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.
There is an evident superiority in this class over the preceding one, in
the greater prominence of the anterior extremity, where there are two or
more feelers, with which eyes more or less developed are connected; and
though there is nothing that can be properly called a head, yet there
can be no hesitation as to the distinction between the front and hind
ends of the body.

[Illustration: Limpet, Patella, cut transversely: _a_, foot; _b_, gills;
_c_, mantle; _d_, shell; _e_, heart; _f_, main cavity, with intestines.]

The third and highest class of Mollusks has been called Cephalopoda, in
reference again to a special feature of their structure. They have long
arms or feelers around the head, serving as organs of locomotion, by
which they propel themselves through the water with a velocity that is
quite extraordinary, when compared with the sluggishness of the other
Mollusks. In these animals the head is distinctly marked,--being
separated, by a contraction or depression behind it, from the rest of
the body. The feelers, so prominent on the anterior extremity of
the Gasteropoda, are suppressed in Cephalopoda, and the eyes are
conseq