THE NERVOUS SYSTEM. 



The nervous system of the squid (Plates II flg. 15, III fig. 16 and Text 



Eigs. 15 and 16) is well centralized and is almost perfectly bilaterally symmetrical. 



The nerve cells are collected in thirty-one ganglia, all but one of which are 



paired, as follows: 



cerebral \ 



pleural / 



,1 > forming the oesophageal ring or brain. 



visceral / 



propedal . closely connected with the pedal and cerebral ganglia. 



optic . . . closely connected with the pleural ganglia. 



suprabuccal l connected with the cerebral and pedal ganglia and 



infrabuccal ) forming the buccal ring. 



first brachial 



second „ , j. ■, -^^ ^-, , , ,. , „ . . 



, . , ( connected with the propedal ganglia and forming the 



„ ^, " ( brachial nerve ring, 

 fourth „ * 



fifth 



stellate connected with the visceral ganglia. 



branchial „ 



splanchnic unpaired „ „ „ infrabuccal ganglia. 



The eight ganglia which form the oesophageal ring are so completely fused 

 that it is almost or quite impossible to determine the bounderies of individual 

 ganglia. Moreover, each member of three pairs of ganglia, the propedal, infrabuccal, 

 and suprabuccal, unites with its fellow in the median line forming a median gang- 

 lion. A similar fusion exists in the cerebral, pedal, and visceral ganglia so that 

 each pair of these and of the above ■ mentioned ganglia is indiscriminately termed 

 a ganglion or a pair of ganglia. 



The oesophageal ring is enplosed by the skull and surrounds the oesophageal 

 venous sinus and the structures that lie within the sinus. The top of the ring 

 is formed by the cerebral ganglia, the sides by the pleural ganglia, and the 

 bottom by the pedal ganglia in front and the visceral ganglia behind. The pedal 

 ganglion lies directly below the cerebral and pleural ganglia and in front of the 



