Slug

Slug, often known as a land slug, is a popular name for any terrestrial gastropod mollusc that appears to lack a shell. In addition, the word "slug" is frequently included in the common name of any gastropod mollusc, especially sea slugs and semislugs, that has no shell, a greatly reduced shell, or only a small internal shell (this is in contrast to the common name snail, which applies to gastropods that have a coiled shell large enough that they can fully retract their soft parts into it). 


Land slugs are in a variety of taxonomic groups that belong to several quite distinct evolutionary lineages that also include snails. Therefore, despite a superficial similarity in the general body form, the different slug groups are not closely related to one another.  

As a result of the several distinct occurrences of the shell-less state as a result of convergent evolution, the category "slug" is polyphyletic.