Slugs
Gastropod species that lack a conspicuous shell are commonly called slugs rather than snails. Some species of slugs have a reduced shell, some have only an internal vestige that serves mainly as a calcium repository, and others have no shell at all. Other than that there is little morphological difference between slugs and snails. There are however important differences in habitats and behaviour. A shell-less animal is much more maneuverable and compressible, so even quite large land slugs can take advantage of habitats or retreats with very little space, retreats that would be inaccessible to a similar-sized snail. Slugs squeeze themselves into confined spaces such as under loose bark on trees or under stone slabs, logs or wooden boards lying on the ground. In such retreats they are in less danger from either predators or desiccation, and often those also are suitable places for laying their eggs.
Slugs as a group are far from monophyletic; biologically speaking "slug" is a term of convenience with little taxonomic significance. The reduction or loss of the shell has evolved many times independently within several very different lineages of gastropods. The various taxa of land and sea gastropods with slug morphology occur within numerous higher taxonomic groups of shelled species; such independent slug taxa are not in general closely related to one another.
Read more about this topic: Snail