Annelids are vermiform, bilaterally symmetrical, triploblastic and schizocoelomate animals whose body is metamerically segmented and covered with a layer of cuticle. Body bears segmentally arranged setae or chetae. Acron is divided into prostomium and peristomeum and the posterior terminal segment is called pygidium. Blood vascular system is closed type and a respiratory pigment is usually present. Excretion takes place by nephridia.
Body length varies from less than 1 mm to more than 3 m. The Giant Earthworm (Megascolides australis) can attain 1 m length. The South American leech, Haementeriaghilianii, can reach more than 40 cm in length. There are more than 9000 species of annelids in the world.
Polychaetes are mostly marine but some are freshwater or brackish water species. Commonly found from the intertidal zone to depths of over 5,000 m. The polychaetes are so named because of the numerous setae or chaetae they bear. They have about 8,000 species. Each body segment has a pair of locomotory organs called parapodia that bear many bristles called chetae, because of which polychaetes are sometimes referred to as bristle worms. Body size varies from 1 mm to about one metre. Polychaetes differ from other annelids in having a well differentiated head with sense organs, such as eyes, antennae and sensory palps and there is no clitellum on body. Polychaetes are dioecious but have no permanent sex organs and no gonoducts. Some polychaetes are free swimming pelagic forms, some are active burrowers and crawlers, while others are sedentary, living in tubes or burrows and feeding on planktons and detritus. Based on their habitat they are divided into two subclasses:
1. Subclass ERRANTIA
They are free swimming or crawling worms in which body possesses numerous, identical segments and well-developed parapodia and setae. There is a definite head, with an eversible pharynx having jaws or teeth. The group includes swimming, crawling, burrowing and tube-dwelling members. Ex. Nereis; Polynoe; Aphrodite; Glycera; Eunice.
2. Subclass SEDENTARIA
They are tube dwelling worms that display a high degree of segmental differentiation. Parapodia are reduced, with or without modified setae. Prostomium without sensory structures but has tentacles and palps or other feeding structures. Teeth or jaws are generally absent. Ex. Arenicola; Terebella; Amphitrite; Serpula.
They are worms without a distinct head and parapodia or suckers. There is a clitellum which produces a cocoon during breeding season. Pharynx is not eversible. They are hermaphrodite and there is no larval stage. There are about 10,000 species. Locomotion is by small setae and burrowing by coelomic hydraulic skeleton. Most of them are mud ingesting and detritus feeders but few are predaceous, e.g. Agriodrilus and Phagodrilus. There are two orders under class Oligochaeta.
1. Order ARCHIOLIGOCHAETA (=TUBIFICIDA)
Freshwater forms with worm-like body and setae present in bundles. Eye spots may be present. Gizzard is poorly developed. Generally live in muddy tubes. Ex. Tubifex; Dero; Chaetogaster; Aelosoma; Nais; Stylaria; Enchytraeus.
2. Order NEOLIGOCHAETA (=HAPLOTAXIDA)
Commonly known as earthworms, they are terrestrial worms with well developed gizzard and without eye spots. Clitellum well developed and female genital opening on the 14th segment. Minute setae are used in peristaltic locomotion and hydraulic skeleton is used for burrowing in mud. Ex. Pheretima; Lumbricus; Eudrilus; Moniligaster; megascolides.
They are ectoparasites of vertebrates having definite number of segments and annuli on each segment and without parapodia. Suckers are present for attachment and locomotion. They are hermaphrodite and deposit their eggs in cocoons. There are 4 orders as follows:
1. Order ACANTHOBDELLIDA, Ex. Acanthobdella. Parasitic on salmons.
2. Order RHYNCHOBDELLIDA, Ex. Pontobdella. Aquatic leeches.
3. Order GNATHOBDELLIDA, Ex. Hirudo, Hirudinaria. Common cattle leeches.
4. Order PHARYNGOBDELLIDA, Ex. Erpobdella. Terrestrial predatory leeches.
They are primitive marine worms, without distinct segmentation, parapodia and setae. There are 2 or 3 tentacles on prostomium. Anus is terminal. Larva is a typical trochophore. Ex. Polygordius; Protodrilus, Nerilla; Dinophilus.