Clitellata
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| Clitellata | ||||||||
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 Earthworm
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Branchiobdellae |
Clitellata is a class of Annelid worms, characterized by having a clitellum - the 'collar' that forms a reproductive cocoon during part of their life cycle. The clitellates comprise around 8,000 species.
Contrary to the class of Polychaeta, they do not have parapodia and their head is less developed.
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[edit] Habitats
Most clitellata live on land or in freshwater.
[edit] Reproduction
All clitellata are hermaphrodites. During reproduction, the clitellum secretes a coat which hardens. The worm then creeps out backward from the coat and deposits either fertilized zygotes or both ovae and sperms into the coat, which is then packed into a cocoon. The zygotes then evolve further directly in the cocoon without passing through a larva stadium (as opposed to other annelids, e.g. polychaeta.) This mechanism is considered to be apomorphic (newer in evolution).[1]
[edit] Subclasses
- Branchiobdella - formerly in Hirudinea
- Hirudinea (leeches)
- Oligochaeta (earthworms - aquatic microdriles + terrestrial megadriles)
The Acanthobdellidea are sometimes moved out of the Hirudinea as a distinct subclass too. Overall. clitellate phylogeny is not well resolved, in particular the relationship of leeches and earthworms, which in the present delimitation do not seem to form natural groups.
[edit] References
- ^ Reichardt, Anna Katharina (2006). Systematische Zoologie, 63, 67-68.
[edit] External links
