General Characters of Annelida

General Characters of Annelida:

1. Annelida are mostly aquatic; marine or freshwater, some terrestrial, bun-owing or living in tubes, sedentary or free-living. Some are commensal and parasitic also.

2. Body triploblastic, bilaterally symmetrical, elongated and vermiform.

3. Body metamerically segmented; externally by transverse grooves and internally by septa into a number of divisions; each division is called a segment, metamere or somite.

4. Organ-system grade of body organisation.

5. Outer covering of the body is cuticle secreted by the underlying epidermis.

6. Body wall is contractile, consisting of an outer epidermis, circular and longitudinal muscles.

7. Appendages when present are un-jointed.

8. Locomotory organs are segmentally arranged paired setae or chaetae in most of the cases.

9. Presence of a true schizocoelous coelom usually divided into a large number of compartments by inter-segmental septa.

10. Alimentary canal is tube-like, complete, extending from mouth to anus.

11. Respiration occurs through general body surface, in some cases by gills also.

12. Blood vascular system is closed type, blood is red due to the presence of haemoglobin or erythrocruorin found dissolved in the plasma.

13. Excretion by segmentally arranged nephridia which usually communicate the coelom to exterior.

14. Nervous system with a pair of cerebral ganglia, the brain and a double nerve cord having segmentally arranged ganglia.

15. The tactile organs, taste buds, statocysts, photoreceptor cells and eyes are the receptor organs.

16. Usually monoecious, i.e., hermaphrodite; dioecious or unisexual forms also present.

17. Development direct in monoecious forms but indirect in dioecious forms.

18. A free-swimming trochophore larval stage is characteristic in case of indirect development, while in others this stage is passed during the development.

19. Asexual reproduction also occurs in some forms.