58 
NATURE OF THE 
The internal shell of the common sepia is large and 
broad, and composed wholly of the carbonate of lime, it 
is well known by the name of cuttle-fish bone. Its 
structure is extremely curious, and deserves particular 
attention, as establishing the universality of the prin- 
ciple which regulates the formation of shells, whether 
external or internal, and from which structures differing 
much in their outward appearance may result. It is 
composed of an immense number of thin calcareous 
plates, arranged parallel to one another, and connected 
by thousands of minute hollow pillars of the same cal- 
careous material, passing perpendicularly between the 
adjacent surfaces. This shell is not adherent to any 
internal part of the animal which has produced it, but 
is enclosed in a capsule, and appears like a foreign body 
impacted in the midst of organs with which at first 
sight it appears to have no relation. It no doubt is of 
use in giving mechanical support to the soft substance 
of the body, and especially to the surrounding muscular 
flesh, and thus probably contributes to the high energy 
which the animal displays in all its movements. It has 
been regarded as an internal skeleton, but it certainly 
has no pretensions to such a designation, for, although 
enveloped by the mantle, it is still formed by that 
organ, and the material of which it is composed, still 
carbonate of lime. On both these accounts it must be 
considered as a true shell, and classed among the pro- 
ductions of the integuments. It differs indeed altogether 
from bony structures, which are composed of a different 
kind of material, and formed on principles of growth 
