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 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.