Slug, or land slug, is a
common name for any apparently shell-less
terrestrial gastropod mollusc. The word slug is also often used as part of the common name of any
gastropod mollusc that has no shell, a very reduced shell, or only a small internal shell, particularly
sea slugs and
semislugs (this is in contrast to the common name
snail, which applies to gastropods that have a coiled shell large enough that the animal can fully retract its soft parts into the shell).
Various
taxonomic families of land slugs form part of several quite different evolutionary lineages, which also include snails. Thus, the various families of slugs are not closely related, despite a superficial similarity in the overall body form. The shell-less condition has arisen many times independently during the evolutionary past, and thus the category "slug" is a
polyphyletic one.