an, without + hapsis, arch): These reptiles lack openings or
fenestrae in the temporal region of the skull. The turtles represent this lineage
today.
Recent evidence suggests that the ancipsid lineage does not have close evolutionary
ties to other reptiles. Changes have occurred in their long evolutionary history. The
fundamental skull and shell is found in 200-million- year-old fossils. Evidence of
the anapsid lineage was found in 245-million-year-old rocks from south Africa.
2. Diapsida (Or. di, two): A second group of reptiles are diapsid. They have upper
and lower openings in the temporal region of the skull. Some taxonomists believe
that this condition has a single lineage. Some divide this group into two subclasses.
a) Subclass Lepidosauria: It includes modern snakes, lizards and tuataras.
b) Subclass Archosauria: Extensive evolutionary radiation occurred in it in the
Mesozoic era. It includes the dinosaurs. Most archosaurs are now extinct.
Living archosours are crocodilians and birds. Birds are closely related to
dinosaurs.
3. Synapsids (syn, with): They possess a single dorsal opening in the temporal region
the skull. All of the synapsids have become extinct. But are most important to
evolutionary point of view. A group of synapsids is therapsids. It gave rise to the
mammals.

Cladistic Interpretation of the Amniotic Lineage

Cladistic Method
Cladistic taxonomic methods have reexamined and reinterpreted the amniotic lineage.
Cladistics believes that the amniotic lineage is monophyletic. The birds and the mammals
have a common ancestor with the reptiles. It is the rule of the cladistic analysis that
animals with most recent common ancestor must be placed in a particular taxon. Birds
and mammals have common ancestor with reptiles. But traditional classification does not
include birds and mammals in class Reptilia. According to cladistic interpretations, birds
should be classified as “reptiles” with dinosaurs. Similarly, cladistic interpretations also
develop close relationship between mammals and ancient synapsid reptiles.

Evolutionary systematics
1. Evolutionary systematists disagree that these cladistic interpretation. These explain
that the birds and mammals have important morphological, behavioral and
ecological characteristics. For example, feathers and endothermy in the birds, hair,

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 mammary glands and endothermy in mammals. These characteristics suggest that
birds and mammals should be placed in separate classes. Evolutionary systematists
take them as important characters. They conclude that these characteristics have
great importance in the taxonomy of these groups.

9.23 Survey of the Reptiles

Reptiles have following characteristics:
1. Their skull has one surface (condyle) for articulation with the first neck vertebra.
2. Respiration takes place by lungs.
3. They have metanephric (kidney Formed From the lower part of the middle part of
ancestral kidney) kidneys.
4. They have internal fertilization and amniotic eggs.
4. Reptiles have dry skin. It has keratinized epidermal scales. Keratin is a resistant
protein. It is found in epidermally derived structures of amniotes. It is chemically
bonded to phospholipids. Therefore, it prevents loss of water through body
surfaces.
5. They are found on all continents except Antarctica. However, they are most
abundant on tropical and subtropical environments. There are 17 orders of reptiles.

9.23.1 Order testudines (chelonia) turtles (L. testudo, tortoise)

1. Teeth are absent in adults. They are replaced bv a horny beak. The have keratinized
beak.
2. They have short broad body.
3. Their shell consists or a dorsal carapace and ventral plastron.
4. Turtles have 225 species. They have a bony shell.
5. Their limbs articulate internally with the ribs.

Example: Turtles

Shell in turtles
Carapace: The dorsal portion of the shell is the carapace. Carapace is formed bv the
fusion of vertebrae, ribs and bones in the dermis of skin. Keratin covers the bone of the
carapace.

Plastron: The ventral portion or the shell is plastron. It is formed from bones of pectoral
girdle and dermal bone. Keratin also covers it. The shell of some turtle has flexible areas
or hinges. These hinges attach the anterior and posterior edges of the plastron. The hinges
close the openings of the shell during withdrawal of body into the shell.

Turtles have eight cervical vertebrae. These vertebrae can form an S-shaped structure. It
draws the head into the shell (Figure 9.33).

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Figure 9.33: Structure of turtle

Reproduction and life span

Turtles have long life spans. Most turtles reach at sexual maturity after seven or eight years.
They can live 14 or more years. The age of large tortoises of the Galapagos Islands may be
above 100 years. All turtles are oviparous. Females use their hind limbs to dig nests in the
soil. There they lay eggs. Their clutches contain 5 to 100 eggs. They cover the eggs with and
soil. Development takes from four weeks to one year. The parent does not attend to the eggs
during this time. The young are independent of the parent at hatching.

Turtles in danger
Turtules have slow rates of growth. Therefore, they have long juvenile periods.
Therefore, they have high mortality rates. Thus turtles are becoming extinct. Turtle
conservation programs have been started in recent years. Dogs and other animals are
hunting young turtles. It has severely threatened some species of turtles. These species
are sea turtles. They make nest on certain beaches year after year. Conservation
programme of sea turtles is difficult. Theyhave ranges of thousands of square kilometers
of ocean. The protectiveareas include waters of different countries.

9.23.2 Order Rhynchocephalia (Rhynchos. Snout + Kephole, Head)

Rhynchocephalia has only one surviving species called tuatara (Sphenodon pundalus). It is
lizard like reptile. It has remained unchanged since it evolved from its extinct relatives. Its
relatives were present at the beginning of the Mesozoic era nearly 200 million years ago.

Distinctive features: Tooth attachment and structure distinguish the tuatara from other
reptiles. They have two rows of teeth on the upper jaw and a single row of teeth in the
lower jaw. They bite the birds with these teeth. This biting can decapitate a small bird. It
was widely distributed in New Zealand. The population of tuatara is affected by human

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influences and domestic animals. It is now present only on far off offshore islands. It is
protected by New Zealand law. It is oviparous and it shares underground burrows with
ground nesting seabirds. Tuataras come out of their burrows at dusk and dawn. They feed
on insects or sometimes on vertebrates.

9.23.3 Order Squamata (Snakes and Lizards) (Squama, Scale + Ata, Hear)
1. They have specific characteristics of the skull and Jaws. Temporal arch are reduced
or absent. They quadrate bone is moveable or fixed.
2. They are the most successful and diverse group or living reptiles.
Examples: Snakes, lizards, worm lizards

The order Squamata is divided into three suborders. Ancestral member of these suborders
originated in the lepidosaur lineage about 150 million years ago.

a) Suborder Sauria: The Lizards

1. Suborder Sauria have about 3, 300 species of lizards.
2. The lizards have two pairs of legs.
3. Their upper and lower jaws arc united anteriorly.
4. Few lizards are legless. But they retain remanants of a pectoral girdle and
Sternum.
5. Lizards vary in length. Their length may be from a few centimeters to 3m.
6. Many lizards live on surface substrates. They move down under rocks or logs
when necessary. Others are burrowers or tree dwellers.
7. Most lizards are oviparous. Some are oviviparous or viviparous. They
deposit eggs under rocks or debris or in burrows.

Geckos
Geckos are commonly found on the walls of houses in semitropical areas. Their body is
short and stout. They are nocturnal. They produce the sound of clicking. They have large
eyes, their pupil contract and narrow during the day. It widens at night. This is an
adaptation for night vision. They have adhesive disks on their digits. These disks help in
clinging to trees and walls (Figure 9.34).

Figure 9.34: Geckos

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Iguanas
Iguanas have heavy bodies. They have short necks and distinct heads. This group
includes the marine iguanas of the Galapagos Islands and the flying dragons (Draco) of
South East Asia. Darco has lateral folds of skin. It is supported by ribs. The ribs of Draco
can expand to form gliding surface. It can glide 30m or more.

Chameleons
It is another group of iguanas. It is found in Africa and India. They use a long sticky
tongue to capture insects. Anolis, the “pet-store chameleon”is also an iguanid. But it is
not true chameleon. Chamelons and Anolis are well known for their ability to change
color. They change their body colour in response to illumination, temperature or their
behavioral state.

Venomous Lizards
They are the gila monster (Heloderma suspectum) and the Mexican beaded lizard
southwestern North America (Heloderma horridum). These heavy bodied lizards live in
southwestern America. The surface of their teeth have groove. Venom is releascd into
these grooves. The lizard chews the prey and introduces venom into it. Lizard bites are
not fatal to humans.

b) Suborder Serpentes
There are about 2, 300 species in the suborder Serpentes. Majority of snakes are not
dangerous to humans. But three hundred species are venomous. 30, 000 to 40, 000
people the from snake bites each year widely. Most of these deaths are in Southeast
Asia. In the United States about one hundred people die each other from snake
bites.

Body Structure
i) Snakes are elongated and lack limbs. Vestigial pelvic girdles and appendages are
sometimes present.
ii) They contain more than two hundred vertebrae and pairs of ribs. Joints between
vertebrae make the body very flexible.
iii) Snakes have adaptations in skull for swallowing large prey. Their upper jaws are
moveable on the skull. The upper and lower jaws are loosely joined. Therefore,
each half of the jaw can move independently.
iv) They have different mechanism for focusing the eyes and the morphology of the
retina than lizards.
v) Their body is elongated and narrow. Therefore, left lung is reduced and
gallbladder, right kidney and gonads are displaced.
vi) Most snakes are oviparous. A few species give to live young like the New World
boas and garter snakes.

Evolution of Snakes
Zoologists debate the evolutionary origin of the snakes. The earliest fossils of snakes are
135-million-year-old Cretaceous period. Some zoologists believe that the earliest snakes

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were borrowers. Loss of appendages and changes in structure take place for borrowing
habit. It is believed that early snakes were aquatic or they lived in densely vegetated
areas. Therefore, lost the legs.
c) Suborder Amphisbaenia: Worm Lizards (amphi , douhle+boen, walk)
This order has about 13 species. They are specialized borrowers and live in soils.
They are found in Africa, South America, the Carribean West Indies and the
Middle East. Most of them are legles. Their skulls are wedge or shovel shaped. A
single median tooth is present in the upper jaw. It distinguishes amphisbaenians
from all other vertebrates.The skin of amphisbaenians has ring like folds ,called
annuli. Their skin in loosely attaches to the body wall. Skin bulges outward by the
contraction of muscles. It forms an anchor against a burrow wall. Amphishaenians
move easily forward and backward. Thus, they are named as amphisbeanians. They
feed on worms and small insects. They are oviparous.

9.23.4 Order Crocodilia (Krokodeitos, Lizard)

The order Crocodilia has 21 species. Dinosaurs and crocodilian, are derived from
thearchosaurs. They has certain special skull characteristics:

1. They have openings in the skull in front of the eve.

2. They have triangular eye orbits.
3. They have laterally compressed teeth.
Examples: Lining crocodilians are alligators, crocodiles, gavials and caimans.

Adaptations in crocodiles: Little changes take place in crocodilians over their 170-
million year history.
i) Snout: Their snout is elongated. It is used to capture prey by a sideways movement
of the head. The nostrils are at the tips of the snout. Thus animal can breath mostly
submerged.
ii) Air Passage Way: Air passage ways open into rear of the mouth and throat. There
is a flap of Tissue near the hack of the tongue. It forms a watertight seal. Therefore,
breathing takes place without inhaling water in the mouth. Secondary palate is a
plate of hone. It separates the nasal and mouth passageways.
iii) Tail: They have muscular elongate compressed tail. It is used or swimming. It is
also used for offense and defense.
iv) Digestive System: Teeth are used only for seizing prey. Food is swallowed as a
whole. But crocodiles hold the large prey. It rotates it and tears it into small pieces
a he stomach is gizzard like. The crocodilians swallow rocks and other objects.
They use these rocks for breaking the ingested Food.
iv) Reproduction: Crocodilians are oviparous. They display parental care like birds.
Nesting behavior and parental care shows that both birds and crocodiles have
common ancestor.

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9.24 Evolutionary Pressures in Reptiles
The reptiles have striking adaptations for terrestrial life.
Adaptations in Chuckwalla (Sauromalus obesus)

It lives in the deserts of Southwestern United States. It has following adaptations:

i) Adaptation in Summer: It can survive in late summer at temperate above 40
degree centigrade. Chukwallas browse on plants. These plants wither. Therefore,
chuckwallas aestivate to withstand these hot and dry conditions. Chukwallas
disappear below ground during aestivation.
ii) Adaptation for Winter:
Temperature becomes model ate during the winter. But little rain falls. Thus lift in
the desert is still not possible. Therefore, summer sleep of chuckwalla enters into
winter sleep. Rain fall started in March. Therefore, greenery and appears in the
desert. The chukwalla comes out from sleep. The chuckwalla browses and drinks
water. It stores large amount of water under its skin.
iii) Defense from Predators: Predators cannot prey chukwallas easily. If threatened, a
chukwalla enters into rock crevice. It inflates lungs with air. It increases its forms a
wedge entrance or the rock walls. There is friction of its the rocks. Therefore,
chukwalla cannot be removed from rock.

9.25 External Structure and Locomotion

9.25.1 Skin
Skin of reptiles has no respiration functions. Reptilian skin is thick, dry or keratinized.
Scales are modified for various functions. For example, the snakes have large belly
scales. The scales provide contact with the substrate during locomotion. Reptilian skin is
less glandular than that of amphibians. Skin glands secrete phermones. These
pheromones functions in sex recognition and defense (Figure 9.35).

Figure 9.35: Skin of Reptiles

The chromatophores of reptiles are dermal in origin. Cryptic coloration, mimicry and
aposematic coloration occur in reptiles. Colors also function in sex recongnition and
thermoregulation.

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Ecdysis
The process in which reptiles periodically shed their outer epidermal layers, is called ecdysis.
All reptiles undergo ecdysis. The blood supply to the skin does not move in the epidermis.
The outer epidermal cells lose contact with the blood supply and die. The lymph moves
between moves the inner and outer epidermal layers. It loosens the epidermis. Ecdysis begins
in the head region. The epidermal layers come off in one piece in many lizards and snakes. In
other lizards, epidermal layers broken into small pieces. The frequency of ecdysis is different
in different species. It occurs more in young than in adults.

9.25.2 Support and Movement

Skeleton
The reptailians are inherited skeleton from ancient amphibians. The skeletons of reptiles
show many notifications. The skeleton is highly ossified. Thus it provides greater
support.

1. Skull: Their skull is longer than that of amphibians. They have secondary palate.
Secondary palate partially separates the nasal passages from the mouth cavity.
Palate was evolved in archosaurs. It was an adaptation for breathing when the
mouth is full of water or blood. It is also present in other reptiles. They also have
longer snouts. It increases the sense of olfaction.
2. Vertebrae: Reptiles have more cervical vertebrae than amphibians. The first two
cervical vertebrae are atlas and axis. They provide greater freedom of movement to
head. An atlas articulates with a single condyle on the skull. It helps in nodding.
Axis is modified for rotational movements. They have different number of cervical
vertebrae. It provides additional neck flexibility.
3. Ribs: The ribs of reptiles may be highly modified. The ribs of snakes have
muscular connections to large belly scales. It helps in locomotion. The cervical
vertebrae of cobras are attached with some special ribs. Cobra flares these ribs in
aggressive displays.
4. Pelvic girdle: The pelvic girdle is attached to the vertebral column by two or more
sacral vertebrae.
5. Autotomy: The caudal vertebrae ot man often possess a vertical fracture plane. If a
lizard is grasped by the tail, caudal vertebrae are broken. Therefore a portion of the
tail is lost. The loss of tail is called autotomy. Autotomy is an adaptation that
allows a lizard to escape from a predator. Sometimes, the predator runs away from
lizard after seeing its broken moving tail. The lizard later regenerates the lost
portion of the tail.

9.25.3 Locomotion
There are three types of locomotion in reptiles:

1. Locomotion in primitive reptiles is similar to salamanders. The body move low

between paired appendages. The appendages extend laterally and move in the
horizontal plane.

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2. The limbs of other reptiles are elongated and slender. They remain closer to the
body. The knee and elbow .joints rotate posteriorly. Thus, the body moves higher
from the ground. Thus the legs support the both vertically.
3. Many prehistoric reptiles were bipedal. The walking on the hind limbs is called
bipedalism. They had a narrow pelvis. Ihey have a heavy outstretched tail for
balance. Bipedal locomotion treed the front appendages. Thus these appendages are
used capturing of prey or flight in some animals.

9.25.4 Nutrition and the Digestive System

Tongue: Most reptiles are carnivores. But turtles eat almost all the things. The tongues of
the turtles and crocodilians are non-protrusible. It helps in swallowing. Some lizards and
the tuatara have sticky tongues. It is used or capturing the prey. The extended tongue of
chameleons exceeds their body length.

Modification in snakes for swallowing

The skulls of snakes are greatly modified for feeding. The bones of the skull and jaws are
attached loosely. These bones move away from for ingestion of prey. In this way, snakes
can ingest larger than a snake’s normal head size. The bones of the upper jaw on the
skull. The halves of both of the upper and lower jaws are attached loosely by ligments at
anterior side. Therefore, each half of the upper and lower jaws can move independently.
Opposite sides of the upper and lower jaws are moved forward and retracted alternately
alter the capturing of prey. Their teeth are posteriorly pointed. These teeth prevent the
prey from escaping. They also force the food into the esophagus. The glottis is much
forward in snake. Thus they can breathe during swallowing of prey (Figure 9.36).

Figure 9.36: Feeding adaptation of snake (b) Skull of viper (c) Maxillary bones

Biting Apparatus and Biting Mechanism

1. Vipers
Vipers possess hollow fangs. These fangs are present on the maxillary bone at the
anterior margin of the upper jaw. These fangs are connected to venom glands. The
Maxillary Bone of the vipers is hinged. It can moved backwards. Thus when the

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 snake mouth is closed the fangs fold back and it lie along the upper jaw. When the
mouth opens, the maxillary bone rotates. It swings down the fangs. Thus the fangs
project outward from the mouth. Now vipers may strike at objects (Figure 9.37).

Figure 9.37: Biting apparatus in snakes

2. Rear fanged snakes

Rear-fanged snakes have groove in rear teeth. Venom is passed through grooves
and injected into the prey during swallowing.These snakes usually do not bite.
Therefore, they are harmless to humans. However, the African boomslang
(Dispholidus typus) have killed men.
3. Coral snake, sea snake and cobra
The fangs of coral snakes, sea snakes and cobras are attached to the upper jaw. It
remains in an erect position in opened mouth. The fangs lit into a pocket in the
outer gum of the lower jaw when the mouth is closed. Fangs have a groove or it is
hollow. The muscles of the venom glands contract and inject venom into the fangs.
Some cobras can spit venom at its prey. This venom may cause blindness.

Venom
Venom glands are modified salivary glands. The venoms of most snakes are mixtures of
neurotoxins and hemotoxins.
1. Neurotoxin: Neurotoxin attacks on nerve centers. It causes respiratory paralysis.
The venoms of coral snakes, cobras and sea snakes are neurotoxins.
2. Hemotoxins: Hemotoxins break blood cells. It attacks blood vessel linings. The
venoms of vipers are primarily hemotoxins.

9.25.5 Circulation
Gas exchange and temperature regulation

Circulatory System
Th circulatory system of reptiles is based on amphibians. The blood of reptiles must
travel under high pressures.

1. The reptiles possess two atria. These atria are completely separated in the adult.
Veins from the body and lungs open into them. The sinus venosus is absent in
reptiles except in turtles. It has become a patch of cells and act as a pacemaker.

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2. The ventricle of most reptiles is incompletely divided. The ventricular septum is
complete only in crocodilians.
3. The ventral aorta and the conus arteriosus divide during development. They form
three major arteries that leave the heart.

a) A pulmonary artery: It leaves the ventral side of the ventricle. It takes blood to
the lungs.
b) Two systemic arteries: One systemic artery arises from the ventral side of the
heart. Second systemic artery arises from the dorsal side of the heart. It takes blood
to the lower body and the head.

Circulation of Blood
The deoxygenated blood enters into the ventricle from the right atrium. It leaves the heart
through the pulmonary artery and moves to the lungs. Pulmonary veins bring oxygenated
blood from lungs and transfer it into left atrium. Blood then enters into the ventricle from.
It leaves the heart through left and right systemic arteries.

Mixing of Blood: An adaptation

There is incomplete separation of the ventricle in most reptiles. The pulmonary artery
contracts and some blood moves from pulmonary circuit to the systemic circuit. All
reptiles do not breathe constantly. Therefore, the movement of blood from pulmonary
circuit to systemic circuit has advantage for reptiles. The breathing by lung stops when
turtles withdraw into their shells. They also stop breathing during diving. During periods
of apnea (“no breathing“), blood flow to the lungs is limited. It conserves energy. It
allows more efficient use of the pulmonary oxygen supply.

9.25.6 Gas Exchange

Reptiles exchange respiratory gases through internal respiratory surfaces. Thus they do
not lose large quantities of water. Larynx is present in them. However, vocal cords are
absent in them. Cartilages support the respiratory passages of reptiles. Their lungs are
partitioned into sponge like interconnected chambers. Lung chambers provide the large
surface area or gas exchange.

9.25.6 Mechanism of Respiration in Most Reptiles

Negative pressure mechanism is responsible for lung ventilation. A posterior movement
of the ribs and the body wall expands the body cavity. It decreases pressure in the lungs.
Thus lung draws air into the lungs. Elastic recoil of the lungs and forward movements of
the ribs and body wall compress the lungs. Thus air is expelled out of it.

Mechanism of Respiration in Turtles

The ribs of turtles are a part of their shell. Thus movements of the body wall and ribs are
impossible. Therefore, turtles exhale by contracting muscles. These contractions force the
viscera (Internal organs) upward and compress the lungs. They inhale bv contracting
muscles that increase the volume of the visceral cavity. It creates negative pressure lung.
This pressure draws air into the lungs.

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Temperature Regulation
The terrestrial animals face high temperature (65 to 70°C). This temperature is not
suitable for life. Thus temperature regulation is important in terrestrial animals.

1. Ectotherms: The animals which use external heat sources for thermoregulation arc
called ectotherms. Most reptiles are ectotherms.
2. Endotherms: The animals which generate internal heat during metabolism are
called endotherms. Some reptiles like monitor lizards and brooding Indian pythons
arc endotherms. Female pythons coil around their eggs. It raises its body
temperature by 7.3oC above the air temperature. It uses metabolic heat to raise this
temperature.

Reptiles regulate their body temperature by following methods.

1. Heat regulation by hibernation and aestivation: Some reptiles can survive in
wide temperature fluctuations (2 to 41o C or some turtles). However, body
temperatures are regulated within a narrow range between 25 and 37oC. If they are
unable to maintain this range, they remain within the range in this retreat.
2. Behavioral methods of heat regulation: Most thermoregulatory activities of
reptiles are behavioral. A lizard orients itself at right angles to the sun’s rays to
warm itself. It presses its body tightly on a warm surface to absorb heat by
conduction. A lizard orients its body parallel to the sun’s rays to cool itself. It seeks
shade or burrows. It take its body erect prostrate (legs extended and tail arched) to
reduce conduction from warm surfaces. Many reptiles are nocturnal in hot climates.
3. Physiological methods of heat regulation: Various physiological mechanisms
also regulate body temperature. Some reptiles use panting for releasing neat.
Panting releases heat through evaporative cooling. Marine iguanas absorb heat by
basking in the sun. It divert blood to the skin and arm up quickly. Marine iguanas
reduce heart rate and blood flow to the skin during ing into the ocean. It slows
down heat loss. Chromatophores also help in emperature regulation. Dispersed
chromatophores increase the rate of heat bsorption.
4. Heat regulation by torpor: Many temperate reptiles withstand cold winter
temperatures by entering into torpor. Torpor is an inactive stage. The body
temperatures and metabolic rates decrease during torpor. The body temperatures of
reptiles in torpor are not regulated. It is a difference from the true hibernators.
5. Heat regulation by hibernacula: The solitary reptiles migrate to a common site
and spend winter there. These animals clumped together. Heat loss from these
groups is called hibernacula. Exposed surface area reduces hibernacula.
Sometimes, the animals can freeze and die in cooler winter. Death from freezing is
an important cause of mortality for temperate reptiles.

9.25.7 Nervous and Sensory Functions

The brain of reptiles is similar to the brains of other vertebrates. The cerebral hemspheres
are larger than amphibians. This increase of size of brains has improved the smell. The
optic lobes and the cerebellum are also enlarged. It shows that reptiles much depend on
vision. They have better coordination of muscle functions.

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Sense Organs
The reptiles have complex sensory systems. It is evidenced by a chameleon’s method of
feeding. It has protruding eyes. Its eyes move independently. Each eye has different field
of view. Initially, the brain keeps both images separate. But when they an insect, both
eyes converge on the prey. As a result binocular vision is formed. It helps chameleon to
determine whether the insect is within range of the chameleon s tongue.

1. Eyes
a) Focusing Mechanism: Vision is the dominant sense in most reptiles. Their
eves are similar to amphibians. Snakes moves the lens forward for focusing
the nearby objects. Iris contract and places pressure on itrcous body. Vitreous
both is gel-like in the posterior region of the eye. The displacement of this
gel pushes the lens forward. All other reptiles have different method to locus
on nearby objects. Their ciliary muscles press the ciliary body against the
lens. It changes the shape of lens from elliptical to more spherical. The
spherical lens is used for focusing on nearby object. Reptiles have a greater
number of cones than amphibians. Thus they have well-developed color
vision.
b) Protection: The eyes of reptiles have upper and lower eyelids, a nictitating
membrane and a blood sinus. These structures protect and cleanse the surface
of an eye. In snakes and some lizards, the upper and lower eyelids fuse in the
embryo. It forms a protective window of clear skin called the spectacle. The
blood sinus is present at the base of the nictitating membrane. It swells with
blood and force debris to the corner of the eye. It is rubbed out from this
corner. Horned lizards rupture this sinus and blood come out from it. It is a
defensive act to confuse the predators.
c) Median Eye: Some reptiles possess a median (parietal) eye. This eye is
developed from outgrowths of the roof of the forebrain. In the tuatara, this
eye has a lens, nerve and a retina. The parietal eye is less developed in other
reptiles. Parietal eyes are covered by skin. Thus it cannot form images.
However, parietal eye can differentiate light and dark periods. Thus it is used
to locate the position of the sun.
2. Ear
The structure of reptilian ears varies. The ears of snakes detect substrate
vibrations.They lack a middle ear cavity, an auditory tube and a tvmpanic
membrane. A bone of the jaw articulates with the stapes. The jaws and stapes
receive substrate vibrations. Snakes can also detect airborne vibrations. In other
reptiles, a tympanic membrane is present on the surface. Or it may be in a small
depression in the head. The inner ear of reptiles is similar to amphibians.
3. Olfactory Senses
Olfactory senses are better developed in reptiles than amphibians. They have partial
secondary palate. It provides more surfaces for olfactory epithelium. Many reptiles
possess blind- ending pouches. This pouch opens into the mouth cavity through the
secondary palate. These pouches are called Jacobson’s (vomeronasal) organs.
These organs are present in diapsid reptiles. However, they are best developed in

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 the squamates. Jacobson’s organs develop in embnonie crocodilians. But it
degenerate in adults. Anapsids (turtles) lack these olfactory organs. The protrusible,
forked tongues of snakes and lizards are accessory olfactory organs. It detects
chemicals present in air. A snake’s tongue come out and then moves to the
Jacobson’s organ. Jacobson detect odor molecules. Tuataras use Jacobson’s organs
to taste objects present in its mouth.
4. Pit Organs
Rattlesnakes and other pit vipers have pit organs. Pit organs are present on each
side of the face between the eye and nostril. It is a heat-sensitive organ. Pit organs
form depressions. These depressions are lined with sensory epithellium. These are
used to protect objects with temperatures different from the snake’s surroundings.
Pit vipers are nocturnal. Their pit organs help them to locate small, warm-blooded
prey.

9.25.8 Excretion and Osmoregulation

Excretion

Excretory Organs: The kidneys of embryonic reptiles are similar fishes and amphibians.
Terrestrial animals have larger body size. They have higher metabolic rates. However,
kidneys are capable of processing wastes with little water loss. Their kidneys have in
many nephrons. The functional unit reptiles are called metanephric kidneys. Their
function depends on a circulatory system. It delivers more blood to kidney at greater
pressures. Thus kidney filters large quantities of blood.

Mechanism of Excretion: Most reptiles excrete uric acid. It is nontoxic and insoluble in
w tier. It precipitates in the excretory system. The urinary bladder or the cloacal absorb
water. The uric acid is stored in them in a paste like form. Nontoxic uric acid be stored in
egg membranes. Thus it has made possible the development of embryos in terrestrial
environments.

Osmoregulation
There are many adaptations in reptiles to reduce water loss by evaporation. These are:
1. Their excretory system reabsorbs water.
2. They have internal respiratory surfaces.
3. They have impermeable exposed surfaces.
4. The behaviors that help regulate temperature also help conserve water.
5. Most reptiles are nocturnal. They do not come out hot day time. The burrowing at
day time reduces water loss.
6. When water is available, many reptiles store large quantities of water in lymphatic
spaces. Lymphatic spaces are present under the skin or in the urinary bladder.
7. Many lizards possess salt glands below the eyes. These glands remove excess salt
from the body.

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9.25.9 Reproduction and Development
Vertebrates have internal fertilization and the amniotic egg. It has adapted them
completely on land. The amniotic egg is not completely independent of water. Pores are
present in the eggshell. They allow the gas exchange. But it allows water to evaporate.
Amniotic eggs require a large amount of energy expenditures. This energy is provided by
parents in the form of stored food. Parental care occurs in present in some reptiles. They
maintain high humidity around the eggs.

Fertilization
Reptiles have internal fertilization. Fertilization occurs in the reproductive tract of the
female. Then protective egg membranes are formed around the eggs. All male reptiles
possess an intermittent organ. It transfers sperm into the female reproductive tract.
Intermittent organs are absent in tuataras. Lizards and snakes possess paired hemipenes at
the base of the tail. Hemipenes are erected by turning inside out, like a linger of a glove.

Gonads lie in the abdominal cavity. A pair of ducts transfers sperms into the cloaca in
males. The female may store sperms in seminal receptacle after copulation. Secretions of
the seminal receptacle nourish the sperm. Sperm may be stored for up to four years in
some turtles, and up to six years in some snakes. Sperm can be stored for winter in
temperate latitudes. The individuals grouped in hibemacula in the fall and copulation take
place. Female stores sperms. Fertilization and development occur in thu spring.
Fertilization occurs in the upper regions of the oviduct. Oviduct opens into cloaca.
Glandular regions of the oviduct secrete albumen and the eggshell. The shell is tough and
flexible. The egg shell is calcareous and rigid in some crocodilians.

Parthenogenesis
Parthenogenesis occurs in six families of lizards and one species of snakes. In these
species, no males are present. Populations of parthenogenetic females have higher
reproductive rate than bisexual populations. A large population of reptiles died in the cold
winter. The surviving reptiles can repopulate rapidly in winter.

Reproductive Behaviour
Reptiles have complex reproductive behaviour. Males actively seek females. Courtship
behaviour helps in sexual recognition. It is involved in physiological preparation for it
production.
a) Some males display head bobbing. These males have bright patches of color on the
throat and enlarged folds of skin.
b) Courtship in snakes is based on tactile stimulation. The male displays tail-waving
activity. It brings it chin along the female. Then it entwines his body around her.
Then male produces wavelike contractions that pass from posterior to verior side of
the body.
c) Recent research indicates that lizards and snakes also use sex pheromones.
d) Voclizations are important only in crocodilians. During the breeding season, males
bark or cough. It is a territorial warning to other males. Roaring vocalizations also
tract females and mating occurs in the water.

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Parental Care
Most reptiles freely lay eggs. They do not care about them. Turtles bury their eggs on the
ground or in plant debris. Other reptiles lay their eggs under rocks, in debris, or in
burrows. About one hundred species of reptiles show parental care of eggs.

One example is the American alligator, Alligator mississippiensis. The female builds a
nest or mud and vegetation. It is about 1 m high and 2 m in diameter. She hollows out the
cell of the mound. She partially fills it with mud and debris. She deposits her eggs in the
cavity and then covers the eggs. Temperature within the nest influences the sex of the
hatchlings. Temperature at or below 31.50 C produce females offspring. Temperatures
between 32.5 and 33° C produce male offspring. Temperatures around 32°C result in
both male and female offspring. Similar temperature effects on sex determination are
found in some lizards and many turtles. The female remains near the nest throughout the
development and protect the eggs from predation. She frees young from the egg shell.
Then she picks them up in her mouth, and transfers them into water. She forms shallow
pools for the young and remain with them for up to two years. The female feeds on small
vertebrates and invertebrates and drops the food for young. The young scraps this food.

9.26 Further Phylogenyetic Considerations in Reptiles

The archosaur and synapsid lineages of ancient reptiles diverged from ancient amniotes
about 280 million years ago.

Archosaur Lineage
The dinosaurs belonged to archosaur lineage. This lineage gave rise to crocodilians. It
also gave rise to two groups of fliers.
1. Pterosaurs (pteros, wing + saurus, lizard): Their size is ranged from sparrow
size to animals with wing spans of 13 m. They developed an elongation of the
fourth finger. It supports their membranous wings. Their sternum was adapted for
the attachment of flight muscles. Their bones were hollow to lighten the skeleton
for attachment of flight muscles. These adaptations are paralleled to the adaptations
in the birds.
2. Birds: Birds are the descendants of the second lineage of flying archosaurs.

Synapsid Lineage

Synapsid lineage gave rise to the mammals. The legs of synapsids were long. They held
their bodies above the ground. Teeth and jaws were adapted for chewing and tearing.
Additional bones were incorporated into the middle ear. These mammal-like
characteristics developed between the Carboniferous and Triassic periods. It flnally gave
rise to mammals.

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Self Assessment Questions
Q: Fill in the blanks.
i. ………….. eggs have extra embroynic membranes (Amniotic)
ii. The birds and the mammals have a common ancestor with the
………………. (Reptiles)
iii. Rhynchocephalia has only one surviving species called ………………
(Tuatara)
iv. The order Squamata is divided into ………….. suborders (Three)
v. Most lizards are ……………….. (Oviparous)

Q: Answer the following.

i. Write about structure of turtle.
ii. Write few lines about iguanas.
iii. How snakes evolution takes place.
iv. What are venom glands?
v. What is ecdysis?

9.27 Birds and Adaptations for Aerial Mode of Life (Flight

Adaptations); Flightless and Flying Birds with Examples;
Evolutionary Origin of Birds With Reference to
Archaeopteryx the Birds Feather Flight & Endothermy,
Evolutionary Prospective and Phylogenetic Relationships
Birds are traditionally classified as class Aves. Birds have adaptations for flight. The
major characteristics of this class are:
1. Their appendages are modified to wings.
2. Their body is covered by leathers.
3. Birds are endotherms.
4. They have high metabolic rate.
5. Their vertebral column is modified or flight.
6. Flues are lightened by numerous air spaces.
7. Modern birds possess a horny bill and they lack teeth.

9.27.1 Evolution of Birds from Reptiles

There are close similarities between birds and reptiles. That is why, birds are called as
glorified reptiles. Followings are the possible ancestor of birds:
1. Archosaurs ancestor: The ancestor of the birds is ancient archosaurs. Other flying
reptiles in this evolutionary lineage are pterosaurs and pterodactyls. But these are
not regarded as ancestor of the modern birds due to:
a) These reptiles lost the clavicles long before the appearance of birds. Clavicle
is an important avian characteristic. Clavicle is used or the attachment of

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 flight muscles in birds. Thus pterosaurs and pterodactyls were not strong
living animals like birds.
b) They did not have feather wings. Their flight surface was composed of
membranous fold of skin.
2. Saurischian ancestor: Therefore, these flying reptiles are not taken as ancestor.
Thus modern birds were derived from the Saurischian lineage of dinosaurs. This
lineage also includes bipedal carnivorous dinosaurs like Tryannosaurus. According
to cladistic interpretations the birds are modern dinosaurs.

Ancient birds and the evolution of flight

An important vertebrate fossil was found in Bavaria, Germany in 1861. It is named as
Archaeopteryx (archaios, ancient + pteron wing). It lived during the Jurassic period about
150 million Years ago. It has following characteristics:
1. It was pigeon-sized animal.
2. It had a long reptilian tail.
3. It has clawed fingers.
4. The complete head of this specimen was not preserved. The imprints feathers on
the tail and on short were found. It has rounded wings.

These characters are the main evidence that this was the fossil of an ancient bird. A
complete fossil archaeopteryx was discovered sixteen years later. It has teeth in beak like
jaws. Later four other fossils of Archaeopteryx were discovered. These fossils supported
the ideas of reptilian ancestry for birds.

There are two hypotheses about the origin of flight in birds:

1. Evolution of Gliding
According to this hypothesis gliding was most primitive than flight. Most
zoologists consider Archaeopteryx as the oldest bird fossil. It is very close to the
main line of evolution between the reptiles and birds. The lifestyle of
Archaeopteryx supports the hypotheses on the origin of flight. The clavicles (wish
bones) of Archaeopteryx were well developed. Clavicle provides points of
attachment for wing muscles. The sternum, wing, bones and other sites for night
muscle attachment were less developed than in modern birds. These observations
indicate that Archaeopteryx as a glider. It used flapping flight for short distances.

Some zoologists think that the clawed digits of the wings were used to climb trees
and cling to branches. The ancestor of birds jumps from branch to branch, or
branch to ground. Then it started gliding. Later, weak flapping started. It
supplemented gliding. Finally, winng-powered flight evolved.
2. Evolution of flight
According to this hypothesis flight was more primitive than gliding. Some
zoologists studied the hind limb structure of the earliest birds. It suggests that they
were bipedal.They run and hop on the ground. They used their wings for capturing
flying insects from air and ground. Their teeth and claws resembled talons (claw of

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 eagle) of modern predatory birds. They used these teeth and claw for grasping prey.
They also started flight over short distances. This flight led to wing-powered flight.
Two fossil evidences support this hypothesis:

i) Fossil of Sinornis
A second ancient bird Sinornis was discovered in China. It supported the view that
archacopteryx was closely related to ancestral bird stocks. Sinorirs fossils are 135
million Years old. It has only 15 million years younger than Archaeopteryx.
Sinornis has some primitive dinosaur like characteristics. It also had characteristics
similar to modern birds. These characteristics are:
1. It had shortened body and tail
2. Its sternum has a large surface area for flight muscles.
3. The claws were reduced.
4. The forelimbs were modified for folding of wings at rest.

These characteristics indicate that powered flight was well developed in birds 135
million years ago.
ii) Fossil of Eoalulavis
Another fossil of bird, Eoalulavis was discovered from early Cretaceous deposits in
Spain. It was found in 115 million year old rocks.It provided additional information
about the evolution of flight. It had a wing span of 17 cm. The fossil of Eoalulavis
had a wing structure called an Alula. Alula is also present in many modern birds. It
is used in slow hovering flight. Its presence in this fossil indicates that complex
flight was slow, hovering. This flight evolved at least 115 million years ago.

9.27.2 Diversity of Modern Birds

1. Archaeopteryx, Sinornis and Eoalulovis are transition between reptiles and birds.
But zoologists do not know about the direct ancestor of modern birds.
2. Many fossils of birds have been found between the period of 100 million and 70
million years ago. Some of these birds were large flightless birds. Others were
adapted for swimming and diving. Some were fliers. Most birds like
Archaeopteryx, had reptile like teeth. Most of the lineages of these fossils have
become extinct. ‘Ilk. dinosaurs were also belonged to this lineage. They became
extinct at the end of the Mesozoic era.
3. Some of the few birds survived. These birds enter into the Tertiary period. They
were the ancestors of modern toothless birds. The phvlogeny of modern birds is
controversial. The adaptive radiation has produced about 9,100 species of living
birds. They are divided into about 27 orders. This classification is based or
characteristic behaviors, songs, anatomical differences and ecological niches.

Evolutionary Pressures in Birds

Every body system of a bird shows some adaptation for flight. These adaptations are
endothermy, feathers, acute senses, long, flexible necks and lightweight hones.

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9.28 External Structure and Locomotion
9.28.1 Feathers
Functions of Feathers
The covering of feathers on a bird is called the plumage. Feathers have three primary
functions:
1. They are essential for flight. They form the flight surfaces. It provides lift. This
surface also helps in steering.
2. They prevent excessive heat loss. Thus birds are endothermic. They maintain of
high metabolic rates.
3. Feathers also have roles in courtship, incubation and waterproofing.

Development of Feathers
The development of feathers is similar to the epidermal scales of reptiles. an evidence of
evolutionary ties between reptiles and birds. The inner pulp cavity of feathers contains
dermal elements like blood vessels. Blood provides nutrients and pigments for the
growing feather. The blood supply is cut off in mature feather. Thus the feathers become
dead, keratinized epidermal structures. Feathers are embedded in epidermal invaginations
of the skin called feather follicles.

Types of Feathers
There are different types or feathers:
1. Contour Feathers: These are most obvious feathers. They cover the body, wings
and tail. Contour feathers consist of a vane. The vane has inner and outer webs and
a supportive shaft. Feather barbs are the branches of the shaft. The barbules are the
branches of the barbs. Barbules of adjacent barbs overlap one another. The hook
like hamuli (sing.,hamulus) locks the barbules. Interlocking barbs keep contour
feathers Finn and smooth.
2. Down Feathers: Down feathers are present on the skin. They are insulating
feathers.
3. Filoplumes: They are also called pinfeathers. They have sensory functions.

Maintenance of Feathers
Birds maintain a clean plumage. It removes the parasites from the feathers and skin.

1. Preening: Preening keeps the feathers smooth, clean and in place. Preening is done
by rubbing the bill over the feathers. The dislodged stimuli can be rehooked with
the help of bill. Oil glands are present at the base of the tail of many birds.
Secretions of these oil glands spread over the feathers during Preening. It makes the
plumage water repellent and slimy. The secretions also lubricate the bill and legs. It
prevents the shafing.
2. Anting: It is a maintenance behavior in some birds. It is more common in many
songbirds. In this case, the bird picks up ants in the bill and rubs them over the
feathers. The ants secrete formic acid. This forming acid is toxic to feather mites.

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Colour Patterns in Feathers
There are two factors involved in coloration in birds:

1. Feather Pigments: Feather pigments are deposited during feather formation. These
pigments produce most colors in the plumage bird.
2. Structural Colours: Structural colors are appeared due to irregularities on the
surface of the leather. These irregularities diffract white light and produce colours.
For example, blue feathers do not have blue pigment. A porous non-pigmented
outer layer is present on a barb. This layer reflects blue wavelengths of light. The
other wavelengths pass into the barb. They are absorbed by the dark pigment
melanin.
3. Iridescence: The flattening and twisting of harbules causes the diffraction of light.
It causes iridescence. The interchanging colors on the neck and back of
hummingbirds and grackles are examples of iridescence. Color patterns are used in
cryptic coloration, species and sex recognition and sexual attraction.

Molting of Feathers
Mature feathers are damaged and become useless. Thus all birds periodically shed and
replace their feathers. The process of shedding and replacement of feathers is called
molting. The timing of molt periods varies in different taxa. Following is a typical
molting pattern for song birds:

1. Juvenile Molt: A chick is covered with down feathers after hatching. Juvenile
feathers replace the down feather during juvenile molt.
2. Post Juvenile: A post juvenile molt occurs at the sexual maturity. It produces
plumage similar to adult.
3. Prenuptial Molt: Prenuptial molt occurs at sexual maturity. It occurs in late winter
or early spring before the breeding season.
4. Postnuptial Molt: It occurs between July and October. Flight feathers are lost in a
sequence. Thus the flight continuous during molt. However, many ducks, coots and
rails cannot fly during molt periods. They hide in thick marsh grasses.

9.28.2 The Skeleton

1. Hollow and Light Bones

The bones of most birds are light weighted. But these bones are strong. Some
bones like humerus have large air spaces. They also have internal strutting. It
increase the strength of bone. Birds also have a reduced number of skull bones.
They have a lighter, keratinized sheath called bill: It has replaced the teeth. There
different adaptations for reduction of body weight in some birds. For example,
some aquatic birds (e.g.. loons) have dense bones. These bones reduce buoyancy
during diving.
2. Adaptation for Feeding and Nesting
The flight appendages cannot be used for nesting materials or feeding young. Birds
use bill, flexible neck and feet for these activities. The cervical vertebrae have

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 saddle-shaped articular surface. It provides greater freedom of movement. The first
cervical vertebra (the atlas) has a single point of articulation with the occipital
condyle of skull. It allows a high degree of rotational movement between the skull
and the neck. This flexibility allows the bill and neck to function as a fifth
appendage.
3. Adaptation in Vertebrae
The birds have strong pelvic girdle, vertebral column and ribs for flight. The
thoracic region of the vertebral column contains ribs. These ribs are attached to
thoracic vertebrae.

Most ribs have posteriorly directed uncinate processes. These processes overlap the
next rib and strengthen the ribs. Uncinate processes are also present on the ribs of
most reptiles. Posterior to the thoracic region is the lumbar region. The synsacrum
is forrmed by the fusion of the posterior thoracic vertebrae, all the lumbar and
sacral vertebrae and the anterior caudal vertebrae. Fusion of these bones maintains
the proper flight posture. It also supports the hind limbs during landing, hopping,
and walking. The posterior caudal vertebrae are fused to form a pygostyle.
Pvgostyle supports the tail feathers. Tail feathers are uses in steering.
4. Adaptations in Sternum: The sternum of most birds develops a large median keel.
Keel is used for the attachment of flight muscles. Keel is absent in some flightless
hinds like ostriches.
5. Adaptations in Appendages and Mechanism of Perching
The appendages of birds are also modified. Some bones of the front appendages are
lost. Or they fused to form points of attachment for flight feathers. The rear
appendages are used for hopping, walking, running and perching. Perching tendons
starts from the toes. These pass through the back of the ankle joint and are attached
to the muscles of the lower leg. The ankle joint is flexed during perching. Thus the
tension on the perching tendons increases and the foot grips the perch. This is an
automatic grasp. It can perch even during sleeping. The muscles of the lower leg
can increase the tension on these tendons. Eagle grasps a fish in its talons (claw) by
this mechanism.

9.28.3 Muscles
The flight muscles are the largest and strongest muscles of birds. They are attached to the
sternum and clavicles their other end is attached to the humerus. The muscles of most
birds are adapted physiologically for flight. Flight muscles must contract quickly. They
are fatigued very slowly. These muscles have many mitochondria and produce large
quantities of ATP. ATPs provide energy for flight. Domestic fowl have massive amounts
of muscle (“white meat”). The humans like it as food. These muscles contain fibers for
rapid contraction. But they have few mitochondria and poor vascularization. Therefore,
they are poorly adapted for flight.

Flight
The wing of birds is adapted for different kinds of flight. Birds can glides or flap their
wings. But in all cases they use the same mechanism (Figure 9.38).

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Figure 9.38: Flight Muscles

Mechanism of Flight

1. Upward Lift
Wings of birds form an airfoil. The upper surface of the wing is slightly convex.
The lower surface is flat or slightly concave. Air moves faster over the wing than
under it. It decreases air pressure on the upper surface of the wing. Thus it lifts the
bird upward. The lift of the wings must be more than the weight of the bird. The air
in front of the bird creates resistance. The force of propulsion of the bird must
overcome this resistance for moving forward.
2. Turbulence of Air
The force of propulsion increases the angle of leading edge of the wing. As a result,
the oncoming air (the angle of attack) increases lift. The bird moves upward and its
angle of attack keep on increasing. The flow of air over the upper surface becomes
turbulent with the increase of angle of attack. It reduces lift. Two adaptations in
birds reduce this turbulence:
a) The leading edge of wings has slots. Air flow rapidly through these slots and
reduce turbulence.
b) Some birds also have alula on the anterior margin of the wing. The alula is a
group of small feathers supported by medial digit.
The slotting of the feathers and the presence of an alula reduce turbulence. The
angle of attack increases during takeoff and landing, hovering flight. Thus alula is
elevated. The angle of attack decreases during soaring and fast flight. It reduces
slot.
3. Strokes of Wings
The distal part of the wing generates propulsive force for flight. Distal end of wing
is farther from the shoulder joint. Therefore, the distal part of the wing moves
farther and faster than the proximal part of the wing. I he wings produce two types
of strokes.
a) Power Stroke: The down stroke is called ilowa stroke. During power stroke,
the leading edge of the distal part of the wing moves slightly downward. It

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 creates thrust like airplane. Feathers overlap on a wing. Therefore, air presses
the leathers at the wing margins together during the down stroke. The
leathers do not allow air to pass between them. It increases the lift and
propulsive flutes.
b) Recovery Stroke: The upstroke is called recovery stroke. During recovery
stroke the distal part of the wing is oriented upward. It decreases resistance.
Feathers slightly separated from each other on the upstroke. It allows air to
pass between them. It reduces the resistance during recovery stroke.

Role of Tail in Flight

The tail of a bird acts as balancing, steering, and braking structures during flight. It also
enhances the lift of wings during low-speed flight.
1. The birds spread the tail during horizontal night. It increases lift at the rear side of
the bird. As a result bird dips the head for descent.
2. The closing the tail leathers has the opposite effect.
3. Tilting of tail sideways turns the bird.
4. Tail acts as an air brake. The tail moves downward during landing of bird.
5. In some species like sunbirds and widow bird, the tails of the males have special
ornamentation. It attracts females and improves chance or reproduction.

Types of Flight
Different birds use different kinds of flight during different times.
1. Gliding Flight: The wings remain stationary during gliding night and the bird
loses altitude. Water fowl use gliding flight for landing.
2. Flapping Flight: Flapping flight generates the power for flight. It is the most
common type of flying. Many variations are present in wing shape. Therefore,
flapping patterns is different in different species.
3. Soaring Flight: Soaring flight allows some birds to remain in air without spending
much energy. Wings remain stationary during soaring. The birduses updrafts and
air currents to gain altitude. Hawks and vultures are soaring birds. They encircle
the mountain valleys. They soar downwind to pick up speed. Then the turn upwind
to gain altitude. Then the bird slows down and starts losing altitude. It turns
downwind again. The wings of many soaring bids are wide and slotted. It increases
their activities at relatively low speeds. Oceanic soarers like frigate birds have long
narrow wings. These wings provide maximum lift at high speeds. But it reduces
their activities. It also makes takeoff and landing difficult.
4. Hovering Flight: Hummingbirds perform hovering flight. They fan their wings
beck and forth hover in still air by (50 to 80 beats per second). They remain
suspended in front of a flower or feeding station during hovering.

9.28.4 Nutrition and the Digestive System

Birds feel great appetites. This appetite support a high metabolic lats. High metabolic rate
is necessary for endotherim and flight. For example, humming birds feed during the day.
But they can’t maintain metabolism at night. Therefore, they become torpid at night.

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They reduce temperature and respiration rate. They again become active and feed in the
morning (Figure 9.39).

1. Bill and Tongue

The bills and tongues of birds are modified for different feeding habits and food
sources.
For example:
a) The tongue of woodpecker is barbed. It is used or extracting grubs (larvae of
insect) from the bark of trees.
b) Sapsuckers dig holes in trees. It uses a brush like tongue for licking the sap
from the holes.
c) Hummingbirds are nectar feeders. Their tongues roll to form a tube for
extracting nectar from flowers.

Figure 9.39: Digestive System of Pigeon

2. Crop
Crop is present in man birds. Crop is a diverticulum of the esophagus. It is a
storage structure. It allows birds to quicklv ingest large quantity of food. Then they
move to save places and digest the food. The crop of pigeons produces “pigeon’s
milk.”
Pigeon’s mill is a cheesy secretion. It is formed by the proliferation and sloughing
of the lining or the crop Young pigeons feed on pigeon’s milk till they are able to
eat grain. Vultures and birds of prey also use their esophagus for storage. The
insectivorous birds feed throughout the dav. Therefore, crop is less developed in
them.
3. Stomach
The stomach of birds has two regions. These regions are proventriculus and
ventriculus.
a) Proventriculus: It secretes gastric juices. Gastric juice initiates digestion.
b) Ventriculus or (gizzard): It has muscular walls to crush the seeds and other
hard materials. Birds swallow, sand and stone in gizzard. These stones help
in digestion.

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4. Intestine
Most of digestion and absorption occurs in the small intestine. Intestine receives
secretions from the pancreas and liver. These secretions help in digestion. Paired
ceca are present at the union of the large and small intestine. Ceca are blind-ending
sacs. They contain bacteria. These bacteria help in the digestion of cellulose.
5. Cloaca
Cloaca is common opening for urinogenital duct and anus. Cloaca is present in
most of the birds. It eliminates undigested food. However, owls form pellets of
bone, fir and feathers. It ejected these pellets through the mouth.

9.28.5 Feeding Habits in Birds

Birds are grouped on the basis of their feeding habits. These groupings are are artificial.
Birds may eat different kinds of food at different stages in their life history. They can
change their diets according to availability of food. For example, Robins feed largely on
worms and other invertebrates when these foods are available. Robins may feed on
berries (fruits like tomato) in the winter.

The feeding habits of some birds are harmful to man. Birds damage orchard and grain
crops. Millions of birds arc collected in local area due to flocking and roosting habits in
some birds. They destroy fields of grain. Recent monocultural practices line increased the
problem. The birds form large flocks in these areas.

Birds of prey have minimum impact on populations of poultry and game birds and on
fisheries. It has been mistaken though that they are responsible for these loses. Thus
human are killing them by poison or shooting them.

9.28.6 Circulation
Gas exchange and Temperature Regulation

Blood circulation
The circulatory system of birds is similar to reptiles. but the heart of birds has completely
separated atria and ventricles. Thus they have separate pulmonary and systemic circuits.
Therefore, oxygenated blood does not mix with less oxygenated blood. The sinus venosus
has gradually decreased in size during evolution of vertebrates. It is a separate chamber in
fishes, amphibians and turtles. It receives blood from the venous system. In other reptiles,
sinus venous is a group of cells in the right atrium. It acts as pacemaker in their heart. In
birds, the sinus venosus is also present as a patch of pacemaker tissue in the right atrium.
The bird heart is relatively large. It beats rapidly. The rate of heart of humming bird is 1,
000 per minute in stress condition. Larger birds have smaller hearts and slower heart rate.
For example, the heart rate of an ostrich is between 38 and I 76 beats per minute.

The birds are endotherms. Therefore, they need large quantity of blood or flight. Birds
have following adaptations far supply of blood:
1. They have a large heart.
2. They have rapid heart rate.

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3. They have complete separation of highly oxygenated blood from less oxygenated
blood.

Gas Exchange

Respiratory Organ
The respiratory system of birds is complex and efficient. It consists of external nares,
pharynx, trachea air sacs and lungs.
a) External nares open into nasal passageways and pharynx.
b) Pharynx opens into trachea. Bones and cartilage support the trachea. A special
voice box syrinx is present in the birds. It is located where the trachea divides into
bronchi. The muscles of the syrinx and bronchi and properties or the trachea
produce bird vocalizations.
c) The bronchi open into air sacs. The air sacs occupy much of the body. They extend
into some of the bones of the skeletal muscles. The air sacs and bronchi connect to
the lungs.
c) The lungs of the birds are made or small air tubes called parabronchi. Parabronchi
divides to form air capillaries. These air capillaries are associated with capillary
beds for gas exchange.

Mechanism of Respiration
Two factors are involved in inspiration and expiration:
1. Increasing and decreasing volume of the thorax
2. Alternate expansion and compression of air sacs occur during flight and other
activities. The movement of the sternum and posterior ribs during breathing
compresses the thoracic air sacs. The contraction of flight muscles distorts the
furcula. Alternate distortion and recoiling compress and expand the air sacs
between the two shafts of bones.

Two ventilatory cycles move air through the respiratory system or a bird. The process of
respiration is divided into two cycles:
1. Cycle 1: Air moves into the abdominal air sacs during inspiration. Air is already
present in the lung. This air moves into the thoracic air sacs through parabronchi.
The air in the thoracic air sacs moves out of the respiratory system during
expiration. Now the air in the abdominal air sacs moves into parahronchi.
2. Cycle 2: At the next inspiration, the air moves into the thoracic air sacs. It is
expelled during expiration.

9.28.7 Comparison of Respiration in Birds and other Tetrapods

The birds have a greater rate of oxygen consumption due to high metabolic rate. The
respiratory cycle in other tetrapods is a simple back-and-forth cycle. Ventilation does not
c place during expiration. Thus much “dead air– remains in the lungs. But the birds has
unique system of air sacs and parabronchi.Thus, there is continuous movement ofoxygen-
rich air over respiratory surfaces during both inspiration and expiration. The quantity of
“
dead-air“ in the lungs is much lesser than other vertebrates.

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 The respiratory system of birds is more efficient than other tetrapod. It supports high in
metabolic rates. Similarly, the oxygen tension is low at high altitudes. The birds live and
fly at high altitudes due to this efficient gas exchange system. Thus, birds geese fly can
reach the Himalayas at 9, 200 m during migrations.

Other Tetrapods (Amphibians,

Birds Reptile and mammals)
Tetrapods have comparatively lesser
metabolic rate.
The birds have a greater rate of
oxygen consumption due to
high metabolic rate. The respiratory cycle in other
Birds have one way flow of 1. tetrapods is a simple back-and-forth
gases. Ventilation does not take cycle.
place during expiration.
Parabrochi are present. 2. Parabrochi are absent.

1. Alveoli are absent. Alveoli are present.

The birds have unique system of 3.

air sacs and parabronchi. Thus
2. there is continuous movement 4.
of oxvgen-rich air over
respiratory surfaces during both
3. Much dead air remains in the lungs.
inspiration and expiration. The
quantity of “dead air– in the
lungs is much lesser than other
4. vertebrates.
5.

The respiratory system of birds

is more efficient than tetrapod.
It supports high metabolic rates.
Similarly, the oxygen tension is
low at high altitudes. Their respiratory system is not much
5. efficient.

Thermoregulation
Birds maintain body temperatures between 38o and 45oC. Lethal temperatures for birds
are lower than 32oCand higher than 47° C. There are following adaptations for the
rtnoregulation in birds:
1. Resting bird fluffs its feathers in a cold day. It increases their insulating properties
and dead air space within them.

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2. Birds also tucks its bill into its feathers. It reduces heat loss from the respiratory tract.
3. The most exposed parts of a bird are the feet and tarsi. They are not covered by
flesh n muscles. They are also not supplied with rich blood supply. Temperatures in
these structures can drop near freezing point. It prevents heat loss. There is a
countercurrent heat exchange between the arm blood flowing to the legs and feet
and the cooler blood flowing to the body from the legs and feet. It prevents
excesive heat loss at the feet. Thus heat is returned to the body.
4. Shivering also generates heat in extreme cold.
5. Increases in metabolism during winter months require additional food. Some birds
become torpid. Their body temperatures drop on cool nights. For example, the
body temperatures of whippoorwill drop to 16 C from 40 C. Thus its respiratory
rate become very slow.
6. Muscular activity during flight produces large quantities of heat. Birds can
dissipate this heathy panting.
7. The vascular membranes from the floor of the mouth we fluttered. It increases the
evaporative heat loss.

9.28.8 Nervous and Sensory Systems

There are many sensory adaptations in birds.

Brain
1. Fore Brain: The forebrain of birds is much larger than reptiles. Birds have large
cerebral hemispheres and corpus striatum. Corpus striatum is a region of gray matter.
The corpus striatum functions in visual learning. feeding, courtship and nesting. A
pineal body is present on the roof of the forebrain. It stimulates ovarian development
and regulates other functions controlled by light and dark periods. The optic tectum
(the roof of the midbrain) and corpus striatum integrates sensory functions.
2. Mid Brain: The midbrain also receives sensory input from the eye.
3. Hind Brain: The hindbrain includes the cerebellum and medulla oblongata.
Medulla oblongata coordinates motor activities. It regulates heart and respiratory
rates.

Sense Organs
1. Eye
Vision is an important sense for most birds. Most structures of bird eyes are similar
to other vertebrates. But eyes of bird are much larger relative to body size than
other vertebrates. Their eyes are flattened in an anteroposterior direction. The eyes
of birds of prey has e a bulging cornea. Therefore, their eyes protrude anteriorly.
Birds have a unique double-focusing mechanism. Pad like structures control the
curvature of the lens. The ciliary muscles change the curvature of the cornea. The
bird collpse has double and instantaneous focusing mechanism. They remain
focused on a fish and jump on it.
Ratina: The retina of birds is thick. It contains both rods and cones becomes active
in loss light intensities and cones become active in high light intensities. Cones are

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 especially concentrated (1,000,000/ mm) at fovea. Fovea is a focal point of retina.
Some birds have two foveae per eye.
a) Search Fovea: One fovea is called search fovea. It is present at the center of
the retina. It gives the bird a wide angle of monocular vision.
b) Pursuit Fovea: The other foveae is present at the posterior margin of the
retina. It is called pursuit foveae. It functions with the posterior foveae of the
other eye and form binocular vision. Binocular vision produces depth
perception. It is necessary to capture prey. Other birds use the “search fovea”
to observe the landscape below them during flight. They use pursuit fovea or
depth perception. It is needed during landing on a branch of a tree.
Formation of Vision: The position of the eyes on the head also influences the
degree of binocular vision.
a) Monocular Vision: The eyes of the pigeons are located on the sides of their
head. It gives them a nearly 360 monocular field. But it gives a narrow
binocular field. They do not have to pursue their food. A wide monocular
field of view keeps them alert from predators during feeding on the ground.
b) Binocular Vision: Theeyes of the hawks and is are further forward on the
head. This increases their binocular field of view and decreases their
monocular field of view.
2. Olfaction
Olfaction plays a minor role in the lives of most birds. The olfactory epithelium is
poorly developed.
External nares open near the base of the beak. But turkey vultures have well
developed sense of olfaction. It locates their dead and dying prey largely by smell.
3. Ear
Most birds have well-developed hearing. Loose, delicate feathers called auriculars
cover the external ear opening. Middle ear and inner ear are similar to reptiles. The
sensitivity of the avian ear (100 to 15,000 Hz) is similar to the human ear ( 16 to
20,000 Hz).

9.28.9 Excretion and Osmoregulation

Birds and reptiles face similar excretory and osmoregulatory problems. Birds excrete uric
acid. It is temporarily stored in the cloaca. Water is also reabsorbed in the cloaca. The
excretion or uric acid conserves water and promotes embryo development in terrestrial
environments. Some birds have supraorhital salt glands. These glands remove excess
sodium chlorlde from the body through the nasal openings. These glands are especially
important in marine birds. These birds drink seawater and feed on invertebrates containing
large quantities of salt in their tissues. Salt glands can secrete salt in a solution. This solution
is two to three times more concentrated than other body fluids. Thus salt glands compensate
for the inability of kidney to remove concentrate salts in the urine.

9.28.10 Reproduction and Development

The sexual activities of birds include establishing territories finding mates, constructing
nests, incubating eggs, and feeding young. All birds are oviparous. Their gonads are
present in the dorsal abdominal region, next to the kidney.

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Male Reproductive Organs
Testes are paired. Coiled tubules vasa differentia transfers sperm to the cloaca. The vasa
differentia enlarges to form seminal vesicle. It stores sperms temporarily and maturation
sperm occurs before mating. Testes are enlarged during the breeding season. Birds have
no intermittent organ (copulatory organ). Males mount on female and the sperm are
transferred by cloaca contact.

Female Reproductive Organs

Two ovaries are formed during development in females. But only the left ovary fully
develops. A large funnel-shaped opening (the ostium) or the oviduct envelops the ovary.
It receives eggs after ovulation. The eggs are fertilized in the upper portions of the
oviduct. The albumen is secreted by glandular regions of the oviduct. Albumen surrounds
zygote. A shell gland present in the lower region or the oviduct. It secretes shell around
the egg. The oviduct opens into the cloaca.

Mating Territories
Many birds establish territories prior to mating. The size and function of territory vary
greatly among species. Territories allow birds to mate without interference. They provide
nest locations and sometimes food resources for adults and offspring. Breeding birds
defend their territories. They expel intruders of the same sex and species. Threats arc
common. But actual fight is minimum.

Mating
The mate is attracted to its territory during mating. Different birds have different methods
attraction of females:
1. Male woodpeckers drum on trees to attract females.
2. Male ruffed grouse fan their wings on logs. Its sound can be heard from many
miles.
3. Cranes have a courtship dance. This dance includes stepping, bowing, stretching
and jumping displays. The female gives readiness signals and mating starts. Mating
occurs quickly but repeatedly. Thus all the eggs are fertilized.

Reproductive Behaviours
1. Monogamy
Most birds are monogamous. A single male pairs with a single female during the
breeding season. Some birds (swans, geese, eagles) pair for life. Frequent mating
strengthens the pair bonds. Monogamy is common when resources are widely and
evenly distributed. Thus one bird cannot control all these resources.
Monogamy has also advantages. Both parents participate in nest building and care
of the young. One parent incubates and protects the eggs or chicks. The other
parent searches food.
2. Polygyny
Some birds are polygynous. Males mate with more Man one female. The females
care for the eggs and chicks. Polygyny occurs in species whose young are less
dependent at hatching. Sometimes, the resource are scattered in patch. It attracts

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 many females to a small breeding area. Prairie chickens are polygnous. The males
display groups called leks. The males in the center positions are preferred and
attract the majority of females.
3. Polyandry
A few bird species are polyandrous. In this case, the females mate with more than
one male. Female in spotted sandpipers birds are larger than males. The female
establish her territories. She defends her territory from other females. She lays eggs
for each male. The male builds a nest in the territory. If a male loses his eggs to a
predator. the female replaces them. Polyandry produces more eggs than
monogamous mating. It has advantages when food is plentiful but the chances of
successfully rearing young are low due to predation.

Nesting Activities
The nesting behavior of birds is species specific. Some birds choose nest sites away from
other members of their species. Some birds nest in large flocks. Unfortunately, nesting
behaviour is the cause of extinction of some species of birds.

Nest construction begins after pair formation. The female initiates this instinctive
behavior. A few birds do not make nests. Emperor penguins breed on the Snow and ice of
Antarctica. Thus no nest materials are available. Their single egg is incubated on the web
of the foot. The foot has a fold of abdominal skin.

Caring of Eggs and Hatching

The group of eggs laid and chicks produced by a female is called a clutch. Different
species have different clutch size.
a) Most birds incubate their eggs. Some birds have a featherless. vascularized
incubation or brood patch. Brood patch keep the eggs at temperatures between 33
and 37″ C. Birds often turn the eggs. It prevents the attachment of egg membranes.
Thus it prevents the deforming of the embryo.
b) Some birds spray cool water on the eggs. It makes the eggs humid.
c) The Egyptian plover carries water in the breast feathers. The incubation period lasts
between 10 and 80 days.

Hatching: The young-bird develops an air sac one or two days before hatching. This air
sac penetrates into the blunt end of its egg. It inflates lungs of young and it starts
hatching. The young bird pecks the shell and hatching occurs.

Caring of Young
There can be two types of young:

1. Artificial Young
Some birds are helpless at hatching. They depend on their parents for hatching. The
young are entirely dependent on their parents are called altricial. They are often
naked at hatching. Endothermy is not developed at start in altricial young. Thus

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 they are brooded constantly at first. They gross rapidly. When they leave the nest,
they are nearly as large as their parents.
2. Precocial Young
Precocial young are alert and lively at hatching. They are covered with down
feathers. They can walk, run, swim, and feed themselves. Usually, one parent is
present to lead the young to food and shelter.

Feeding of Young
Young altricial birds have huge appetites. One or both parents continually search food for
it. The young may consume a mass of food that equals their own weight each day. Adults
bring food to the nest Or they regurgitate food stored in the crop or esophagus. The adults
produce vocal signals or color patterns on the bills or throats. It initates feeding responses
in the young. Parents feed gaping mouths. Many young have brightly colored mouth
linings or spots. It attracts parent’s attention. The first-hatched young have large size. It
can stretch its neck higher. Therefore, it is given food first

Life span of Birds

Life is brief for birds. About 50% of young leave the nest. Remaining birds died. Most
bird has life span of 10 to 20 years in captivity. But this life span is shorter in natural
conditions. The average American robin lives 1.3 years. The average black-capped
chickadee lives less than 1 year. Mortality is high in the first year due to preditors and
adverse weather.

9.28.11 Migration and Navigation

The periodic round trips of birds between breeding and non-breeding areas are
called migration. Most migrations are annual. They have nesting areas in northern
regions and wintering grounds in the south. 70% of the earth’s land is in the Northern
Hemisphere. Therefore, Migration is more important for species found in the Northern
Hemisphere. Migrations involve east/west movements or altitude changes. Migration
allows birds to leave the climatic extremes. Thus they secure adequate food, shelter, and
space throughout the year.

Stimulus for Migration

Birds migrate in response to species-specific physiological conditions. Following factors
stimulate the migrations
1. Innate (genetic) clocks and environmental factors influence the migration.
2. The photoperiod is an important migratory sign for many birds.
3. The changing photoperiod starts seasonal changes in development of gonads. These
changes act as migratory stimuli.
4. Increasing day length in the spring promotes development of gonads. The
decreasing day length in the fall initiates regression of gonads. The changing
photoperiod also promote fat deposition in many birds. This fat acts as an energy
reserve for migration.
5. It is believed that the anterior lobe of the pituitary gland and the pineal body are
involved in photoperiod responses.

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Mechanism of Migration
The mechanics of migration are species specific. Some birds are long- distance migrants.
‘[hey store fat equal to 50% of their body weight. They make non-stop journeys. Other
species start migration slowly. They begin their journeys early and stop frequently for
feeding and resting. Many birds can fly at altitudes greater than 1,000 m. Thus do not hit
the tall obstacles. Many birds have very specific migration routes.

Navigation
Birds use two forms of navigation:
1. Rout based Navigation
It involves following the track of landmarks during an outward journey. Those
landmarks are used in a return trip.
2. Location based Navigation
In this case direction of the destination is followed from the information available
at site of start of the journey. It involves the use of sun compasses and other
celestial. The magnetic field of the earth is also followed.
i) Sun Compass
The lenses of birds are transparent or ultraviolet light. Their photoreceptors
respond to ultraviolet radiations. Thus they orient themselves according to
position of sun. This orientation sign is called a sun compass. The sun
moves between sunrise and sunset. Birds use internal clocks to sense the sun
rises in the east. The sun is overhead at noon. It sets in the West.

The biological clocks of migratory birds can be changed. In an experiment birds ready for
northward migration is held in a laboratory. The laboratory sunrise occur later than the
natural sunrise. After that they are released to natural light conditions. They fly in a
direction they sense to be north. But it is really northwest. Night migratory birds can also
use the sun. They fly in the proper direction from the sunset.

iii) Celestial Cues

Celestial cues other than the sun are also used for navigation. The North Star lines up
with the axis of rotation of the earth in the Northern Hemisphere. The angle between the
North Star and the horizon decreases as we move toward the equator. The latitudes of the
earth can be determined by this method. Birds also use similar information to determine
latitude.

i) Magnetic Compass
Some zoologists believe that the birds use magnetic compasses to detect the magnetic
field the earth. They determine their direction from this magnetic field. If the magnets are
strapped to the heads of pigeons, it disorient the birds. European robins and the garden
warbler use the earth’s magnetic field for orientation. But magnetic receptors are not
found in any of the birds. There were early reports that magnetic iron and magnetite a e
present in the head and necks of pigeons. But it does not help in greater understanding of
magnetic compasses. Further experiments can not prove the magnetic properties in tl ese
regions. Magnetic iron is also found in bacteria and other animal tissues. But they not

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dead associated with a magnetic sense. Some zoologists believe that he pineal body of
pigeons use the sun compass in responses to magnetic fields.

Many navigational mechanisms are present in the birds. It suggests that birds use d
Ilerent sources of information in different circumstances.

Self Assessment Questions

Q: Fill in the blanks.
i. The ancestor of the birds is ancient ………………….. (Archosaurs)
ii. Every body system of a bird shows some adaptation for …………… (Flight)
iii. The covering of feathers on a bird is called the ………………….. (Plumage)
iv. The force of ……………… increases the angle of leading edge of the wing
(Propulsion)
v. The circulatory system of birds is similar to ……………….(Reptiles)

Q: Answer the following.

i. What do you know about Archaeopteryx?
ii. What are contour and down feathers?
iii. How molting takes place.
iv. What is the role of tail during flight in birds?
v. What is parabronchi?

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Summary/Key Points
1. Characteristic features of Chordata are a notochord, a dorsal hollow nerve cord,
pharyngeal slits, and a post-anal tail. Chordata contains two clades of
invertebrates: Urochordata (tunicates) and Cephalochordata (lancelets), together
with the vertebrates in Vertebrata.
2. Reptiles are air-breathing, cold-blooded vertebrates that have scaly bodies rather
than hair or feathers; most reptile species are egg-laying, though certain
“squamates” — lizards, snakes and worm-lizards — give birth to live young.
3. All amphibians begin their life in water with gills and tails. As they grow, they
develop lungs and legs for their life on land. ... There are more than 4,000 different
kinds of amphibians. Members of this animal class are frogs, toads, salamanders,
newts, and caecilians or blindworms.
4. Birds are a group of warm-blooded vertebrates constituting the class Aves,
characterized by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a
high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton.

333
References
 "Morphology of the Vertebrates". University of California Museum of
Paleontology. Retrieved 23 September 2008.
 Delabre, Christiane; et al. (2002). "Complete Mitochondrial DNA of the Hagfish,
Eptatretus burgeri: The Comparative Analysis of Mitochondrial DNA Sequences
Strongly Supports the Cyclostome Monophyly". Molecular Phylogenetics and
Evolution. 22 (2): 184–192
 Dzik, J. (June 1999). "Organic membranous skeleton of the Precambrian
metazoans from Namibia". Geology. 27 (6): 519–522.
 Blair, J. E.; Hedges, S. B. (November 2005). "Molecular Phylogeny and
Divergence Times of Deuterostome Animals". Molecular Biology and Evolution. 22
(11): 2275–2284.
 Nielsen, C. (July 2012). "The authorship of higher chordate taxa". Zoologica
Scripta. 41 (4): 435–436
 Goujet, Daniel F (16 February 2015), "Placodermi (Armoured Fishes)", ELS, John
Wiley & Sons, Ltd, pp. 1–7,

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