GALLERY.] NATURAL HISTORY. 107 
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 
the edge of the mouth of the shell; while those produced 
by the alternations of the periods of rest and developement 
are always in concentric lines parallel with the edge of its 
mouth. This method of formation and enlargement allows 
only the inner surface and the edge of the mouth of the 
shell, which is immediately applied to the surface of the 
animal, to be within its influence, as each deposition of 
new matter removes the part which it covers from the ani¬ 
mal’s control; consequently all the shelly appendages, 
&c., on the surface, except those on the immediate 
edge, which were used by the animal to protect its fleshy 
