Phylum Annelida consists of segmented worms called annelids. They have a segmented body with repeated units called metameres or segments. Annelids include earthworms, leeches. They exhibit a variety of lifestyles, including free-living, parasitic, and predatory.
General Characteristics:
1) Habitat: They are free living, found in aquatic both marine and freshwater and terrestrial environments.
2) Shape: Body is soft, vermiform and more or less elongated.
3) Organization: This species exhibit organ system level of body organization.
4) Germ Layer: They are triploblastic animals.
5) Symmetry. Body is bilaterally symmetrical.
6) Coelom: They have true coelom, lined by mesoderm.
7) Digestion: Complete digestive system, running straight from mouth to anus. Digestion completely extracellular.
8) Respiration: Gas exchange either by general body surface or by gills in some tube dwellers.
9) Excretion: Excretion done through nephridia.
10) Circulation: Closed blood vascular system with dorsal and ventral longitudinal vessels connected by smaller vessels.
11)Locomotion: Locomotory organs are setae or parapodia.
12) Reproduction: Sexual Reproduction seen. Fertilization is internal.
13) Development: Development direct in species of class Oligochaeta and Hirudinea and indirect in Polychaeta.
14) Larval stage: Trochophore is the larval stage if present.
15) Metamerisation: Body is metamerically segmented.
Example: Nereis sp. (Ragworm), Pheretima sp. (Earthworm). Hirudo sp. (Leech), Hirudinaria granularia (Cattle leech) etc.
Classes of Annelida
There are three classes in phylum Annelida.
- Polychaeta
- Oligochaeta
- Hirudinea
Class 1: Polychaeta
- Habitat: Exclusively all animals are marine.
- Body: Body elongated, segmented with identical cylindrical body segments.
- Head: Anterior end modified into a head.
- Setae: Numerous setae on the trunk segment, hence called Polychaeta.
- Parapodia: Parapodia basically biramous, paddle like outgrowth bears in each body segment in a pair act as locomotory organ.
- Gills: Highly vascularised gills are present.
- Protonephridia: Protonephridia present act as excretory organ.
- Gonochoristic: Sexes are seperate in most species.
- Fertilization: Fertilization is external in sea water.
- Trochophore: Trochophore larval stage in life cycle.
- Clitellum: Clitellum absent.
Example: Nereis sp. (Rag worms); Aphrodite sp. (Sea mouse)
Class 2: Oligochaeta
- Habitat: Most species are found in freshwater or terrestrial habitats, few are marine.
- Body: Streamlined body with well developed segmentation.
- Head: Head indistinct.
- Setae: Setae are chitinous, thin, slightly curved bristles help in in locomotion.
- Parapodia: Parapodia and cirri absent.
- Gas exchange: Take place by diffusion through moist body wall.
- Meta nephridia: Excretion through meta nephridia.
- Hermaphrodites: They are hermaphrodites.
- Fertilization: Cross fertilization occurs externally.
- Larval Stage: No larval stage in the life cycle.”
- Clitellum: Clitellum present.
Example: Pheretima sp. (Earthworm),
Class 3: Hirudinea
- Habitat: They are either free living ectoparasites, found in both freshwater and marine environment.
- Body: Body consists of definite and limited number of segments.
- Head: Presence of an anterior sucker, which is crucial for attachment and feeding.
- Locomotory organ: Setae and Parapodia absent.
- Respiration: Respiration take place through their moist, thin, vascular skin.
- Metanephridia: Excretory organ include segmentally arranged 10 to 17 pairs of metanephridia.
- Hermaphrodite: They are hermaphrodite species.
- Fertilization: Fertilization is internal.
- Larval stage: No larval stage in the life cycle.
- Clitellum: Clitellum present.
Example: Hirudo sp. (Leech), Hirudinaria sp.
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