Aplacophora
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| Aplacophora | |
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| Scientific classification | |
| Kingdom: | Animalia |
| Subkingdom: | Metazoa |
| Phylum: | Mollusca |
| Class: | Aplacophora |
| Subclasses and families | |
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Aplacophora is a class of small, deep-water, exclusively benthic, shell-less marine mollusks found in all oceans of the world. The class comprises 28 families and about 320 species. Aplacophorans are cylindrical and worm-like, and most very small, being no longer than 5 centimetres (2.0 in); some species, however, can reach a length of 30 centimetres (12 in).
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[edit] Habitat
Aplacophorans mainly burrow into the substrate in water regions deeper than 20 metres (66 ft). These species may reach densities up to 4 or 5 specimens per m². They are typically either carnivores or scavengers.
[edit] Description
Aplacophorans are worm-like burrowing animals, with little resemblance to most other molluscs. They have no shell, although small calcified spicules are embedded in the skin. Most also lack a foot, although some species have some small ridges on the underside that might represent a vestigial foot. The mantle cavity is reduced to a simple cloaca, into which the anus and excretory organs empty, and is located at the far end of the animal. The head is rudimentary, and has no eyes or tentacles.[1]
The relationship with other molluscs, however, is apparent from some features of the digestive system; aplacophorans possess both a radula and a style. Some species are hermaphroditic, but most have two sexes, and reproduce by external fertilisation. During development, the mantle cavity of the larva curls up and closes, creating the worm-like form of the adult.[1]
[edit] Taxonomy
This class was once classified as sea cucumbers in the echinoderms. In 1987, they were officially recognized as molluscs and given their own class. This class is polyphletic, and consists of two clades: the Solenogastres and the caudofoveata.[2]
[edit] References
- ^ a b Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. p. 389. ISBN 0-03-056747-5.
- ^ Giribet; Okusu, A; Lindgren, A.R.; Huff, S.W.; Schrödl, M; Nishiguchi, M.K. (May 2006). "Evidence for a clade composed of molluscs with serially repeated structures: monoplacophorans are related to chitons" (Free full text). Proceedings of the National Academy of Sciences of the United States of America 103 (20): 7723–7728. doi:. PMID 16675549. PMC 1472512. http://www.pnas.org/cgi/pmidlookup?view=long&pmid=16675549.
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