Semi-slugs, also spelled semislugs, are land gastropods whose shells are too small for them to retract into, but not quite vestigial. The shell of some semi-slugs may not be easily visible on casual inspection, because the shell may be covered over with the mantle.
This is a type of gastropod that is intermediate between a slug (without an external shell) and a land snail (with a large enough shell to retract completely into).
There exist a number of gastropod families that have semi-slug species.Breure A. S. H. (2010). "The rediscovery of a semi-slug: Coloniconcha prima Pilsbry, 1933 (Gastropoda, Pleurodontidae) from Hispaniola". Basteria 74(4-6): 78-86. There exist about 1,000 species of semi-slugs in comparison to about only 500 species of slugs.Burton D. W. (1982). "How to be sluggish". Tuatara 25(2): 48–63. HTM.
Examples Semi-slugs have a worldwide distribution and have evolved in several families; genera include: Palearctic and Nearctic family Parmacellidae: Cryptella family Vitrinidae: Eucobresia, Semilimax, Vitrina, Vitrinobrachium Asia - Pacific family Ariophantidae: Parmarion, Ratnadvipia, Varadia family Helicarionidae: Attenborougharion, Helicarion, Howearion, Ibycus, Parmellops, Ubiquitarion Semi-slugs also exist but are exceptional in the Camaenidae. Neotropics family Amphibulimidae: Amphibulima, Gaeotis family Xanthonychidae: Cryptostrakon, Semiconchula, Xanthonyx family Pleurodontidae: Coloniconcha prima Tropical Africa family Urocyclidae: Gymnarion
See also "Leatherleaf slugs" - Veronicellidae
References