764 MOLLUSCA—CONCHIFERA.,. 
to attach their tendinous threads or byssus to rocks or marine bodies. As 
the movements of this class are thus nearly reduced to those of their muscu- 
lar attachment to the shell and their muscular cloak, these parts are much 
developed. The thickness of the muscle which attaches the oyster to its 
shell, and the amplitude of the mantle in all the Conchifera, are well 
known. The disposition of the first of these has afforded characters for 
the determination of groups. In the oyster, for instance, there is but one 
muscle, which traverses, in some measure, the whole body to attach it to 
the valves of the shell. In others, such as the genera Venus and Tellina, 
che muscles of attachment are two in number, and attached to the lateral 
extremities of the shell; and in a third group, these muscles seem di- 
vided, as in the Anodonta, into three or four muscles of attachment. 
The muscles of attachment are generally thick, composed of straight 
vertical fibres, and at their place of junction with the shell acquire 
a remarkable hardness. Their use is to shut the valves by contrac- 
tion; when they are relaxed, the ligament at the hinge suffices by its elasti- 
city to open them. It is remarkable, that during the life of the animal, 
these muscles really change their place, without ceasing, for an instant, to 
attach the animal to the shell. They become obliterated, dried up, and 
detached, by almost imperceptible degrees on one side; while on the other, 
they increase by the addition of new fibres; and this is done in such a man- 
ner that they always preserve the same relative position as the shell in- 
creases in size from age. When the animal is removed from the shell, the 
muscles of attachment always leave on its internal surface impressions 
which show their situation, their number, and the displacement which they 
have undergone. Among the Conchifera, the animal never has a shell, or 
other hard part internally. The body is always soft, often oval, more or 
less compressed, and the mouth is generally situate towards the lowest part 
of the shell, on the left side of the hinge. All the Conchifera are aquatic. 
Some races live in fresh water, and others in the sea. The greater part are 
free; but some are fixed upon marine bodies by their shell, and others at- 
tached by bony filaments, or a byssus. Lamarck divides the class Conchi- 
fera into two orders, viz. Order first—Monomyarta. With but one muscle 
of attachment; shell marked interiorly with one subcentral muscular im- 
pression. Order second—Dimyaria. With at least two muscles of attach- 
ment; shell marked interiorly with two separate and lateral muscular im 
pressions. It is not necessary to detail here all the arrangements proposed 
for this class cf animals. They were included by Linneus among his Vermes 
testacea, and form the class of Mollusca acephaia in the Régne Animal of 
M. Cuvier. The older naturalists, who arranged the testaceous animals as 
one great family by the from of their testaceous covering, took their charac- 
ters wholly form the shell; and this department of science, including the 
testaceous coverings of the preceding class, form the branch of science 
termed Conchology. 
