Semi-slug

Semi-slugs, also spelled semislugs, are land gastropods whose shells are too small for them to retract into, but not quite vestigial.[1] The shell of some semi-slugs may not be easily visible on casual inspection, because the shell may be covered over with the mantle.

Drawing of two views of Cryptella canariensis from the Canary Islands

Shell of Cryptella canariensis

An unidentified semi-slug from Uganda

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-slugs species.[2] There exist about 1,000 species of semi-slugs in comparison to about only 500 species of slugs.[1]

Examples

Semi-slugs from the Neotropics:

family Amphibulimidae: Amphibulima and Gaeotis[2]
family Xanthonychidae: Cryptostrakon, Semiconchula and Xanthonyx[2]
family Pleurodontidae: Coloniconcha prima[2]
Semi-slugs do exist but are exceptional in the Camaenidae.[2]

References

Burton D. W. (1982). "How to be sluggish". Tuatara 25(2): 48-63. HTM.
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.

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[Burton_1982-1] Burton D. W. (1982). "How to be sluggish". Tuatara 25(2): 48-63. HTM.

[Breure_2010-2] 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.