Annelids are commonly called as ring worms or segmented worms.
Characteristic feature of phylum Annelida:
1. Forms: Annelids may be aquatic, ectoparasitic or free – living or burrowing in moist soil.
2. Body symmetry: They are bilaterally symmetrical.
3. Body coelom: They are true coelomates.
4. Segmentation: Body is metamerically segmented and has a special region called clitellum.
5. Digestive system: Alimentary canal is complete.
6. Locomotion: Locomotion takes place with the help of setae (earthworm), parapodia (Nereis) or suckers (leech). Well developed longitudinal and circular muscles help in locomotion.
7. Nervous system: It consists of nerve ring and ventral solid and ganglionated nerve cord.
8. Reproduction: Mostly are hermaphrodites and few are dioecious (Nereis).
9. Respiration: Exchange of gases takes place through body wall.
10. Circulation: Circulatory system is of closed type. Excretion and osmoregulation is carried out with help of nephridia. e.g. Nereis (Aquatic annelid), Pheretima (Earthworm), Hirudinaria (Leech).
