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Unit 2: Phylum Annelida to Phylum Echinodermata
2.1 Phylum Annelida: General Characters and Classification up to classes,
Economic importance of annelids with reference to Earthworm and Leech
• Phylum Annelida : -
1. Phylum Annelida is a very broad phylum belonging to the kingdom Animalia.
2. The Annelids are found in aquatic as well as terrestrial environments.
3. These are bilaterally symmetrical invertebrate organisms.
4. Their segmented body distinguishes them from any other organism.
General Characters of Phylum Annelida:-
1. The Annelids are found in aquatic, fresh water as well as marine water, some are
terrestrial which live in burrows.
2. Body is elongated, vermiform and bilaterally symmetrical.
3. Body is segmented. Externally, segments are shown by transverse grooves and internally
by muscular partitions called septa.
4. The outer most covering of the body is called cuticle secreted by underlying epidermal
cells.
5. The body wall is composed of circular and longitudinal muscles.
6. Setae or chaetae are the locomotary organs.
7. They are triploblastic animals.
8. All annelids are true coelomate animals. The body cavity or coelomic cavity is lying
between the two layers of mesoderm.
9. Respiration is by general body surface.
10. Annelids shows well developed blood vascular system and it is of a closed type.
11. Nephidia are the excretory organs which communicate the coelom with the exterior end.
12. Nervous system consists of cerebral ganglion, circumpharyngeal connectives and double
ventral nerve cord with segmental ganglia.
13. Sexes may be seperated or united (hermaphrodite) e.g. Oligochaeta and ahirudinea.
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Classification of Phylum Annelida up to classes: -
The phylum Annelida has four main classes, such as,
1. Polychaeta
2. Oligochaeta
3. Hirudinea
4. Archiannelida
1. Polychaeta: -
1. The body is elongated and divided into segments.
2. They are found in the marine environment.
3. These are true coelomates, bilaterally symmetrical worms.
4. They excrete through metanephridia and protonephridia.
5. Fertilization is external.
6. They have a well-developed nervous system.
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7. The circulatory system is closed type.
8. They are hermaphrodites.
9. They might possess fin-like appendages called parapodia.
10. The organisms belonging to this group lack clitellum and are dioecious.
Eg., Nereis, Aphrodite, Chaetopterus, Terebella, Amphitrite.