TTPB MOLLUSCA. 



327 



truded, and two posterior, the branchial and anal openings. 

 The mantle around these latter frequently becomes prolonged 

 so that two tubes, or siphons as they are termed, are formed, 

 sometimes in contact with one another {Pholas), sometimes 

 quite separate ( Venus), sometimes capable of retraction within 

 the shell, sometimes so large as to be incapable of retraction 

 (Mya). 



Fig. 147. — A, Mya are^iaraa with the siphons slightly expanded; B, inner sur- 

 face of the right valve of the shell of Mya. 

 aa = impression of anterior ad- pa = impression of posterior ad- 



ductor muscle. ductor muscle. 



I = ligament. pi = pallial line. 



m = mantle edge. s = siphon. 



p = foot. si = siphoual impression. 



In conformity with the form of the mantle-lobes the shell 

 consists of two similar portions or valves, lying on the sides 

 of the body and united along the dorsal mid-line by a hinge. 

 The hinge is formed by a ligament, as it is termed, which is 

 really a portion of the shell substance, and consists of an 

 external portion continuous with the epidermis of the shell 

 and an internal elastic portion, frequently calcified to a cer- 

 tain extent, and continuous with the middle layer (prismatic 

 layer) of the shell. When at rest the two valves of the shell 

 are kept apart along the ventral line by the elasticity of the 

 hinge-ligament, and it is only by the application of force that 



