ANODONTA. 271 



a similar scar of the posterior adductor. The adductors 

 run across from shell to shell, and their contraction draws 

 the shells together. Inside each adductor scar is a smaller 

 round scar, caused by the anterior and posterior retractors^ 

 which serve to draw the foot into the shell. Lastly, near 

 the anterior adductor scar is a small protractor scar, the 

 muscle serving to draw the foot forward. The attachments- 

 of the muscles shift outwards and downwards as the shells 

 grow. 



Fig. 190. — Internal View of Right Shell of Anodonta. 

 f^Ad nat,) 



Umbo. 



Hinge. 

 Anterior Retractor. / / ^ . 



Posterior 

 Retractor. 



T. iiiiiii ■■— ■■■iiffliiiiiiiiiii II I Pallial Line. 



Protractor. 



When the shells are forced open they expose a large 

 mantle-cavity. This is bounded dorsally by the body of the 

 animal and laterally by the lateral mantle folds ; ventrally 

 it is widely open to the exterior, except when the shells are 

 shut. The mantle-flaps line the inner surface of the shells, 

 which they secrete. The free edges are pressed together, 

 except at the posterior end, where they diverge to form a 

 large inhalent openings then meet, and again diverge to form 

 the smaller exhalent opening. 



In the centre of the mantle-cavity a large muscular foot 

 depends downwards and on occasion it can be protruded 



Sensory outwards between the shells. Embedded in the 



foot, near the pedal ganglia^ are the otocysts, but 



Anodonta has no eyes. There is a pair of osphradia or 



sense-organs of an olfactory nature at the base of the gills, 



innervated from the visceral ganglia. 



