406 PHILOSOPHY OF ZOOLOGY. 



The characters furnished by the skin and its appendices 

 are extensively employed in the systematical arrangement 

 of iriolluscous animals. Nearly all those characters which 

 distinguish the species, and many of those on which gene- 

 ra are established, are derived from the form of the shell, 

 the tentacula, or the colour. This last character, however, 

 is one on which little dependence should be placed. 



There is nothing peculiar in the muscular system of this 

 class of animals. Where the muscles are inserted in the 

 skin, as is usually the case, that organ is in some cases 

 strengthened by condensed cellular substance, and even ac- 

 quires a coriaceous density. 



Molluscous animals preserve themselves in a state of rest, 

 chiefly by suction and cementation. The organ which acts 

 as a sucker, is in some cases simple, soft, and muscular, as 

 the foot of the snail, while in others it is compound, and 

 strengthened internally by hard parts, as in the arms of the 

 cuttle fish. The force with which some animals adhere is 

 very considerable, and is strikingly displayed, for example, 

 when we attempt to detach a limpet from the rock. 



The rest, which is maintained by cementation, in some 

 cases depends on a glairy secretion, which glues the body 

 of the animal to the substance to which it is disposed to be 

 attached. By such an expedient, the shells of snails adhere 

 to rocks, stones, and plants. It is probable that the bi- 

 valve shells of the genus Cyclas, which readily adhere to 

 the side of a glass, secure their temporary attachment by 

 means of their glutinous cuticle. In other animals threads 

 are produced (termed a Byssus) from particular glands, 

 and while one extremity is glued to the rock, the other re- 

 mains in connection with the animal. But there is an at- 

 tachment more durable than any of these, which takes 

 place in some shells, they being cemented to rocks or stones 



