MOLLUSCA. 405 



formed, forbids us to expect their existence. Yet the shell 

 cannot be considered as dead matter, so long as it remains 

 an connection with the living animal. In those animals in 

 which the shell is external, there are muscles which connect 

 the animal with its internal surface, and the bond of union 

 being a substance soluble in water, the muscle can be de- 

 tached by maceration. The analogy between shell and 

 bone is here obvious, although in the one case the connec- 

 tion between the muscle and the bone is permanent, in the 

 other, between the muscle and shell, temporary, or frequent- 

 ly changed during the life of the animal. But the vitahty 

 of the shell, if I may use the expression, is demonstrated, 

 from the changes which it undergoes when detached from 

 the animal : The plates of animal matter harden : the epi- 

 dermis dries, cracks, and falls off; and in many cases the 

 colours fade or disappear. We confess ourselves unable to 

 point out the means employed by the animal to prevent 

 these changes from taking place, by any process similar to 

 circulation. 



When the shelly covering consists of two or more pieces, 

 they are joined together, as the articulated bones in the high- 

 er classes of animals, by ligaments. These, in some cases, 

 are of great thickness and strength, and, in consequence of 

 their elasticity, assist in the motion of the different parts. 



In the mulluscous 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 

 periods of growth. 



