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sea slug
Sea slug is a common name which is applied to some marine invertebrates that more or less resemble terrestrial slugs. Most creatures known as sea slugs are actually snails, i.e. they are sea snails (marine gastropod mollusks) that over evolutionary time have lost their shells, or have a greatly reduced shell or an internal shell. The name "sea slug" is most often applied to nudibranchs, as well as to a paraphyletic set of other marine gastropods without obvious shells.〔Thompson, T. E. 1976. Biology of opisthobranch molluscs, vol. 1, 207 pp., 21 pls. Ray Society, no. 151.〕
The phrase "sea slug" is, however, also sometimes applied to taxa in other phyla, such as the sea cucumbers, which are not mollusks but echinoderms. Although the other animals sometimes called "sea slugs" are not gastropods, they are nonetheless soft-bodied, and their overall shape is slug-like.
==Shell-less marine gastropods==
The name "sea slug" is often applied to numerous different evolutionary lineages of marine gastropod molluscs or sea snails, specifically those gastropods that are either not conchiferous (shell-bearing) or appear not to be.〔(【引用サイトリンク】 title=Mating behaviour in the sea slug Elysia timida )〕 In evolutionary terms, losing the shell altogether, having a small internal shell, or having a shell so small that the soft parts of the animal cannot retract into it, are all features that have evolved many times independently within the class Gastropoda, on land and in the sea; these features often cause a gastropod to end up labeled with the common name "slug".
Nudibranchs (clade Nudibranchia) are a large group of marine gastropods which have no shell at all. These may be the most familiar sort of sea slug, at least to scuba divers; nudibranchs, although most are not large, are often very eye-catching because so many of species have brilliant coloration. In addition to nudibranchs, a number of other taxa of marine gastropods (some easily mistaken for nudibranchs) are also often called "sea slugs".〔(Thompson, T. E., & Brown, G.H., 1984. Biology of opisthobranch Molluscs. Vol. 2. Ray Society; London. 1-229, p.49 )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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