Slug

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 semi slugs (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). 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 as an example of convergent evolution, and thus the category "slug" is polyphyletic. Of the six orders of Pulmonata, two – the Onchidiacea and Soleolifera – solely comprise slugs. A third group, the Sigmurethra, contains various clades of snails, semi-slugs (i.e. snails whose shells are too small for them to retract fully into) and slugs. The taxonomy of this group is in the process of being revised in light of DNA sequencing. It appears that pulmonates are paraphyletic and basal to the opisthobranchs, which are a terminal branch of the tree. The family Ellobiidae are also polyphyletic. Subinfraorder Orthurethra Superfamily Achatinelloidea Gulick, 1873 Superfamily Cochlicopoidea Pilsbry, 1900 Superfamily Partuloidea Pilsbry, 1900 Superfamily Pupilloidea Turton, 1831 Subinfraorder Sigmurethra Superfamily Acavoidea Pilsbry, 1895 Superfamily Achatinoidea Swainson, 1840 Superfamily Aillyoidea Baker, 1960 Superfamily Arionoidea J.E. Gray in Turnton, 1840 Superfamily Athoracophoroidea Family Athoracophoridae Superfamily Orthalicoidea Subfamily Bulimulinae Superfamily Camaenoidea Pilsbry, 1895 Superfamily Clausilioidea Mörch, 1864 Superfamily Dyakioidea Gude & Woodward, 1921 Superfamily Gastrodontoidea Tryon, 1866 Superfamily Helicoidea Rafinesque, 1815 Superfamily Helixarionoidea Bourguignat, 1877 Superfamily Limacoidea Rafinesque, 1815 Superfamily Oleacinoidea H.