MOLLUSCA. 9 



cuticle, exhibit. Those shells which are thus concealed 

 are in general white ; those which are more exposed are 

 frequently coloured. The colouring, however, does not de- 

 pend on the direct exposure to the light, as some have im- 

 agined, for many shells which are destitute of a cuticle are 

 white, while many of those covered by a dense cuticle are 

 finely variegated beneath. 



Between the skin and the shell neither vessels nor nerves 

 have been traced ; and the manner in which the latter is 

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

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

 in 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 vitality 

 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. The difficulty, however, is felt in contem- 

 plating the functions of hairs, nails, and feathers, in verte- 

 brated animals. 



When the shelly covering consists of two or more pieces, 



