118 STRUCTURE OP GASTEROPODS AND BIVALVES. 
On either side, a little beliind 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 commonly 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, like the Snail and Slug, crawl 
upon a fleshy disk on the under side of their bodies; and 
the number of distinct 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, whilst 
others are destitute of them. The shells are composed of a 
single piece, or are univalve , except in one tribe; 
and they have usually more or less of a spiral 
formation (fig. 60). The animals of this class all 
possess a distinct head; and this is generally 
furnished with eyes, as well as with tentacula. 
They have often a powerful masticating ap¬ 
paratus, and are voracious in their habits ; 
Pig. 60.—shell some of them feed upon vegetable matter, others 
OF Paludina. npon animals> 
113. The Acephalous Mollusca are divided into two groups, 
—those which form shells, and those which do not. The 
former are termed Conchifera, or shell-bearing animals; and 
this class includes all the Mollusca that form a shell composed 
of two parts or valves fitted together (which shell is termed 
bivalve ), as well as some others whose general structure is the 
same, but whose shell is formed in several pieces, or multivalve. 
The two valves of a bivalve shell (fig. 61) are connected by a 
hinge, where they are united by a ligament, which, by its 
elasticity, keeps them apart while it holds them together. This 
is their usual condition when the animal is alive; and in this 
manner the water which is required for their respiration, and 
also to convey their supply of food, has free access to the internal 
