114 MOLLUSCA. 



salivary glands, the ducts from which extend on either side of 

 the esophagus to the mouth. Further back, on the right side 

 of the esophagus, is the dehcate pancreas. 



Trace the esophagus to the stomach and the intestine to the 

 anus. 



Nervous System. — Most of the gangha are grouped around the 

 esophagus, about three-quarters of an inch posterior to the base 

 of the proboscis. They are all brown and accordingly conspic- 

 uous. Cut around its base so the proboscis may be turned back, 

 and the ganglia on the ventral surface of the esophagus may be 

 seen. Carefully pick away the tissue that covers the ganglia and 

 notice on the ventral side of the esophagus: 



1. The small but conspicuous buccal ganglia. These are 

 united with each other and with the cerebral ganglia and send 

 nerves to the mouth apparatus. 



2. The large pedal ganglia, fused together but distinctly 

 paired, lying posterior to the buccal ganglia and sending nerves 

 to the two sides of the foot. Each is united by connectives with 

 the corresponding cerebral and pleural ganglia. 



From the dorsal side a number of ganglia may be seen, more 

 of which lie to the right than to the left of the median line. 



1. On the left side there are two ganglia that are in rather 

 close union with each other. The most anterior, the left cerebral, 

 is the larger of the two. The left pleural joins it posteriorly and 

 ventrally and extends nearly to the ventral side of the esophagus. 



2. On the right side four ganglia may be distinguished. The 

 right cerebral and right pleural are fused to form one mass, but 

 there is a marked constriction between them. Posteriorly and 

 dorsally the right pleural is connected by a connective with the 

 right visceral, which lies very close to it. The remaining ganglion, 

 the left visceral, w^hich is almost in contact with the right pleural 

 and right pedal ganglia, lies ventrally and to the right of the 

 right visceral ganglion. It is connected with the left pleural 

 ganglion by a connective that runs behind the pedal ganglia. 

 There seems also to be a connection with the right pleural gan- 

 glion, but this must be considered a secondary connection. Do 

 you understand why this ganglion occupies this position? 



