140 



COMPA RA TIVE ANA TOMY 



CHAP. 



-cms to have moved up to the right pleural ganglion, or to have fused with it, so 

 that the nerve running to the branchial ganglion rises direct from the right pleural 

 ganglion. 



The nervous system of the Ptfropoda thccosomata, which we derive from Cephala- 

 xpiilir, bears a general correspondence to that of the latter, especially in the fact 

 that the pleural ganglia shift near to or fuse with the cerebral ganglia. The 

 ph-uro visceral connectives are so much shortened that the ganglia occurring in 

 their course lie close to the cerebral and pedal ganglia. There are usually two such 



Ki<;. l Hi. Nervous System of Bulla hydatis (after Vayssiere). 



I, Buccal ganglion; 2, cerebral ganglion; 3, pleural ganglion; 4, 

 j>edal ganglion ; 5, part of the right pleural ganglion (?) ; 7, eye ; 

 8, cerebral commissure ; !, pedal commissure ; 10, auditory vesicle ; 



II, right parietal ganglion ; 12, 13, 14, visceral ; 15, branchial 

 ganglia. 



Fiu. 1-20. Nervous System 

 of Aplysia, diagram, combined 

 from several sources. 1, 

 Buccal ; 2, cerebral ; 3, pleural ; 

 4, pedal ; 5, right parietal ; ti, 

 visceral ganglion ; 7, osph ra- 

 dium ; 8, genital ganglion ; 9, 

 branchial ganglion. 



ganglia (the right parietal and a visceral ganglion ?), less frequently three (two 

 intestinal and one visceral ganglion ?). The pedal ganglia also innervate the fins, 

 which correspond with the parapodia of the Cephalaspiilo . 



Fig. 120 represents the nervous system of Aphjsia, one of the AnaspidcK. The 

 two cerebral ganglia have moved close to each other in the middle line. The pleural 

 ganglia here, unlike those of the Cephalaspidcc. lie close to the pedal ganglia, so 

 that the pleuropedal connectives are much shortened. The pedal commissure is 

 double, the anterior commissure is, relatively speaking, short and thick, the posterior 

 long and thin. The long pleurovisceral connectives run back from the pleural 



