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