. Animal physiology. Physiology, Comparative. 118 STRUCTURE OF GASTEROPODS AND BIVALVES. On cither side, a little behind the head, the mantle is extended into a fin-like expansion, by the aid of which the animal can swim through the water. The hinder part of the body is usually inclosed, more or less completely, in a shell, which is conmionly of extreme thinness and delicacy. The head is not furnished with long arms, to grasp the food; but it has a number of minute sucking disks, by which it can lay firm hold of whatever it attacks : whilst its powerful rasp-like tongue is set to work upon it.
. Animal physiology. Physiology, Comparative. 118 STRUCTURE OF GASTEROPODS AND BIVALVES. On cither side, a little behind the head, the mantle is extended into a fin-like expansion, by the aid of which the animal can swim through the water. The hinder part of the body is usually inclosed, more or less completely, in a shell, which is conmionly of extreme thinness and delicacy. The head is not furnished with long arms, to grasp the food; but it has a number of minute sucking disks, by which it can lay firm hold of whatever it attacks : whilst its powerful rasp-like tongue is set to work upon it.—The class Gasteropoda con- tains those animals which, hke the Snail and Slug, crawl upon a fleshy disk on the under side of their bodies; and the number of distmct forms which it includes is very large. The greater part of them are inhabitants of the sea-shore^ rivers, lakes, &c.; some have the power of swimming freely through the open sea; and the proportion of those that breathe air and live on land, is comparatively small. The general structure of the animals of this group has been already described (§ 108). Some of them form shells, wMst others are destitute of them. The shells are composed of a smglo piece, or are univalve, except in one tribe ; and they have usually more or less of a spiral formation (fig. GO). The animals of this class all ])ossess a distinct head ; and this is generally I'urnished with eyes, as well as with tentacula. They have often a powerful masticating ap- paratus, and are voracious in their habits ; Fir. some of them feed upon vegetable matter, others OF PALumsA. ^^^^^^^ animals. 11 .'5. The Acephalous MoUusca are divided into two groups, —those which form shells, and those which do not. The former are termed Conchifera, or shell-hearing animals ; and tliis class includes all the MoUusca that form a shell composed of two parts or valves fitted together (which shell is termed hivalve), as well as some others whose general struct
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