4 
SYSTEMATIC BISTKIBUTION. 
and blunts compared with the size of the shell. 
They are always destitute of colour^ for the animal 
does not deposit the colouring matter until after it 
has been hatched ; and it is therefore generally easy 
to distinguish in the young shell (and sometimes also 
in the adult) that part of the top of the spire which 
formed the shell of the animal when in the egg. 
The shell is formed by the hardening of the animal 
matter^ which is secreted by certain glands on the 
surface of the body, by means of chalky matter, 
also secreted by similar glands. It has been stated 
that the unhatched animal, very shortly after it 
is formed, begins to make its shell; and when it 
is hatched, deposits on the edge of the mouth of the 
little shell which covered its body in the egg, a small 
quantity of the mucous secretion. This dries, and is 
then lined with some mucous matter, intermixed with 
calcareous particles; and when this hardens, it again 
places on its edge another very thin layer of the mu- 
cous secretion, and again lines it as before. The 
mucous secretion first deposited forms the outer coat 
of the shell, which is called the periostraca or epi-- 
dermis^ some persons having regarded it as similar 
to the epidermis of the human skin. It is of use 
in protecting it from injury; while the mucous 
matter mixed with lime, which is placed within it, 
forms the substance of the shell itself, and consti- 
tutes, with the calcareous matter already existing, 
the crystalline structure of the shell. This depo- 
sition of mucus, and of mucus mixed with calcareous 
matter, goes on as the animal grows and feels the 
want of a larger shell for its protection : the shell Is, 
