VII MOLLUSCA—THE NERVOUS SYSTEM 133 



1. Two cerebral ganglia near or above the resophagus, which 

 aie connected by a cerebral commissure. 



2. Two pedal ganglia below the oesophagus, connected with each 

 other by a pedal commissure, and with the cerebral ganglia by two 

 cerebropedal connectives. 



The cerebral and pedal ganglia with the commissures and con- 

 nectives belonging to them form a ring encircling the oesophagus, 

 which may be compared with the oesophageal ring of the Annulata 

 and Arthropoda. 



3. Two pleural or pallial ganglia (between the cerebral and 

 pedal ganglia), which are connected with the cerebral ganglia by two 

 eerebropleural, and with the pedal ganglia by two pleuropedal con- 

 nectives. 



4. A simple or complex visceral ganglion lying below the in- 

 testine, united to the pleural ganglia by two pleurovisceral con- 

 nectives. 



5. A ganglion, which may be called parietal, almost always occurs 

 in the course of each pleurovisceral connective. The parietal ganglion 

 divides the connective into two parts, an anterior pleuroparietal and 

 a posterior viseeroparietal connective. 



The cerebral, pedal, and pleural ganglia are (with unimportant 

 exceptions) always arranged symmetrically to the median plane in all 

 Gastropoda. The pleurovisceral connectives and their ganglia, how- 

 ever, are only found in such a position in some Gastropoda. In fact, 

 only in the Opisthobranchia (including the Pteropoda but excepting ^cteow) 

 and the Fidmonata are they symmetrical, in the sense that the right 

 connective and its ganglion lie entirely on the right, and the left 

 connective and its ganglion entirely on the left side of the body. 

 The Opistliohrancliui and Pulmonata are therefore called euthyneurous 

 Gastropoda. 



In the Prosohranchia and Adceon, the pleurovisceral connectives are 

 asymmetrical, inasmuch as they cross one another, the connective 

 springing from the right pleural ganglion running aver the intestine to 

 the left before joining the visceral ganglion, while the connective 

 from the left pleural ganglion runs under the intestine to the right side 

 of the body. In consequence of this crossing, the parietal ganglion of 

 the connective which springs from the right pleural ganglion becomes 

 the supraintestinal ganglion, which lies on the left side, and the 

 parietal ganglion of the connective springing from the left pleural 

 ganglion becomes the infra-intestinal ganglion which lies on the right 

 side. The Prosobraiichiaund Adceon are thus streptoneurous Gastropoda. 



The Areas of Innervation of the various Ganglia. 



1. The cerebral ganglia innervate the eyes, the auditory organs, 

 the tentacles, the snout or proboscis, the lips, the motor muscles of the 

 proboscis and buccal mass, and the body walls lying at the base of the 



