BIVALVES. 147 



The animals belonging to the bivalve shells 

 are acephalous mollusca, and have not a distinct 

 head ; they have no eyes, and the mouth, which 

 is hidden under the mantle, is only a simple open- 

 ing for the reception of food, without proboscis, 

 jaws, or any hard parts fitted for mastication. 

 This mouth is surrounded by four flattened 

 moveable expansions, which partake of the na- 

 ture of tentacula. The branchice,* or gills, con- 

 sist of two leaves or expansions on each side of 

 the mollusk, and extend the length of its body. 

 The mantle is large, sometimes it is quite open, 

 and bordered with contractile, irritable fila- 

 ments ; in some instances it is joined in front, 

 forming tubular elongations, called siphons, 

 which conduct the water to the mouth and 

 branchiae. The muscles are generally very 

 thick and strong, and hard at the place of attach- 

 ment to the shell ; those which close the valves 

 are called the adductor f muscles. Many spe- 

 cies have not the power of locomotion, but are 

 immoveably cemented to rocks or stones : a few 

 are attached by a cartilaginous ligament, others 

 by a byssus. These mollusca have no ventral 

 foot similar to that possessed by some of the 

 cephalous mollusca ; but some have a muscular 

 substance usually tongue-shaped and capable of 

 considerable elongation. This organ enables them 

 to creep, or to effect a kind of leap, by which they 



* Branchiae, is derived from the Greek, jS^ayxm (bran- 

 chia) the gills of fish. 



f Adductor, is derived from adduco, I bring together. 

 L 2 



