BIVALVES. 145 



opening for the reception of food, without pro- 

 boscis, jaws or any hard parts fitted for mastica- 

 tion. This mouth is surrounded by four flat- 

 tened moveable expansions, which partake of the 

 nature of tentacula. The branchice,* or gills, 

 consist of two leaves or expansions on each side, 

 and extend the length of the body. The mantle 

 is large, sometimes it is quite open, and bor- 

 dered with contractile, irritabje filaments : in 

 some instances it is joined in front, forming tu- 

 bular elongations, called siphons, which conduct 

 the water to the mouth and branchise. The mus- 

 cles are generally very thick and strong, and 

 hard at the place of attachment to the shell ; 

 those which close the valves are called the ad- 

 ductor^ muscles. Many species have not the 

 power of locomotion, but are immoveably ce- 

 mented 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 mol- 

 lusca; but some have a muscular substance 

 usually tongue-shaped and capable of conside- 

 rable elongation. This organ enables them to 

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

 change the position of their shells ; sometimes 

 it is transformed into a paw, and sometimes it 

 is employed to fix the silky filaments of a byssus. 

 None of the Bivalves are terrestrial shells, some 

 few are fluviatile. The generic character of the 



* Branchiae, is derived from the Greek, /Jpay#ia (branchia) the 

 gills of fish. 



j" Adductor, is derived from addu co, I bring together. 

 13 



