ANNELIDS

KHRISTINA J. CRUZ
Department of Biological Sciences
College of Arts and Sciences
Central Luzon State University
Science City of Muñoz, Nueva Ecija
 INTRODUCTION

• earthworms, polychaete worms,

and leeches.
• one of the largest animal groups
• All members of the group are to
some extent segmented
• Segmentation is called
METAMERISM.
• METAMERISM is the organization
of the body in a linear series of
similar segments.
• Segments each contain elements of
such body systems as circulatory,
nervous, and excretory tracts.
• Metamerism
–increases the efficiency of body
movement by allowing the effect
of muscle contraction to be
extremely localized
–it makes possible the development
of greater complexity in general
body organization
• Segmentation allows for
flexibility and mobility
because annelids can
bend at segmented parts
• the body wall of annelids is
characterized by being made up
of both circular and longitudinal
muscle fibers surrounded by a
moist cuticle that is secreted by
an epidermal epithelium.
• All annelids except leeches
also have chitinous hair-like
structures, called SETAE,
projecting from their cuticle.
Sometimes the setae are
located on paddle-like
appendages called
PARAPODIA.
3 MAJOR PROTOSTOME PHYLA

• ANNELIDA
• MOLLUSCA
• ARTHROPODA
• The internal organs of
annelids are well
developed.
• includes a closed,
segmentally-arranged
circulatory system.
• The digestive system is a
complete tube with mouth and
anus.
• Gases are exchanged through
the skin, or sometimes through
specialized gills or modified
parapodia.
• Each segment
typically contains a
pair of nephridia.
• The nervous system includes a pair of
cephalic ganglia attached to double
nerve cords that run the length of the
animal along the ventral body wall, with
ganglia and branches in each segment.
• Annelids have some combination of
tactile organs, chemoreceptors,
balance receptors, and photoreceptors;
some forms have fairly well developed
eyes, including lenses.
• Annelids may be monoecious or
dioecious.
• Larva may or may not be present; if
present they are of the trochophore
type. Some forms also reproduce
asexually.
• They are protostomes, with spiral
cleavage.
• Annelids probably originated in
the Precambrian. Definite
annelids appeared in the
Cambrian. Pictured above is
Canadia
 INTRODUCTION
• Canadia and other Cambrian
polychaetes had no jaws, but some
later polychaetes developed hard
jaws, which are sometimes
mineralized with iron oxide. Such
polychaete jaws are fairly common
in the fossil record, and are known
as scolecodonts.
Scolecodonts -- Fossil Polychaete
Worm Jaws
 DOMINANT CHARACTERISTICS
• METAMERISM
• PRESENCE OF LARGE
COMPARTMENTALIZED
COELOM : The coelom is divided
into a series of repeated parts.
• ANNELIDS NEED NO RIGID
SKELETAL PARTS IN ORDER TO
EFFECT MOVEMENT
 CLASSES

»POLYCHAETA
»OLIGOCHAETA
»HIRUDINEA
 POLYCHAETA
• Polychaetes include such forms as sand
worms, tube worms, and clam worms.
• Most have well developed, paired, paddle-
like appendages (parapodia), well
developed sense organs, and numerous
setae (usually on the parapodia;
"polychaete" means "many hairs").
• Polychaetes usually have a well-developed
head, often complete with well-developed
eyes, antennae, and sensory palps.
POLYCHAETA
 POLYCHAETA
• Fertilization is external, and
development proceeds indirectly
through a trochophore larva.
• They lack any permanent sex organs (in
contrast to other kinds of annelids);
gonads appear as swellings during the
breeding season.
 POLYCHAETA
• Polychaetes are a large and extremely
diverse group. Around 10,000 species have
been described.
• Most are marine.
– featherduster worms: sedentary, lives tubes
buried in sand or mud; feed by trapping food
particles in mucus or by ciliary action.
– Others, such as the clam worm, are active,
mobile predators that capture prey in jaws
attached to their pharynges.
– fireworms, graze on gorgonians and stony
corals.
 POLYCHAETA

• They play essential ecological roles,

serving on one hand as predators
on small invertebrates, and on the
other as food for fish and large
invertebrates.
 POLYCHAETA

SUBCLASSES

• ERRANTIA - active and mobile

• SEDENTARIA - immobile, spending
their entire lives burrowed in sediment
 POLYCHAETA

• The errant polychaetes (ERRANTIA)

include some species that are strictly
pelagic (live in the water column), some
that crawl about beneath rocks and
shells, some that are active burrowers in
sand and mud, but also some that
occupy stationary tubes.
Nereis
• sedentary species (SEDENTARIA) construct
and live in semi-permanent burrows or tubes
of various complexity, but some species can
leave the burrow or tube and start a new
home.
• Live in tubes that they make from sticky
proteins secreted near the mouth. Feathery
appendages extending from the tubes trap
food in the water.
• "Christmas-tree worm" Spirobranchus giganteus
are typically confined to their tube and can only
extend their heads from the tube opening.
• All polychates possess a pre-oral head
segment or PROSTOMIUM, equipped
with PROSTOMIAL TENTACLES, PALPS
and EYES.
• Behind the head is a PERISTOMIUM,
with mouth and carries 4 pairs of
PERISTOMIAL TENTACLES (organ of
touch, taste, smell, sight, finding food,
avoiding predators)
POLYCHAETA
Dorsal view
 SETAE
• Serves as holdfast devices when
a worm is in its burrow or moving
over ground
 Oligochaeta
• earthworms and other worms that
live in terrestrial or freshwater
environments.
• Some are marine, but most (94%)
live out of sea water.
 Oligochaeta
• hermaphroditic
• the earthworm extracts nutrients from the
soil as it burrows, excreted from the anus
• Because the nutrients that earthworms
dig up are necessary for fertile soil, these
earthworms are an intricate part of farms,
actually tilling the soil
 Oligochaeta
• In general, oligochaetes lack
obvious cephalic (head) sensory
structures or elaborations.
• they possess a clitellum (a
secretory region that functions in
reproduction)
• Over 3000 species are known
• 115 to 200 SOMITES, separated
by TRANSVERSE GROOVES
• MOUTH is in SOMITE I, overhung
by the fleshy PROSTOMIUM
• ANUS is in the LAST SOMITE
• CLITELLUM is in SOMITES 32 –
37 : it is a conspicuous glandular
swelling that secretes the
materials forming cocoons
• Oligochaetes feed primarily on detritus
and algae.
• Earthworms cycle huge quantities of
soil through their guts, a process that
speeds the turnover of nutrients in soil
and increases productivity.
–They also help to aerate soil.
• Aquatic oligochaetes are important
food for fishes and larger
invertebrates. A few are ectoparastic.
 DIGESTIVE SYSTEM
1. MOUTH
2. BUCCAL CAVITY
3. PHARYNX
4. ESOPHAGUS (joined by CALCIFEROUS
GLANDS)
5. CROP : for storage
6. GIZZARD : with muscular wall for grinding
7. INTESTINE : absorption
8. ANUS
 INTESTINE
• Lined by TYPHLOSOLE : functions for
the increase in surface for digestion and
absorption of food
• The exterior is covered by
CHLORAGOGUE CELLS, which
functions as a LIVER: location of fat and
glycogen synthesis and storage, urea
formation and deamination of amino
acids
Oligochaeta
 Oligochaeta
Earthworm copulation
Oligochaeta
Megascolides australis
Terriswalkeris terraereginae
 System Type Earthworms System

An earthworm does not have a skeleton. It has

bristles on each segment called setae that help
the earthworm move. Earthworms have no
Muscular-Skeletal
limbs. The earthworm has two sets of muscles;
one that makes it long and thin and one that
makes it fat.
An earthworm has a digestive system. It eats dirt,
digesting the plant and animal matter in the dirt
and then eliminates the rest. It has an
esophagus for the food to go down, a crop to
Digestion
store the food in, a gizzard that grinds the food
down, intestines for the food to pass through
and take out nutrients and an anus for the food
to come out.
An earthworm has a nervous system with a simple
Nervous
brain and nerve cord.
System Type Earthworms System

An earthworm has blood and blood

Circulation
vessels with multiple (5) hearts.

An earthworm has no respiratory

organ. It takes in oxygen directly
Respiration through its skin and gives off
carbon dioxide. Its skin is always
moist.
System Type Earthworms System

An earthworm's wastes help to fertilize

the soil. It gets rid of its wastes
Excretion through tubes called nephridia that
lead to pores that allow the wastes
out.

Symmetry An earthworm has bilateral symmetry.

An earthworm is generally earthtones

Coloration such as brown, tan, etc.. It can be
up to eight feet in length!
 An earthworm has both sperm and
eggs within its body and
reproduces sexually. However, the
eggs must be fertilized by the
Reproduction sperm of another worm. An
earthworm lays a batch of eggs at
one time. They do not spend time
raising their young once they are
hatched.
 Hirudinea
• Leeches are unlike other annelids in several
ways:
– They have a fixed number of segments
(usually 34);
– a dorso-ventrally flattened body,
– with anterior and posterior sucker (usually),
– no parapodia, and
– usually no setae.
– The coelom is not subdivided by septa in
most species, and it has been filled with
muscle and connective tissue.
 Hirudinea
• Leeches are hermaphroditic. Development
is direct as in oligochaetes.
• Most leeches are found in freshwater
habitats, but a few are marine and some
are terrestrial (but they require warm, moist
conditions).
• Most are either carnivorous or parasitic.
Medicinal leeches were used for centuries
by physicians to control diseases that were
believed to be caused by an excess of
blood
 Hirudinea
• Approximately 500 species of leeches have
been described
• Leeches have a fixed number of segments in
their bodies. There may be variable numbers
of superficial annuli (external rings on the
skin resembling the segments in the other
classes), but these annuli mask the true
body segments which generally number 34.
The number of segments is remarkably
constant for a group with a great deal of
variation in size.
 Hirudinea

• About three quarters of the members

of this class are ectoparasitic
 Hirudinea
• Leeches are known for sucking the blood
of humans, but most are free-living and eat
small invertebrates, feeding on their blood.
• Most live in freshwater environments,
although a few can live in terrestrial
environments. Like earthworms, leeches
are hermaphroditic and lack appendages.
• To successfully drink the blood, they have
a small sucker in the anterior end and a
larger one in the posterior end.
 HIRUDIN
• Secreted by the leeches that
prevents the coagulation of blood
• Digestion is by proteolytic
enzymes and by slow
decomposition due to the action of
a bacterium (Pseudomonas
hirudinis)
Hirudinea
Hirudinea
DIFFERS FROM OTHER ANNELIDS :

• LACKS SETAE
• PRESENCE OF COPULATORY
ORGANS
• GENITAL OPENINGS
 Oligochaeta Polychaeta Hirudinea
Few - reduced in size Many - varying in Absent
Setae if present length and size

Parapodia Absent, although

arrangement of setae
Highly developed and
obvious
Absent

roughly correspond
to parapodial
location

Head Reduced and lacking

appendages
Well developed and
complex, often with
Reduced and lacking
appendages
many appendages

Suckers Absent Absent Present

 Oligochaeta Polychaeta Hirudinea
Sex Hermaphroditic Separate Hermaphroditic

Gonads Essentially 2 pairs Peritoneal lining Several paired testes

of testes and 1 seasonally and a single ovary
pair of ovaries proliferates sex
cells

Fertilization Internal External Internal

Larval type Absent Trochophore Absent

Jaws Absent Very well developed Present in some

Metamerizat Numerous Variable, but many Number of segments

relatively
ion constant, although
number of annuli
vary
 Key Features of Annelida
Body metamerically segmented; symmetry bilateral.
· Body wall with outer circular and inner longitudinal
muscle layers; outer transparent moist cuticle
secreted by epithelium.
· Chitinous setae (absent in leeches), often present
on fleshy appendages called parapodia.
· Coelom well developed and divided by septa
(except in leeches); coelomic fluid supplies
turgidity and acts as a hydrostatic skeleton.
· Blood system closed and segmentally arranged.
• Digestive system complete and not
metamerically arranged.
· Respiratory gas exchange through skin,
gills or parapodia.
· Excretory system typically a pair of
nephridia for each metamere.
· Nervous and sensory systems present.
· Hermaphroditic or separate sexes;
asexual reproduction by budding in some.