107 
GALLERY.] NATURAL HISTORY. 
itself; and it mends them in the same manner as it forms 
its shell, that is to say, by depositing first a coat of ani¬ 
mal matter, which is moulded on the body until it is dry, 
and then lining it with mucous matter mixed with chalk 
to harden it. But as the animal is usually very desirous 
of getting the repairs done as quickly as possible, and is 
most probably damaged by the injury the shell has received, 
and also wants the support that the already formed shell 
gives to it during its growth, these repairs are generally 
irregular, much more roughly executed than the shell 
itself, and commonly destitute of regular colour. (See p. 
86, Cases 3 and 4.) 
The shell of the unhatched animal is generally of a self 
or uniform colour, but after the animal is hatched the surface 
becomes more or less varied or ornamented. The particles 
which produce this colouring of the surface are deposited 
while the shell is increasing in size, immediately un¬ 
der the outer mucous coat, ( periostraca ,) and as these par¬ 
ticles are only secreted by peculiar glands, the colour is 
always disposed in a particular manner in each of the spe¬ 
cies, the glands being gradually enlarged and gradually 
separated, but not changed in position by the growth of 
the animal. All the variations exhibited in the colouring 
of the different species., or in the different individuals of 
the same species, are produced by the different position, or 
by the permanence or temporary interruption of the action 
of these glands. 
By far the greater number of these animals are pro¬ 
vided with two of these shells or valves, which are often 
nearly alike in size and form, and are hence called Bivalves , 
as the shells of the Conchifera , where one of the valves is 
placed on each side of the body, and they are united to¬ 
gether by a ligament behind. In others, as those of the 
Brackiopodes, the two valves are separate, one on the 
upper surface or back, and the other on the under surface 
of the body. In others, as in the shells of Gasteropodes , the 
two valves are so unequal that the smaller one merely 
acts as a lid to close the mouth of the larger, when the ani¬ 
mal is retracted into it; hence it has been called an oper¬ 
culum. This smaller valve, or operculum, is generally 
cartilaginous, either wholly formed of animal matter, or 
strengthened by a quantity of calcareous matter deposited 
on one or both of its surfaces. Sometimes this valve is 
