THE CLAM AND OTHER BIVALVES 



159 



two thick, short muscles (Fig. 82, 2,3; Fig. 83, 2 a) extend 

 across from valve to valve, and resist the spreading action 

 of the hinge-ligament. 



The Mantle and the Mantle-Cavity. When the valves are 

 shut they inclose a considerable space besides the body 

 proper. Fig. 82 

 represents the 

 most important 

 organs as they 

 might lie in the 

 hollow of the 

 right valve. Fit- 

 ting close to the 

 inner surface 

 of the valves is 

 the mantle (Fig. 

 82,4). Except at 

 the edge, where 

 the mantle- folds 

 (halves) unite, the 

 mantle is quite 

 thin; its chief 

 use is to secrete 

 the calcareous 

 substance of 

 which the shell 



is composed. The shell is deposited in three layers (Fig. 83, 

 3, 4, 5), the outer layer called the periostracum, the middle 

 layer called the prismatic layer, and the inner layer called the 

 nacreous or pearly layer. After the shell is formed at the edge 

 the thin part of the mantle-folds continues to deposit the 

 nacreous layer, which is sometimes called mother-of-pearl. 



The mantle-folds inclose a mantle- cavity. An opening at the 

 anterior end of the mantle-cavity allows the foot (Fig. 82, 5), 



FIG. 83. Mechanism for opening 

 Mussel-Shell 



A, valves of mussel closed; B, valves of mussel open: 

 1, 2, hinge-ligament; 2 a , adductor muscle contracted; 

 2 b , adductor muscle relaxed ; 3, outer layer of shell ; 

 4, middle layer of shell; 5, inner (mother-of-pearl) 

 layer 



(From Lang's Lehrbuch) 



