General Characters of Phylum Annelida:-
1. Annelida are mostly aquatic inhabitant and some are terrestrial.
2. They are bilaterally symmetrical and triploblastic organism.
3. They are coelomate.
4. Their body is divided into three separate sections, these are: a prosomium, a trunk and a pygidium.
5. Organ system level of body organization is present in them.
6. The complete body of annelids is metamerically segmented (i.e., they show metameric segmentation).
7. Respiration in them mostly occurs through their body surface.
8. Excretory organ in them is Nephridia.
9. Nervous system in them consists of an anterior nerve ring, ganglia and a ventral nerve cord.
10. Digestive system is well-developed in them.
11. They also have a well-developed closed circulatory system.
12. Red coloured pigment hemoglobin is present in them. Thus, they generally show reddish appearance.
13. Annelids exhibits the property of regeneration.
14. Locomotion and movement is usually carried out in annelid by setae.
15. They are mostly hermaphrodite.
16. Reproduction in them occurs in both asexual and sexual means.
17. Fertilisation in them can either occur internally or externally.
18. Their development in them is direct without the involvement of any larval stages.
Classification of phylum annelida
Class 1: Polychaeta
1. They have elongated body.
2. Their body has distinct segmentation.
3. They are exclusively marine.
4. They bear a true coelom.
5. They are bilaterally symmetrical and triploblastic.
6. Excretion in them occurs through metanephridia and protonephridia.
7. Fertilization in them occurs externally.
8. Well-defined nervous system is present in them.
9. They have a close circulatory system.
10. They are mainly hermaphroditic.
11. They might bear parapodia which are generally fin-like appendages.
12. They usually lack clitellum.
13. They are dioecious .
Example- Nereis, Syllis
Class 2: Oligochaeta
1. They are mostly terrestrial inhabitant and if aquatic they are found in freshwater.
2. Metameric segmention is seen in their body.
3. Oligochaete do not bear distinct head, eyes and tentacles.
4. Though they are hermaphrodites oraganism still they performs cross-fertilization.
5. Fertilization in them takes place externally.
6. Oligochaete (such as earthworm) are involved in cocoon formation.
7. They have prominent segmented setae.
8. Parapodia are absent in them.
9. A well-defined clitellum is present in them.
10. They are monoecious organisms.
11. Their development does not involves a free larval stage, instead it occurs inside the cocoons.
Example- Pheretima, Tubifex
Class 3: Hirudinea
1. They are mostly marine inhabitant some may be parasitic and terrestrial.
2. Organism of these class have segmented body.
3. These organisms lack tentacles, parapodia, and setae.
4. They are monoecious organisms.
5.Body of these organisms is dorsoventrally or cylindrically flattened.
6. A prominent sucker is present in them at their anterior and posterior region.
7. They usually lay their eggs inside cocoon.
8. Development of these organisms does not involve any larval stage.
9. The mouth in them is present ventrally in the anterior sucker.
10. The anus in them is present dorsally in the posterior sucker.
11. Fertilization in them takes place internally.
12. These organisms are hermaphrodite.
Example- Hirudinaria
Class 4: Archiannelida
1. They are highly exclusive to marine environment.
2. They have much elongated body
3. They does not bear any setae and parapodia.
4. They are also hermaphrodite in nature.
5. They bears tentacles on their prostomium.
Example- Dinophilus, Protodrilus
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