Slugs are gastropods without shells or with very small
shells, in contrast with snails from which they evolved,
which have a prominent shell. Although they undergo torsion
(twisting) during development, their bodies are streamlined
and worm-like, and so show little external evidence of
it. This same basic design developed independently in
several different groups, the largest being the sea slugs
or nudibranchs. Other slugs are found on land, but their
soft, slimy bodies are prone to desiccation, so they are
confined to moist environments. Among the various species
are the grey field slug, Deroceras reticulatus, the garden
slug, Arion hortensis and the banana slug, Ariolimax columbianus.
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