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ANIMAL KINGDOM 37

CHAPTER 4
ANIMAL
4.1 Basis of
KINGDOM
When you look around, you will observe different animals with
Classification different structures and forms. As over a million species of animals
4.2 Classification of have been described till now, the need for classification becomes all
the more important. The classification also helps in assigning a
Animals
systematic position to newly described species.

4.1 BASIS OF CLASSIFICATION

Inspite of differences in structure and form of different animals, there
are fundamental features common to various individuals in relation
to the arrangement of cells, body symmetry, nature of coelom,
patterns of digestive, circulatory or reproductive systems. These
features are used as the basis of animal classification and some of
them are discussed here.

4.1.1 Levels of Organisation

Though all members of Animalia are multicellular, all of them do not
exhibit the same pattern of organisation of cells. For example, in
sponges, the cells are arranged as loose cell aggregates, i.e., they
exhibit cellular level of organisation. Some division of labour
(activities) occur among the cells. In coelenterates, the arrangement of
cells is more complex. Here the cells performing the same function are
arranged into tissues, hence is called tissue level of organisation. A
still higher level of organisation, i.e., organ level is exhibited by
members of Platyhelminthes and other higher phyla where tissues are
grouped together to form organs, each specialised for a particular
function. In animals like Annelids, Arthropods, Molluscs,
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38 BIOLOGY

Echinoderms and Chordates, organs have

associated to form functional systems, each
system concerned with a specific
physiological function. This pattern is called
organ system level of organisation. Organ
systems in different groups of animals exhibit
various patterns of complexities. For
example, the digestive system in
Platyhelminthes has only a single opening
to the outside of the body that serves as
both mouth and anus, and is hence
called incomplete. A complete digestive
Figure 4.1 (a) Radial symmetry system has two openings, mouth and anus.
Similarly, the circulatory system may be of
two types:
(i) open type in which the blood is pumped
out of the heart and the cells and tissues
are directly bathed in it and
(ii) closed type in which the blood is
circulated through a series of vessels of
varying diameters (arteries, veins and
capillaries).

4.1.2 Symmetry
Animals can be categorised on the basis of
their symmetry. Sponges are mostly
asymmetrical, i.e., any plane that passes
Figure 4.1 (b) Bilateral symmetry through the centre does not divide them into
equal halves. When any plane passing
through the central axis of the body divides
the organism into two identical halves, it is
called radial symmetry . Coelenterates,
ctenophores and echinoderms have this
kind of body plan (Figure 4.1a).
Mesoglea Ectoderm Animals like annelids, arthropods, etc.,
Endoderm where the body can be divided into identical
left and right halves in only one plane, exhibit
bilateral symmetry (Figure 4.1b).

4.1.3 Diploblastic and Triploblastic

Organisation
Animals in which the cells are arranged in
two embryonic layers, an external ectoderm
Mesoderm
(a) (b) and an internal endoderm, are called
diploblastic animals, e.g., coelenterates. An
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 ANIMAL KINGDOM 39

undiff e rent iated

Figure 4.2 Showing germinal layers : layer, mesoglea, is present in between the
(a) Diploblastic (b) Triploblastic ectoderm and the endoderm (Figure 4.2a).

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Those animals in which the developing embryo has a third germinal

layer, mesoderm, in between the ectoderm and endoderm, are called
triploblastic animals (platyhelminthes to chordates, Figure 4.2b).

4.1.4 Coelom
Presence or absence of a cavity between the
body wall and the gut wall is very important
in classification. The body cavity, which is
lined by mesoderm is called coelom.
Animals possessing coelom are called
coelomates, e.g., annelids, molluscs,
arthropods, echinoderms, hemichordates and
chordates (Figure 4.3a). In some animals, the
body cavity is not lined by mesoderm, instead,
the mesoderm is present as scattered pouches
in between the ectoderm and endoderm.
Such a body cavity is called pseudocoelom
and the animals possessing them are called
pseudocoelomates, e.g., aschelminthes
(Figure 4.3b). The animals in which the body
Figure 4.3 Diagrammatic sectional view of :
cavity is absent are called acoelomates, e.g., (a) Coelomate (b) Pseudocoelomate
platyhelminthes (Figure 4.3c). (c) Acoelomate

4.1.5 Segmentation
In some animals, the body is externally and internally divided into
segments with a serial repetition of at least some organs. For example,
in earthworm, the body shows this pattern called metameric
segmentation and the phenomenon is known as metamerism.

4.1.6 Notochord
Notochord is a mesodermally derived rod-like structure formed on the
dorsal side during embryonic development in some animals. Animals
with notochord are called chordates and those animals which do not
form this structure are called non-chordates, e.g., porifera to
echinoderms.

4.2 CLASSIFICATION OF ANIMALS

The broad classification of Animalia based on common fundamental

features as mentioned in the preceding sections is given in Figure
4.4.

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ANIMAL KINGDOM 41

*Echinodermata exhibits radial or bilateral symmetry depending on the stage.

Figure 4.4 Broad classification of Kingdom Animalia based on common fundamental features
The important characteristic features of
the different phyla are described.

4.2.1 Phylum – Porifera

Members of this phylum are commonly known
as sponges. They are generally marine and mostly
asymmetrical animals (Figure 4.5). These are
primitive multicellular animals and have
cellular level of organisation. Sponges have a
water transport or canal system. Water enters
through minute pores (ostia) in the body wall
into a central cavity, spongocoel, from where it
goes out through the osculum. This pathway
(b) of water transport is helpful in food gathering,
(a)
respiratory exchange and removal of waste.
Choanocytes or collar cells line the spongocoel
and the canals. Digestion is intracellular. The
body is supported by a skeleton made up of
spicules or spongin fibres. Sexes are not
separate (hermaphrodite), i.e., eggs and
sperms are produced by the same individual.
Sponges reproduce asexually by fragmentation
(c)
and sexually by formation of gametes.
Fertilisation is internal and development
Figure 4.5 Examples of Porifera : (a) Sycon is indirect having a larval stage which is
(b) Euspongia (c) Spongilla morphologically distinct from the adult.

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Examples: Sycon (Scypha), Spongilla (Fresh water sponge) and Euspongia

(Bath sponge).

4.2.2 Phylum – Coelenterata (Cnidaria)

They are aquatic, mostly marine, sessile or free-swimming, radially
symmetrical animals (Figure 4.6). The name cnidaria is derived from the

(a) (b)
Figure 4.6 Examples of Coelenterata indicating outline of their body form :
(a) Aurelia (Medusa) (b) Adamsia (Polyp)

cnidoblasts or cnidocytes (which contain the stinging capsules or

nematocysts) present on the tentacles and the body. Cnidoblasts are
used for anchorage, defense and for the capture of prey (Figure 4.7).
Cnidarians exhibit tissue level of organisation and are diploblastic. They
have a central gastro-vascular cavity with a single opening, mouth on
hypostome. Digestion is extracellular and intracellular. Some of the
cnidarians, e.g., corals have a skeleton composed of calcium carbonate.
Cnidarians exhibit two basic body forms called polyp and medusa
(Figure 4.6). The former is a sessile and cylindrical form like Hydra,
Adamsia, etc. whereas, the latter is umbrella-shaped and free-
Figure 4.7
swimming like Aurelia or jelly fish. Those cnidarians which exist in Diagrammatic view of
both forms exhibit alternation of generation (Metagenesis), i.e., polyps Cnidoblast
produce medusae asexually and medusae form the polyps sexually
(e.g., Obelia).
Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone),
Pennatula (Sea-pen), Gorgonia (Sea-fan) and Meandrina (Brain coral).

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4.2.3 Phylum – Ctenophora

Ctenophores, commonly known as sea walnuts or comb
jellies are exclusively marine, radially symmetrical, diploblastic
organisms with tissue level of organisation. The body bears
eight external rows of ciliated comb plates, which help in
locomotion (Figure 4.8). Digestion is both extracellular and
intracellular. Bioluminescence (the property of a living
organism to emit light) is well-marked in ctenophores.
Sexes are not separate. Reproduction takes place only by
sexual means. Fertilisation is external with indirect
development.
Examples: Pleurobrachia and Ctenoplana.

4.2.4 Phylum – Platyhelminthes

They have dorso-ventrally flattened body, hence are called
flatworms (Figure 4.9). These are mostly endoparasites
found in animals including human beings. Flatworms are
Figure 4.8 Example of bilaterally symmetrical, triploblastic and acoelomate animals
Ctenophora with organ level of organisation. Hooks and suckers are
(Pleurobrachia)
present in the parasitic forms. Some of them absorb
nutrients from the host directly through their body surface.
Specialised cells called flame cells help in osmoregulation
and excretion. Sexes are not separate. Fertilisation is
internal and development is through many larval stages.
Some members like Planaria possess high regeneration
capacity.
Examples: Taenia (Tapeworm), Fasciola (Liver fluke).

(a) (b)
Figure 4.9 Examples of Platyhelminthes : (a) Tape worm (b) Liver fluke

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4.2.5 Phylum – Aschelminthes and

Hirudinaria (Blood sucking leech).
The body of the aschelminthes is circular in
cross-section, hence, the name roundworms
(Figure 4.10). They may be freeliving, aquatic
and terrestrial or parasitic in plants and
animals. Roundworms have organ-system
level of body organisation. They are bilaterally
symmetrical, triploblastic and
pseudocoelomate animals. Alimentary canal
is complete with a well- developed muscular
pharynx. An excretory tube removes body
wastes from the body cavity through the
excretory pore. Sexes are separate
(dioecious), i.e., males and females are distinct.
Often females are longer than males.
Fertilisation is internal and development may
be direct (the young ones resemble the adult)
or indirect.
Examples : Ascaris (Roundworm), Wuchereria
(Filaria worm), Ancylostoma (Hookworm).

4.2.6 Phylum – Annelida

They may be aquatic (marine and fresh water)
or terrestrial; free-living, and sometimes
parasitic. They exhibit organ-system level
of body organisation and bilateral symmetry.
They are triploblastic, metamerically
segmented and coelomate animals. Their
body surface is distinctly marked out into
segments or metameres and, hence, the
phylum name Annelida (Latin, annulus : little
ring) (Figure 4.11). They possess longitudinal
and circular muscles which help in
locomotion. Aquatic annelids like Nereis
possess lateral appendages, parapodia,
which help in swimming. A closed circulatory
system is present. Nephridia (sing.
nephridium) help in osmoregulation and
excretion. Neural system consists of paired
ganglia (sing. ganglion) connected by lateral
nerves to a double ventral nerve cord. Nereis,
an aquatic form, is dioecious, but earthworms
and leeches are monoecious. Reproduction is
sexual.
Examples : Nereis, Pheretima (Earthworm)
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Male Female
Figure 4.10 Example of
Aschelminthes:
Roundworm

Figure 4.11 Examples of Annelida : (a) Nereis

(b) Hirudinaria

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4.2.7 Phylum – Arthropoda

This is the largest phylum of Animalia
which includes insects. Over two-thirds of all
named species on earth are arthropods
(Figure 4.12). They have organ-system level
of organisation. They are bilaterally
symmetrical, triploblastic, segmented and
(a) (b) coelomate animals. The body of arthropods
is covered by chitinous exoskeleton. The
body consists of head, thorax and abdomen.
They have jointed appendages (arthros-joint,
poda-appendages). Respiratory organs are
gills, book gills, book lungs or tracheal
system. Circulatory system is of open type.
Sensory organs like antennae, eyes
(compound and simple), statocysts or
balancing organs are present. Excretion
takes place through malpighian tubules.
(c) (d) They are mostly dioecious. Fertilisation is
usually internal. They are mostly
oviparous. Development may be direct or
indirect.
Figure 4.12 Examples of Arthropoda Examples: Economically important insects
: – Apis (Honey bee), Bombyx (Silkworm),
(a) Locust (b) Butterfly
Laccifer (Lac insect)
(c) Scorpion (d) Prawn
Vectors – Anopheles, Culex and Aedes
(Mosquitoes)
Gregarious pest – Locusta (Locust)
Living fossil – Limulus (King crab).

4.2.8 Phylum – Mollusca

This is the second largest animal phylum
(Figure 4.13). Molluscs are terrestrial or aquatic
(marine or fresh water) having an organ-system
level of organisation. They are bilaterally
symmetrical, triploblastic and coelomate
(a) animals. Body is covered by a calcareous
shell and is unsegmented with a distinct
head, muscular foot and visceral hump. A
soft and spongy layer of skin forms a mantle
over the visceral hump. The space between
the hump and the mantle is called the
mantle cavity in which feather like gills are
present. They have respiratory and excretory
(b) functions. The anterior head region has
sensory tentacles. The mouth contains a file-
Figure 4.13 Examples of Mollusca : like rasping organ for feeding, called radula.
(a) Pila (b) Octopus
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They are usually dioecious and oviparous with indirect

development.
Examples: Pila (Apple snail), Pinctada (Pearl oyster), Sepia
(Cuttlefish), Loligo (Squid), Octopus (Devil fish), Aplysia
(Sea- hare), Dentalium (Tusk shell) and Chaetopleura
(Chiton).

4.2.9 Phylum – Echinodermata (a) Animals belonging to phylum

Chordata are fundamentally
These animals have an endoskeleton of calcareous
characterised by the presence of a
ossicles and, hence, the name Echinodermata (Spiny
notochord, a dorsal
bodied, Figure 4.14). All are marine with organ-system
level of organisation. The adult echinoderms are radially
symmetrical but larvae are bilaterally symmetrical. They
are triploblastic and coelomate animals. Digestive system
is complete with mouth on the lower (ventral) side and (b)
anus on the upper (dorsal) side. The most distinctive
feature of echinoderms is the presence of water vascular
system which helps in locomotion, capture and
transport of food and respiration. An excretory system is
absent. Sexes are separate. Reproduction is sexual.
Fertilisation is usually external. Development is indirect
with free-swimming larva.
Examples: Asterias (Star fish), Echinus (Sea urchin),
Antedon
(Sea lily), Cucumaria (Sea cucumber) and Ophiura (Brittle
star).

4.2.10 Phylum – Hemichordata

Hemichordata was earlier considered as a sub-phylum
under phylum Chordata. But now it is placed as a separate
phylum under non-chordata. Hemichordates have a
rudimentary structure in the collar region called
stomochord, a structure similar to notochord.
This phylum consists of a small group of worm-like
marine animals with organ-system level of organisation.
They are bilaterally symmetrical, triploblastic and
coelomate animals. The body is cylindrical and is
composed of an anterior proboscis, a collar and a long
trunk (Figure 4.15). Circulatory system is of open type.
Respiration takes place through gills. Excretory organ is
proboscis gland. Sexes are separate. Fertilisation is
external. Development is indirect.
Examples: Balanoglossus and Saccoglossus.

4.2.11 Phylum – Chordata

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Figure 4.14 Examples of

Echinodermata :
(a) Asterias
(b) Ophiura

Proboscis

Collar

Trunk

Figure 4.15 Balanoglossus

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Nerve cord Notochord hollow nerve cord and paired pharyngeal

gill slits (Figure 4.16). These are bilaterally
symmetrical, triploblastic, coelomate with
organ-system level of organisation. They
possess a post anal tail and a closed
Post-anal part
circulatory system.
Gill slits Table 4.1 presents a comparison of salient
features of chordates and non-chordates.
Figure 4.16 Chordata characteristics

TABLE 4.1 Comparison of Chordates and Non-chordates

S.No. Chordates Non-chordates
1. Notochord present. Notochord absent.
2. Central nervous system is dorsal, Central nervous system is ventral, solid
hollow and single. and double.
3. Pharynx perforated by gill slits. Gill slits are absent.
4. Heart is ventral. Heart is dorsal (if present).
5. A post-anal part (tail) is present. Post-anal tail is absent.

Phylum Chordata is divided into three

subphyla: Urochordata or Tunicata ,
Cephalochordata and Vertebrata.
Subphyla Urochordata and
Cephalochordata are often referred to as
protochordates (Figure 4.17) and are
exclusively marine. In Urochordata, notochord
is present only in larval tail, while in
Cephalochordata, it extends from head to
tail region and is persistent throughout their
life.
Examples: Urochordata – Ascidia, Salpa,
Doliolum; Cephalochordata – Branchiostoma
(Amphioxus or Lancelet).
The members of subphylum Vertebrata
possess notochord during the embryonic
period. The notochord is replaced by a
cartilaginous or bony vertebral column in
the adult. Thus all vertebrates are chordates
Figure 4.17 Ascidia
but all chordates are not vertebrates.
Besides the basic chordate characters,
vertebrates have a ventral muscular heart
with two, three or four chambers, kidneys
for excretion and osmoregulation and
paired appendages which may be fins or
limbs.

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The subphylum Vertebrata is further divided as follows:

Vertebrata
Division

Agnatha Gnathostomata
(lacks jaw) (bears jaw)
Super Class

Pisces Tetrapoda
(bear fins) (bear limbs)

Class Class Class

1. Cyclostomata 1. Chondrichthyes 1. Amphibia
2. Osteichthyes 2. Reptilia
3. Aves
4. Mammals

4.2.11.1 Class – Cyclostomata

All living members of the class Cyclostomata
are ectoparasites on some fishes. They have
an elongated body bearing 6-15 pairs of gill
slits for respiration. Cyclostomes have a
sucking and circular mouth without jaws
(Fig. 4.18). Their body is devoid of scales and
paired fins. Cranium and vertebral
column are cartilaginous. Circulation is of
closed type. Cyclostomes are marine but
migrate for spawning to fresh water. After Figure 4.18 A jawless vertebrate - Petromyzon
spawning, within a few days, they die. Their
larvae, after metamorphosis, return to the
ocean.
Examples: Petromyzon (Lamprey) and Myxine
(Hagfish).
4.2.11.2 Class – Chondrichthyes
They are marine animals with streamlined (a)
body and have cartilaginous
endoskeleton (Figure 4.19). Mouth is
located ventrally. Notochord is persistent
throughout life. Gill slits are separate and
without operculum (gill cover). The skin is
tough, containing minute placoid scales.
Teeth are modified placoid scales which are (b)
backwardly directed. Their jaws are very
powerful. These animals are predaceous. Due Figure 4.19 Example of Cartilaginous fishes :
(a) Scoliodon (b) Pristis
to the absence of air bladder, they have to
swim constantly to avoid sinking.

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ANIMAL KINGDOM 51

Heart is two-chambered (one auricle and one ventricle).

Some of them have electric organs (e.g., Torpedo)
and some possess poison sting (e.g., Trygon). They
are cold-blooded (poikilothermous) animals, i.e., they
lack the capacity to regulate their body temperature.
Sexes are separate. In males pelvic fins bear
claspers. They have internal fertilisation and many
of them are viviparous.
Examples: Scoliodon (Dog fish), Pristis (Saw fish),
Carcharodon (Great white shark), Trygon (Sting ray).
4.2.11.3 Class – Osteichthyes
It includes both marine and fresh water fishes with
bony endoskeleton. Their body is streamlined.
Mouth is mostly terminal (Figure 4.20). They have
four pairs of gills which are covered by an
operculum on each side. Skin is covered with
cycloid/ctenoid scales. Air bladder is present which
(a) (b) regulates buoyancy. Heart is two- chambered (one
auricle and one ventricle). They are
Figure 4.20 Examples of Bony fishes : cold-blooded animals. Sexes are separate.
(a) Hippocampus (b) Catla Fertilisation is usually external. They are mostly
oviparous and development is direct.
Examples: Marine – Exocoetus (Flying fish),
Hippocampus (Sea horse); Freshwater – Labeo (Rohu),
Catla (Katla), Clarias (Magur); Aquarium – Betta
(Fighting fish), Pterophyllum (Angel fish).

4.2.11.4 Class – Amphibia

As the name indicates (Gr., Amphi : dual, bios, life),
(a) amphibians can live in aquatic as well as terrestrial
habitats (Figure 4.21). Most of them have two pairs
of limbs. Body is divisible into head and trunk. Tail
may be present in some. The amphibian skin is
moist (without scales). The eyes have eyelids. A
tympanum represents the ear. Alimentary canal,
urinary and reproductive tracts open into a common
chamber called cloaca which opens to the exterior.
Respiration is by gills, lungs and through skin. The
heart is three- chambered (two auricles and one
ventricle). These are cold-blooded animals. Sexes are
(b)
separate. Fertilisation is external. They are
oviparous and development is indirect.
Figure 4.21 Examples of Amphibia :
(a) Salamandra
Examples: Bufo (Toad), Rana (Frog), Hyla (Tree frog),
(b) Rana Salamandra (Salamander), Ichthyophis (Limbless
amphibia).

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(a) (b) (c) (d)

Figure 4.22 Reptiles: (a) Chameleon (b) Crocodilus (c) Chelone (d) Naja

4.2.11.5 Class – Reptilia

The class name refers to their creeping or crawling mode of
locomotion (Latin, repere or reptum, to creep or crawl). They are
mostly terrestrial animals and their body is covered by dry and
cornified skin, epidermal scales or scutes (Fig. 4.22). They do not
have external ear openings. Tympanum represents ear. Limbs, when
present, are two pairs. Heart is usually three-chambered, but four-
chambered in crocodiles. Reptiles are poikilotherms. Snakes and
lizards shed their scales as skin cast. Sexes are separate. Fertilisation
is internal. They are oviparous and development is direct.
Examples: Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizard),
Calotes (Garden lizard), Crocodilus (Crocodile), Alligator (Alligator).
Hemidactylus (Wall lizard), Poisonous snakes – Naja (Cobra), Bangarus
(Krait), Vipera (Viper).
4.2.11.6 Class – Aves
The characteristic features of Aves (birds) are the presence of
feathers and most of them can fly except flightless birds (e.g., Ostrich).
They possess beak (Figure 4.23). The forelimbs are modified into wings.
The hind limbs generally have scales and are modified for walking,
swimming or clasping the tree branches. Skin is dry without glands
except the oil gland at the base of the tail. Endoskeleton is fully
ossified (bony) and the long bones are hollow with air cavities
(pneumatic). The digestive tract of birds has additional chambers, the
crop and gizzard. Heart is completely four- chambered. They are
warm-blooded (homoiothermous) animals, i.e., they are able to
maintain a constant body temperature. Respiration is by

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ANIMAL KINGDOM 53

(a) (b) (c) (d)

Figure 4.23 Some birds : (a) Neophron (b) Struthio (c) Psittacula (d) Pavo

lungs. Air sacs connected to lungs supplement respiration. Sexes are

separate. Fertilisation is internal. They are oviparous and development
is direct.
Examples : Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio
(Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture).

4.2.11.7 Class – Mammalia

They are found in a variety of habitats – polar ice caps, deserts, mountains,
forests, grasslands and dark caves. Some of them have adapted to fly
or live in water. The most unique mammalian characteristic is the
presence of milk producing glands (mammary glands) by which the
young ones are nourished. They have two pairs of limbs, adapted for
walking, running, climbing, burrowing, swimming or flying (Figure
4.24). The skin of

(c)
(a)

(d)
(b)
Figure 4.24 Some mammals : (a) Ornithorhynchus (b) Macropus (c) Pteropus (d) Balaenoptera

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mammals is unique in possessing hair. External ears or pinnae are

present. Different types of teeth are present in the jaw. Heart is four-
chambered. They are homoiothermous. Respiration is by lungs.
Sexes are separate and fertilisation is internal. They are viviparous
with few exceptions and development is direct.
Examples: Oviparous-Ornithorhynchus (Platypus); Viviparous -
Macropus (Kangaroo), Pteropus (Flying fox), Camelus (Camel), Macaca
(Monkey), Rattus (Rat), Canis (Dog), Felis (Cat), Elephas (Elephant),
Equus (Horse), Delphinus (Common dolphin), Balaenoptera (Blue whale),
Panthera tigris (Tiger), Panthera leo (Lion).
The salient distinguishing features of all phyla under animal
kingdom is comprehensively given in the Table 4.2.

TABLE 4.2 Salient Features of Different Phyla in the Animal Kingdom

Phylum Level of Symme- Coelom Segmen- Digestive Circu- Respi- Distinctive

Organi- try tation System latory ratory Features
sation System System

Cellular Absent Absent Absent Body with pores

Porifera Various Absent Absent
and canals in
walls.
Coelenterata Cnidoblasts
(Cnidaria) Tissue Radial Absent Absent Incomplete Absent Absent
present.

Tissue Radial Absent Absent Incomplete Absent Absent Comb plates for
Ctenophora locomotion.
Organ &
Platyhelm- Organ- Bilateral Absent Absent Incomplete Absent Absent Flat body, suckers.
inthes system
Organ- Pseudo Often worm-
Aschelmin- Bilateral coelo- Absent Complete Absent Absent shaped,
system
thes mate elongated.
Organ- Coelo- Body segment-
Annelida system Bilateral mate Present Complete Present Absent ation like rings.
Organ- Coelo- Exoskeleton of
Bilateral Present Complete Present Present
cu- ticle, jointed
Arthropoda system mate
ap- pendages.
Organ- Bilateral Coelo- Absent Complete Present Present External skeleton
Mollusca system mate of shell usually
present.

Organ- Coelo- Water vascular

Echino- Radial Absent Complete Present Present
system mate system, radial
dermata
symmetry.
Organ- Bilateral Coelo- Worm-like with
Hemi- Absent Complete Present Present
system mate proboscis, collar
chordata
and trunk.
Notochord, dorsal
Organ- Bilateral Coelo- Present Complete Present Present hollow nerve
Chordata
system mate cord, gill slits
with limbs or
fins.

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SUMMARY

The basic fundamental features such as level of organisation,

symmetry, cell organisation, coelom, segmentation, notochord, etc.,
have enabled us to broadly classify the animal kingdom. Besides the
fundamental features, there are many other distinctive characters
which are specific for each phyla or class.
Porifera includes multicellular animals which exhibit cellular level
of organisation and have characteristic flagellated choanocytes. The
coelenterates have tentacles and bear cnidoblasts. They are mostly aquatic,
sessile or free-floating. The ctenophores are marine animals with
comb plates. The platyhelminths have flat body and exhibit bilateral
symmetry. The parasitic forms show distinct suckers and hooks.
Aschelminthes are pseudocoelomates and include parasitic as well as
non-parasitic roundworms.
Annelids are metamerically segmented animals with a true
coelom. The arthropods are the most abundant group of animals
characterised by the presence of jointed appendages. The molluscs
have a soft body surrounded by an external calcareous shell. The
body is covered with external skeleton made of chitin. The
echinoderms possess a spiny skin. Their most distinctive feature is
the presence of water vascular system. The hemichordates are a small
group of worm-like marine animals. They have a cylindrical body with
proboscis, collar and trunk.
Phylum Chordata includes animals which possess a notochord
either throughout or during early embryonic life. Other common
features observed in the chordates are the dorsal, hollow nerve cord
and paired pharyngeal gill slits. Some of the vertebrates do not possess
jaws (Agnatha) whereas most of them possess jaws (Gnathostomata).
Agnatha is represented by the class, Cyclostomata. They are the most
primitive chordates and are ectoparasites on fishes. Gnathostomata has
two super classes, Pisces and Tetrapoda. Classes Chondrichthyes and
Osteichthyes bear fins for locomotion and are grouped under
Pisces. The Chondrichthyes are fishes with cartilaginous endoskeleton
and are marine. Classes, Amphibia, Reptilia, Aves and Mammalia have
two pairs of limbs and are thus grouped under Tetrapoda. The
amphibians have adapted to live both on land and water. Reptiles are
characterised by the presence

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of dry and cornified skin. Limbs are absent in snakes. Fishes,

amphibians and reptiles are poikilothermous (cold-blooded). Aves are
warm-blooded animals with feathers on their bodies and forelimbs
modified into wings for flying. Hind limbs are adapted for walking,
swimming, perching or clasping. The unique features of mammals are
the presence of mammary glands and hairs on the skin. They
commonly exhibit viviparity.

EXERCISES

1. What are the difficulties that you would face in classification of

animals, if common fundamental features are not taken into
account?
2. If you are given a specimen, what are the steps that you would follow
to classify it?
3. How useful is the study of the nature of body cavity and coelom in
the classification of animals?
4. Distinguish between intracellular and extracellular digestion?
5. What is the difference between direct and indirect development?
6. What are the peculiar features that you find in parasitic
platyhelminthes?
7. What are the reasons that you can think of for the arthropods to
constitute the largest group of the animal kingdom?
8. Water vascular system is the characteristic of which group of the
following:
(a) Porifera (b) Ctenophora (c) Echinodermata (d) Chordata
9. “All vertebrates are chordates but all chordates are not vertebrates”.
Justify the statement.
10. How important is the presence of air bladder in Pisces?
11. What are the modifications that are observed in birds that help
them fly?
12. Could the number of eggs or young ones produced by an oviparous
and viviparous mother be equal? Why?
13. Segmentation in the body is first observed in which of the following:
(a) Platyhelminthes (b) Aschelminthes (c) Annelida (d) Arthropoda

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14. Match the following:

(a) Operculum (i) Ctenophora
(b) Parapodia (ii) Mollusca
(c) Scales (iii) Porifera
(d) Comb plates (iv) Reptilia
(e) Radula (v) Annelida
(f) Hairs (vi) Cyclostomata and Chondrichthyes
(g) Choanocytes (vii) Mammalia
(h) Gill slits (viii) Osteichthyes
15. Prepare a list of some animals that are found parasitic on human
beings.

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