140 



COMPARATIVE ANATOMY 



CHAP. 



seems to Lave moved up to the riglit pleural ganglion, or to have fused with it, so 

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

 ganglion. 



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

 spidu; 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 

 pleurovisceral 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 



I'll,. 1111.— Nervous System of Bulla liydatis (after Vayssifere). 



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

 pedal ganglion ; 5, part of the right pleural ganglion (?); 7, eye ; 

 s, cerebral commissure ; 9, pedal conimis.sure ; 10, auditory vesicle ; 



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

 ganglia. 



Fig. 120.— Nervous System 

 of Aplysia, diagi-ani, combined 

 from several sources. 1, 

 Buccal ; 2, cerebral ; 3, pleural ; 

 4, pedal ; 5, right parietal ; 6, 

 visceral ganglion ; 7, osphra- 

 dinm ; 8, genital ganglion ; 9, 

 branchial ganglion. 



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

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

 which correspond with the parapodia of the Cephalaspida:. 



Fig. 120 represents the nervous system of Aplysia, one of the Anaspidai. The 

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

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

 that the jileuropedal 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 



