their
whole organization is based upon the principle of bilateral symmetry, it
is nevertheless very difficult to determine which is the right side and
which the left in these animals, because there is so little prominence
in the two ends of the body that the anterior and posterior extremities
are hardly to be distinguished. Take the Oyster as an example. It has,
like most Acephala, a shell with two valves united by a hinge on the
back, one of these valves being thick and swollen, while the other is
nearly flat. If we lift the shell, we find beneath a soft lining-skin
covering the whole animal and called by naturalists the mantle, from the
inner surface of which arise a double row of gills, forming two pendent
folds on the sides of the body; but at one end of the body these folds
do not meet, but leave an open space, where is the aperture we call the
mouth. This is the only indication of an anterior extremity; but it is
enough to establish a difference between the front and hind ends of
the body, and to serve as a guide in distinguishing the right and
left sides. If now we lift the mantle and gills, we find beneath the
principal organs: the stomach, with a winding alimentary canal; the
heart and liver; the blood-vessels, branching from either side of the
heart to join the gills; and a fleshy muscle passing from one valve
of the shell to the other, enabling the animal by its dilatation or
contraction to open and close its shell at will. A cut across an animal
of this class will show us better the bilateral arrangement of the
parts. In such a section we see the edge of the two shells on either
side; within these the edge of the mantle; then the double rows of
gills; and in the middle the alimentary canal, the heart, and the
blood-vessels branching right and left. Some of these animals have
eye-specks on the edge of the mantle; but this is not a constant
feature. This class of Acephala includes all the Oysters, Clams,
Mussels, and the like. 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