140 



COM PA RA TI VE A NA TOM Y 



CHAP. 



seems to liavc moved up to the right pleural ganglion, or to liave fused with it, so 

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

 ganglion. 



The nervous system of the Ftrropoda thrcosomata, which we derive from C'ephala- 

 spidw, hears a general correspondence to that of the latter, esjjecially 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'lG. 1 1'J.— Nervous System of Bulla hydatis (alter Vayssiere). 



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

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

 8, cerebral coinniissure ; 9, pedal commissure ; 10, auditory vesicle ; 



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

 ganglia. 



Fi(i. IJU.— Nervous System 

 of Aplysia, diagram, combined 

 from several sources. 1, 

 Buccal ; 2, cerebral ; 3, pleural ; 

 4, pedal ; 5, right parietal ; 0, 

 visceral ganglion ; 7, osphra- 

 dinm ; 8, genital ganglion ; 9, 

 branchial ganglion. 



ganglia (the right iiarietal and a visceral ganglion !), less frequently three (two 

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

 which correspond with the parapodia of the Cephalaspidcr. 



Fig. 120 represents the nervous system of ApJysia, one of the Auaspidw. 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 i)Osterior 

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



