Sea slug is a common name for some marine invertebrates with varying levels of resemblance to 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 seemingly lost their shells, due to having a greatly reduced or 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.

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