THE NERVOUS SYSTEM. 



The nervous system of the squid (Plates 11 fig. 15, III fig. 16 and Text 

 Figs. 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 



pedal I 



viscei'al 



propedal 



optic . . 



suprabuccal ) 



infrabuccal ) 



first brachial 



second „ 



third „ 



fourth „ 



fifth 



stellate 



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 tlie 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 enclosed 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 



forming the oesophageal ring or brain. 



closely connected with the pedal and cerebral ganglia, 

 closely connected with the pleural ganglia. 



connected with the cerebral and pedal ganglia and 



forming the buccal ring. 



connected with the propedal ganglia and forming the 

 brachial nerve ring. 



connected with the visceral ganglia. 



