J12 THE FRESHWATER MUSSEL. 



ventral edge of the visceral mass is produced into 

 the wedge-shaped muscular foot. 



3. The alimentary canal. The coils of the intestine are 



seen in the visceral mass, imbedded in the genera- 

 tive gland, and cut across about a half a dozen 

 times. Lying in the pericardial cavity in the median 

 line immediately beneath the dorsal surface is the 

 rectum, in which the typhlosole or fold of its ventral 

 surface is well seen. 



4. The gills lie in the dorsal half of the mantle-cavity, 



between the visceral mass and the mantle. The 

 attachments of their lamellae to the body-wall are 

 well seen. The inner lamella of each inner gill is 

 attached to the base of the visceral mass. 



At the base of each gill is the supra-branchial 

 passage, along which the respiratory stream of 

 water flows backwards to the cloacal cavity. 



5. The circulatory system. 



a. The pericardial cavity is nearly circular in outline ; 



its roof and sides are thick and spongy, forming 

 what is spoken of as Keber's organ. 



b. The anterior aorta lies on the dorsal surface of 



the rectum, and its cavity is slit-like in section. 



c. The vena cava lies in the median plane between 



the excretory organs and the visceral mass. 



d. The afferent branchial vessels lie one on each side, 



just above the attachment of the outer lamella 

 of the inner gill and inner lamella of the outer 

 gill. 



6. The excretory organs are paired, and he between the 

 pericardium and the visceral mass. 



a. The glandular portions, or kidneys proper, are cut 

 across close to their anterior ends. They are 

 seen as a pair of small tubes, with glandular 

 longitudinally folded walls, lying side by side just 



