ANATOMY OF A FRESH-WATER MUSSEL. 47 



anterior lateral teeth in front of them, and posterior lateral teeth 

 behind them. Dreissena polymorpha may, or may not, possess 

 teeth. 



9. The ligament connecting the two valves of the shell together. 

 In Anodonta, according to F. Miiller, this ligament consists of an 

 outer part and an inner part, the former being continuous with 

 the epicuticula of the shell, the latter having a distinct ridge the 

 homologue of that portion produced into teeth in Unto, Spharium, 

 and Pisidium, where it borders the nacreous layer. This ridge, 

 like the teeth, gives attachment to muscles from the foot. The 



Fig. 1 6. Vertical Section through the shell and mantle of Anodonta (after Leydig). 

 Cu, cuticle, 6", prismatic layer; Bl, nacreous layer; Ep\ external epithelium of mantle ; 

 Bd, connective tissue substance ; -/'', internal epithelium of mantle. 



outer part is laminated ; the inner part is radially striated, and 

 consists of radial fibres made up of two different kinds of 

 substances which refract light differently. 



In microscopic structure the shell may be said to consist of 

 three layers, an epicuticula or periostracwnf a prismatic layer, 

 and a nacreous layer. The periostracum is thin, horny, laminated, 

 and consists of conchiolin ; the prismatic layer consists of 

 numerous polygonal prisms united together by conchiolin and 

 * Often erroneously called the epidermis. 



