MOLLUSCA. G 
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, 
