50 
NATURAL HISTORY. 
[EAST. ZOOL. 
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 Cases in the Northern Zoological Gallery.) 
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, im¬ 
mediately under 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 species, 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 tem¬ 
porary interruption of the action of these glands. 
By far the greater number of these animals are provided 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 
together by a ligament behind. In others, as those of the Brachiopodes , 
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, in¬ 
stead of containing the body like the other valve, merely acts as a lid 
to close the mouth of the larger one, when the animal is retracted into 
it; hence it has been called an operculum. 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 altogether wanting, 
especially in those genera which have an expanded mouth compared 
with the size of the remaining shell. 
Most of the Gasteropodous animals, especially the lung-breathing 
kinds, which are not provided with a second valve, prevent the 
evaporation of their juices, during the very hot or cold weather, when 
they are in a torpid state, by secreting a layer of mucus, which forms a 
temporary lid, called an epiphragma, and which is sometimes hardened 
by a quantity of calcareous particles. It falls off when the animal 
recovers from its torpidity, and is formed afresh when required. 
Linnaeus referred all the animals inhabiting shells to five different 
genera, viz., Limax , Ascidia , Anomia , Clio , and Sepia. These 
genera have become the types of five very distinct classes, viz., first, 
those which have a distinct fleshy organ under the body, called the foot, on 
which the animal walks or glides, as the first class, Gasteropoda , which 
have a distinct head and a univalve spiral shell; and the second class, 
the Conchifera , which have the mouth placed at the bottom of the 
bag-like mantle, and are covered with a bivalve shell. The three other 
classes are remarkable for being destitute of any foot, as they are 
attached to other bodies, float about in the sea, or walk on the tentacles 
which surround their head. They consist of the Brachiopoda , which 
have an attached bivalve shell and a pair of spiral arms on the sides of 
the mouth ; the Pteropoda , which have a pair of fins on the sides of 
the head, and a very light univalve shell; and the Cephalopoda , so 
