General characteristics of phylum Annelida:

(i) They are triploblastic, bilaterally symmetrical, soft, elongated and dorsoventrally flattened.

(ii) Their body is metamerically segmented externally by transverse grooves and internally by septa.

(iii) Their body is covered by thin cuticles and exoskeleton is absent. They are true coelomates with closed blood vascular system. There is extensive organ differentiation.

(iv) They have a tube-like alimentary canal, which is complete and extends straight from mouth to anus.

(v) Reproduction is sexual. Sexes may be separate or united (hermaphroditic).

(vi) Segmented nephridia are present for excretion which removes wastes from coelom and blood stream directly to the exterior.

(vii) They are usually found in aquatic, marine or fresh-water habitats. Some are terrestrial and inhabit moist soil and some are free living forms.

Phylum Annelida is divided into three classes:

Class 1- Polychaeta

Example : Nereis.

Class 2- Oligochaeta

Example : Pheretima.

Class 3- Hirudinea

Example : Hirudinaria