10 MOLLUSCA. 



they are joined together, as the articulated bones in the 

 higher classes of animals, by ligaments. These, in some 

 cases, are of great thickness and strength, and, in con- 

 sequence of their elasticity, assist in the motion of the dif- 

 ferent parts. 



In the molluscous animals the skin secretes a viscous, 

 adhesive substance, differing according to the medium in 

 which the animal resides, but in all cases calculated to re- 

 sist its influence. It is probably owing to the lubricating 

 agency of this secretion, that both the cuticle and shell are 

 preserved from decomposition. The skin likewise secretes 

 the colouring matter by which the shells are variegated. 

 The glands from which it proceeds vary much in different 

 individuals, and even in the same individual in different pe- 

 riods of growth. 



The characters furnished by the skin and its appendices 

 are extensively employed in the systematical arrangement 

 of molluscous animals. Nearly all those characters which 

 distinguish the species, and many of those on which genera 

 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 leathery 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, sis 

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



