VENUS MERCENARIA. 121 



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 

 supplies the posterior end of the corresponding lobe of the mantle. 

 Smaller nerves go to the posterior 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 visceral 

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

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

 of the foot. The pedal ganglia are connected with each other 

 by a broad commissure and with the cerebral ganglia by connec- 

 tives. 



By careful dissection it is possible to trace the connectives 

 and many of the nerves. The razor clam, Ensis, is especially 

 favorable for dissections of the nervous system, as the ganglia, 

 connectives, and many important nerves lie very near the 

 surface and can be seen without cutting the tissues above them. 



Make a drawing, indicating the position of the ganglia. 



Digestive System. — This may be traced by following a guarded 

 bristle that has been inserted into the mouth of a specimen that 

 has been killed in hot water (not boiling), or by very carefully 

 picking off the tissue from one side. The intestine where it 

 penetrates the heart has already been seen, and may easily be 

 followed to the anus. 



