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Slug is a common name for an 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, has a very reduced shell, or has 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 the animal can fully retract its soft parts into the shell). Slugs live on land and in saltwater, and one genus, ''Acochlidium'', lives in freshwater. The unadorned word ''slug'', however, is applied primarily to land slugs, whereas slugs from the sea or from fresh water are usually referred to as "sea slugs" or "freshwater slugs". Land gastropods with a shell that is not quite vestigial, but is too small to retract into (like many in the family Urocyclidae), are known as semislugs. Various taxonomic families of slugs form part of several quite different evolutionary lineages, which also include snails. Thus, for example, the various families of land 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 emphatically a polyphyletic one. Slugs, like all other gastropods, undergo torsion (a 180° twisting of the internal organs) during development. Internally, slug anatomy clearly shows the effects of this rotation—but externally, the bodies of land slugs appear more or less symmetrical, except for the positioning of the pneumostome, which is on one side of the animal, normally the right-hand side. The soft, slimy bodies of slugs are prone to desiccation, so land-living slugs are confined to moist environments and must retreat to damp hiding places when the weather is dry. The subsequent information in this article applies to land slugs. ==Taxonomy== Of the six orders of Pulmonata, two – the Onchidiacea and Soleolifera – solely comprise slugs. A third family, 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 the light of DNA sequencing.〔White, T. R.; Conrad, M. M.; Tseng, R.; Balayan, S.; Golding, R.; de Frias Martins, A. M. & Dayrat, B. A. (2011). "Ten new complete mitochondrial genomes of pulmonates (Mollusca: Gastropoda) and their impact on phylogenetic relationships". ''BMC Evolutionary Biology'' 11 (1): 295〕 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. & A. Adams, 1855 * *Superfamily Orthalicoidea Albers-Martens, 1860 * *Superfamily Plectopylidoidea Moellendorf, 1900 * *Superfamily Polygyroidea Pilsbry, 1894 * *Superfamily Punctoidea Morse, 1864 * *Superfamily Rhytidoidea Pilsbry, 1893 * * *Family Rhytididae * *Superfamily Sagdidoidera Pilsbry, 1895 * *Superfamily Staffordioidea Thiele, 1931 * *Superfamily Streptaxoidea J.E. Gray, 1806 * *Superfamily Strophocheiloidea Thiele, 1926 * *Superfamily Parmacelloidea * *Superfamily Zonitoidea Mörch, 1864 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「slug」の詳細全文を読む スポンサード リンク
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