174 SHELLS AND MOLLUSCOUS ANIMALS. 



present a development of skin, which, covering 

 the body more or less, like a mantle, has received 

 that name from naturalists. This mantle, in the 

 greater number, secretes those beautiful shells, 

 whose variety of form and tint is so pleasing to 

 us ; while in other groups we have only the naked 

 animal. 



A large number of the mollusca seem destined 

 to remain near the spot on which they first entered 

 into life, and clinging to the rocks, are fed by the 

 draughts of sea-water which convey to them 

 thousands of minute animals. There are, however, 

 mong the bivalved groups, many which have an 

 instrument of movement admirably adapted to 

 their uses. Naturalists call this part a foot, and 

 in the cockle and some others, this fleshy lump is 

 shaped something like a human leg and foot ; but 

 in most, it is an unsymmetrical mass of flesh. Its 

 muscular structure is, however, very powerful, for 

 it consists of a number of fibres so interwoven as 

 to confer the power of movement in every direc- 

 tion. Thus some bivalved mollusks half open 

 their shells, and raising themselves upon them, 

 urge themselves along in successive movements, 

 by means of this foot, leaving a long tract in the 

 sand or mud which they have passed over. Some, 

 like the cockle, are enabled by this foot to leap to 

 a great height ; while others, again, will by its 

 means delve deep down into the sands. Some, 

 as the scallop, swim along in the water with most 

 wonderful quickness, opening and flapping the 

 valves together as they go; while on the shore 

 they can move onwards or backwards, by using 

 the valves of their shell in a similar manner. 



The mouths of the shell-fish are formed accord- 



