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 hid- 

 den under the mantle, is only a simple opening for 

 the reception of food, without proboscis, jaws, or 

 any hard parts fitted for mastication. This mouth 

 is surrounded by four flattened moveable expan- 

 sions, which partake of the nature of tentacula. 

 The branchiae,* or gills, consist of two leaves or 

 expansions on each side of the mollusk, and ex- 

 tend the length of its body. The mantle is large, 

 sometimes it is quite open, and bordered with con- 

 tractile irritable filaments ; 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 species 

 have not the power of locomotion, but are im- 

 moveably 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 change the 



* Branchiae, is derived from the Greek, Ppayxu* (bran- 

 chia) the gills of fish. 



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

 L 2 



