VENUS 131 



are two openings. The inner one is the opening of the ex- 

 cretory organ. The outer one is the opening of the genital 



duct. 



The genital glands are light colored organs that, during 

 the breeding season, extend through the principal part of the 

 visceral mass. Neither the genital nor the excretory systems 

 can be profitably studied in a general dissection of this form. 

 In Unio the excretory organs are more satisfactory for study. 

 Do you understand the supposed significance of their con- 

 nection with the pericardium? 



Nervous System. — 1. Carefully remove the body wall 

 along the side of the esophagus and notice the cerebral gang- 

 lion of the corresponding side, This is a rounded, slightly 

 yellow organ, about the size of a pin head, lying just pos- 

 terior to the dorsal border of the anterior adductor muscle. 

 (In Unio it is more ventral in position.) The cerebral 

 ganglia of the two sides are united by a commissure that 

 passes anterior to the esophagus. Two connectives leave 

 each cerebral ganglion. One passes posteriorly to join the 

 visceral ganglion of the corresponding side. The other passes 

 into the foot to join the pedal ganglion of the corresponding 



side. 



2. Cut the united lamellae of the inner gills ventral to the 

 posterior adductor muscle. This will expose the visceral 

 ganglia. They are pear-shaped bodies lying just beneath the 

 posterior adductor muscle, connected with each other by a 

 short commissure, and connected with the cerebral ganglia by 

 connectives that may be traced a short distance forward 

 without dissection. A large nerve leaves the posterior end of 

 each ganglion and supplies the posterior end of the corre- 

 sponding lobe of the mantle. Smaller nerves go to the pos- 

 terior adductor muscle and gills. 



3. With a razor or sharp scalpel make a median sagittal 

 section of the foot, extending it some distance into the vis- 

 ceral mass. This will expose the pedal ganglia, that lie just 

 anterior to a loop of the intestine, and dorsal to the muscular 



