The following points highlight the three main classes of the phylum annelida. The classes are: 1. Chaetopoda 2. Hirudinea 3. Archiannelida.
Class # 1. Chaetopoda:
1. Setae develop in skin-sacs and elevated on the Para podia.
2. Segments are usually numerous, well-marked and similar throughout.
3. A large coelom, divided into chambers by septa and not continuous with blood vascular system.
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4. Coelomoducts function as reproductive ducts in many cases.
The class Chaetopoda has been subdivided into two orders:
Order i. Polychaeta:
a. Marine chaetopods with Para podia as locomotory organs.
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b. Head distinct and bears a number of appendages — eyes, tentacles, etc.
c. Clitellum is never present.
d. Sexes distinct, ovaries and testes are metameric in arrangement.
e. Development indirect, larva always trochophore.
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Examples: Neanthes, Aphrodite, Terebella Sabella, etc.
Order ii. Oligochaeta:
a. Land and fresh water chaetopods without appendages.
b. Simple setae for locomotion are present in each segment.
c. Head not distinct, clitellum usually present.
d. Hermaphrodite, gonads of each type never exceeds two pairs.
e. Development direct, metamorphosis always absent.
Examples: Pheretima, Lumbricus, Tubifex, etc.
Class # 2. Hirudinea:
1. Free-living or parasitic, live either on land or in fresh water or in sea.
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2. Body elongated and flattened, devoid of setae and marked externally by more rings than the true segments.
3. The anterior end is modified into a ventrally directed sucker and several hindermost segments fuse to form a powerful posterior sucker, directed downwards.
4. Alimentary tract straight, the mouth and anus opposite and terminal.
5. Coelom is obliterated by connective tissue and is represented only in sinuses containing haemolymph.
6. Hermaphrodite, testes numerous and usually segmentally arranged; the ovaries are a single pair. Genital ducts always present.
7. The penis and vagina are impaired and open to the exterior by respective median aperture.
8. Development always direct.
The class Hirudinea has been subdivided into four orders:
Order i. Acanthobdellida:
a. A short proboscis is present.
b. Anterior sucker absent.
Example: Acanthobdella (a parasite on fish).
Order ii. Rhynchobdellida:
A protrusible proboscis is present.
Examples: Piscida, Pontobdella, etc.
Order iii. Gnathobdellida:
Mouth with more than one toothed jaws.
Example: Hirudinaria.
Order iv. Herpobdellida:
Mouth without true jaws.
Example: Trocheta.
Class # 3. Archiannelida:
1. Primitive, marine worms with narrow, elongated, cylindrical body.
2. True segmentation present, but the segments may not always be distinct externally.
3. Head bears a distinct prostomium with a pair of tentacles and a peristomium with a ventral mouth.
4. Alimentary tract is a straight tube and the anus is terminal.
5. Sense organs probably in the form of paired ciliated grooves.
6. Sexes united or separate.
7. Development with a metamorphosis larva trochophore.
Examples: Polygordius, Protodrilus, etc..