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.
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. source