Slug,
Snail
Slug is a common name that is normally
applied to any gastropod mollusc that lacks a shell,
has a very reduced shell, or has a small internal
shell. (This is in contrast to the common name snail,
applied to gastropods that have a coiled shell large
enough that the soft parts of the animal can retract
fully into it.)
Slugs belong to several different lineages which
also include snails that have shells. The various
families of land slugs are not very 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.
As well as land slugs, there are also many marine
slugs and even one freshwater slug genus (Acochlidium),
but the common name "slug" is most frequently
applied to air-breathing land slugs, while the marine
forms are usually known as sea 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.
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 slugs appear rather 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.
(Source: Wikipedia.org)
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