Geoscience Reference
In-Depth Information
F IG. 38
Two snails of soil and litter showing contrasting forms; the blind snail Cecilioides acicula and the round
snail Discus rotundatus. (Drawn by S. V. Green.)
Apart from variations in absolute size, the identification of snails depends largely
on the ratio of shell height to width, the shape of the mouth, the number, shape, colour
and texture of whorls and whether they are dextrally or sinistrally wound; a shell is
said to be dextral when the coils are clockwise as seen looking down on the apex. The
commonest of the door snails Clausilia bidentata represents one extreme form with
its narrow spire-shaped shell bearing fine chasings on its 12-13 sinestral whorls. At
the other extreme, the rounded snail Discus rotundatus ( Fig. 38 ) is more than twice
as wide as high and the whorls are dextrally coiled.
Identifying snails by their shell characteristics is not at all the same thing as clas-
sifying them into genera and families. Fossils show that some superficially similar
species have descended from quite dissimilar ancestors through convergent evolution.
A natural classification therefore depends primarily on internal anatomical features,
especially of the reproductive system.
Slugs are a good example of a group which are superficially alike but which
are highly polyphyletic in origin. They are all evolved from shelled ancestors but the
British species are considered to have descended from three distinct lineages one of
which has subdivided more recently. The loss of the protective shell is balanced by
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