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