High Quality Content by WIKIPEDIA articles! Slug is a common name for an apparently shell-less terrestrial gastropod mollusk. The word "slug" is also often used as part of the common name of any gastropod mollusc that has no shell, has a very reduced shell, or has only a small internal shell. (This is in contrast to the common name "snail", which is applied to gastropods that have a coiled shell that is large enough that the soft parts of the animal can retract fully into it.)