When named with reference to their double shells,
they are called Bivalves; and with them are associated a host of less
conspicuous animals, known as Ascidians, Brachiopods, and Bryozoa.
[Illustration: Common Mussel, Unio, cut transversely: _a_, foot; _bb_,
gills; _c_, mantle; _d_, shell; _e_, heart; _f_, main cavity, with
intestines.]
The second class in this type is that of Gasteropoda, so named from the
fleshy muscular expansion on which they move, and which is therefore
called a foot: a very inappropriate name; since it has no relation or
resemblance to a foot, though it is used as a locomotive organ. This
class includes all the Snails, Slugs, Cockles, Conchs, Periwinkles,
Whelks, Limpets, and the like. Some of them have no solid covering; but
the greater part are protected by a single shell, and on this account
they are called Univalves, in contradistinction to the Acephala or
Bivalves. These shells, though always single, differ from each other by
an endless variety of form and color,--from the flat simple shell of
the Limpet to the elaborate spiral and brilliant hues of the Cones and
Cowries. Different as is their external covering, however, if we examine
the internal structure of a Gasteropod, we find the same general
arrangement of parts that prevails in the Acephala, showing that both
belong to the same great division of the Animal Kingdom. The mantle
envelops the animal, and lines its single shell as it lined the double
shell of the Oyster; the gills are placed on either side of it; the
stomach, with the winding alimentary canal, is in the centre of the
body; the heart and liver are placed in the same relation to it as in
the Acephala; and though the so-called foot would seem to be a new
feature, it is but a muscular expansion of the ventral side of the body.
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