VENUS MERCENARIA. 97 



Excretory and Genital Systems. — The excretory system con- 

 sists of a pair of dark colored glandular organs that lie beneath 

 the pericardium. Each communicates with the pericardium 

 by a small opening that is not easy to demonstrate in dissections, 

 and with the cloacal chamber by another small opening. 



By turning the two gills (of Venus) dorsally a very small 

 papilla may be seen, just beneath the free border of the inner 

 gill, lying in the cloacal chamber. On the tip of this papilla 

 are two openings. The inner one is the opening of the excretory 

 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 vis- 

 ceral 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 connection 

 with the pericardium? 



Nervous System.— 1. Carefully remove the body- wall by 

 the side of the esophagus and notice the cerebral ganglion of the 

 corresponding side. This is a rounded, slightly yellow organ, 

 about the size of a pin-head, lying just posterior 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 com- 

 missure, 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 

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