CONCHIFERA. 73 



the muscle which attaches the oyster to its shell, and the am- 

 plitude 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 divided, as in theAno- 

 donta, 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 contraction ; when they are relaxed the ligament 

 at the hinge suffices by its elasticity to open them. It is re- 

 markable that during the life of the animal these muscles real- 

 ly 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 manner that they always preserve the 

 same relative position as the shell increases in size from age. 

 When the animal is removed from the shell, these 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 parts internally. The body is always soft, often oval, 

 more or less compressed, and the mouth is generally situate to- 

 wards the lowest part of the shell, on the left side of the hinge. 



All the Conchifera are aquatic. Some races live in fresh wa- 

 ters, and others in the sea. The greater part are free ; but some 

 are fixed upon marine bodies by their shell, and others are at- 

 tached by horny filaments or a byssus. 



Lamarck divides the class Conchifera into two orders, viz. 



Order I. MONOMYAIRA. With but one muscle of attach- 

 ment ; shell marked interiorly with one subcentral muscular im - 

 pression. 



Order II. DIMYATBA. With at least two muscles of at- 



