THE CLAM AND OTHER BIVALVES 



191 



ligament, has a tendency to separate the valves at an acute 

 angle. In life two thick, short muscles 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. 101 represents the most important organs as 



A B 



Fig. 102. Mechanism for opening and closing mussel shell 



A, valves of mussel closed ; B, valves of mussel open ; 1,2, hinge ligament ; 2 a , ad- 

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

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



they might lie in the hollow of the right valve. Fitting 

 close to the inner surface of the valves is the mantle (Fig. 

 101). 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. 102, 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 



1 From Lang's Lehrbuch. 



