118 STRUCTURE OP GASTEROPODS AND BIVALVES. 



On either 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 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 ; 



Fig. GO. SHELL some of them feed upon vegetable matter, others 



OF PALUDINA. ^^ animalg> 



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 



