The document provides an overview of the phylum Annelida, detailing its characteristics, classification, and examples of organisms within the major classes: Polychaeta, Oligochaeta, and Hirudinea. It describes the morphological traits, habitats, and reproductive behaviors of notable examples such as Nereis (a marine predator) and Pheretima (the earthworm), as well as the characteristics of leeches in the Hirudinea class. Key features include segmentation, type of setae, and survival strategies for these organisms.

1. Course I : First semester
PHYLUM ANNELIDA
(Metamerically segmented and coelomate animals)
1. Annelids are mostly free living and marine, freshwater or soil dwellers.
2. These are soft bodied, triploblastic, bilaterally symmetrical and true coelomates.
3. Body is divisible onto head and trunk with a short tail. It is divided into the segments
called as annuli.
4. Excretion is performed by nephridia.
5. Nervous system is well developed, showing brain called as cerebral ganglia.
Classification of Phylum -Annelida:
I. Class – Polychaeta: (animals with many setae):-
1. Setae are many and are bunched together to give rise to lateral parapodia.
2. Polychaets are marine and either free living or tube dwellers, predatory or filter
feeder.
3. Head is distinct with sensory organs like eyes,
tentacles, palps etc.
4. Clitellum is absent.
5. Free swimming larval stage is found called as
trochophore.
6. A typical example is Nereis
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2. Course I : First semester
Nereis: -
1. It is marine and free living predatory organism. It is found at the bottom of rocky
shore in shallow water hidden in the
crevices.
2. Nereis has distinct head with pairs of
eyes, tentacles, palps oral cirri and
anal cirri.
3. Its mouth is guarded by a pair of
claws which helps it to kill prey.
4. It is nocturnal (i.e. active during
night). During the night it feeds on
small mollusks, crustaceans and
worms. Sexes are separate.
5. During breeding period its posterior
parapodia are enlarged and thus it
becomes heteronereis.
II. Class – Oligochaeta: ( animals with few setae):-
1. Setae few and are arranged segmentally.
2. Class includes red worms and earthworms. These are mostly terrestrial or fresh water
forms.
3. Distinct head eyes and tentacles are absent, body is cylindrical.
4. Clitellum is usually present.
5. They are hermaphrodite.
6. A typical example is Pheretima (earthworm)
Pheretima (earthworm):-
1. Earthworms are burrowers found in surface soil
during rainy season. They are nocturnal in habit,
during night they come out of their burrows and
feed on organic reach soil.
2. Body is made up of 100 to 120 segments and is
divided into anterior 13 segments as preclitellar
region; middle 14th to 16th segments as clitellar
region and posterior remaining part as postclitellar
region.
3. Digestive system is complete; mouth is present in
first segment guarded by prostomium and anus is
present in last segment.
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3. Course I : First semester
4. Setae are few and embedded in the skin.
5. Earthworm has integumentary, intestinal and septal nephridia. It is hermaphrodite.
Fertilization is cross and internal and development is direct.
III. Class – Hirudinea: (Class of leeches):-
1. These are mostly ectoparasitic and fresh water forms.
2. Leeches are sanguivorous i.e. feeding on blood of cattle. To keep blood uncoagulated
they secrete an enzyme in its gut called as hirudin; hence the class is named Hirudinea.
3. Head, eyes, tentacles and parapodia are absent.
4. Body made up of definite segment number and is dorsoventrally flattened. Setae are
embedded in skin and are arranged segmentally.
5. A typical example is Hirudinaria (common Indian leech)
Hirudinaria (common Indian leech):-
1. It is found in fresh water tanks, lakes, ponds,
swamps etc. it is ectoparasitic and
sanguivorous sucking the blood of fish, frog,
and cattle and also human.
2. Body is made up of 33 segments and it grows
upto a length of 30 to 35 cm. body is divided
into preclitellar region of first eight
segments; 9th, 10th and 11th segments includes
in clitellar region and remaining 22
segments are called as postclitellar region.
3. A ventral mouth is triradiate and guarded by
an oral sucker. Anus is present in 22nd
segment on dorsal side.
4. Posterior sucker is made up of fusion of last
seven segments. Male genital pore and
female genital pores are present in tenth and
eleventh segment respectively on ventral side.
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