EAST. ZOOL. GAL.] NATURAL HISTORY. 79 
the masses being placed over the gullet, and enveloping it 
like a collar. 
The shell is formed by the hardening of the animal mat¬ 
ter which is secreted by certain glands on the surface 
of the body, by means of chalky matter, which is also 
secreted by similar glands. The unhatched animal, 
very shortly after it is formed, begins to construct its 
shell; and when the animal is hatched, it deposits on the 
edge of the mouth of the little shell which covered its body 
in the egg, a small quantity of mucous secretion. This dries, 
and is then lined with other mucous matter, intermixed with 
calcareous particles, and when this becomes hardened, the ani¬ 
mal again places on its edge another thin layer of the mucous 
secretion, and again lines it as before. The mucous secretion 
first deposited, called Periostraca , forms the outer coat of the 
shell, and is of use in protecting it from injury, while the 
mucous matter mixed with lime, which is placed within, 
forms the substance of the shell itself. This alternate de¬ 
position of mucus, and of mucus mixed with calcareous 
matter, goes on as the animal grows, and feels, from its in¬ 
creasing size, the want of a larger shell for its protection. 
The shell is, in fact, moulded on the body of the animal 
itself as the body grows, and for this reason any inequality 
or irregularity of the body is reproduced on the shell. If 
the irregularity or inequality is of a permanent character, 
such as a canal, tubercles, &c., it causes a permanent groove 
or ridge, running from the apex to the mouth of the shell, or 
even produced beyond it as far as the canal of the mantle 
extends. But besides these permanent inequalities, which 
are generally necessary for the functions of the animal, the 
Mollusca, like most other animals, appear to have seasons of 
activity, or developement, and of rest; and in the season of 
activity there are often expanded from the edge of their man¬ 
tle leaf-like, thread-like, or variously shaped appendages; 
which appendages, as soon as they are formed, generally 
secrete on the edge of the shell, shelly leaf-like expan¬ 
sions, tubular spines, &c., for their protection; these fleshy 
expansions are after a time gradually contracted until the 
period of rest recurs, and the part of the shell that is formed 
during this period is destitute of such appendages on its 
surface. Thus the marks produced by the permanent pro¬ 
jections form ribs, grooves, &c., running from the apex to 
