764 MOLLUSCA-CONCHIFERA. 



to attach their tendinous threads or byssus to rocks or marine bodies. Ag 

 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, 

 the 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 Anodanta, 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 Avith 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 — Monomyaria. 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 of animals. They were included by Linnaeus among his Vermes 

 Sestacca, and form the class of Mollusca acephala in the Regne Animal of 

 jM. 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. 



