314 



THE CEPHALOPODA 



Decapoda there are two on each side, namely, the cerebro-pedal and 

 the cerebro-brachial. The ventral or sub-oesophageal nervous mass 

 is formed by the visceral and pedal centres : these are fairly closely 

 united together (as they are, for instance, in Helix), and are only 

 separated in the middle line to admit of the passage of an aortic 

 vessel which runs dorsad of the visceral and ventrad of the pedal 

 centres ; a similar arrangement occurs in sundry Gastropods. 



The pedal ganglia are divided transversely into two distinct pairs, 

 the anterior or brachial and the posterior or pedal ganglia proper 



Fia. 280. 



Diagram of the nervous system of a female Nautilus pompilivs, ventral aspect, cer, cerebral 

 ganglion ; in, nerves to the mantle ; n.inf.br, posterior branchial nerve ; n.olf, olfactory nerve 

 terminating under the olfactory papilla ; n.sup.br, anterior branchial nerve ; n.visc, genito- 

 branchial nerve, or chief visceral nerve ; n.x, nerve accompanying tlie vena cava, which lies 

 between this and the similar nerve of the right side ; olf.p.p, the right olfactory papilla ; opt, 

 optic ganglion ; ov, the oviduct ; ped, pedal ganglion ; pi, pallial part of the visceral ganglionic 

 commissure ; x and y, ganglion-like enlargements on pedal nerves to the median lobe of the 

 inner circlet of the circumoral tentacular lobes. (After Ray Lankester and Bourne.) 



(Fig. 282, XII, XIV). This division is most marked in the Oigopsida 

 (Ommatostrephes, Spirula, Fig. 268, p.g, etc.), but is less marked in 

 Sepia. In all the Decapoda the brachial centres are divided anteriorly 

 into ten large nerves which pass into the arms, and anastomose with 

 one another at their bases. These centres also have connectives 

 joining them to the anterior and the posterior cerebral lobes (Fig. 

 282). In the Octopoda the brachial and pedal centres are much 

 more closely approximated (Fig. 281), and the former naturally give 

 off only eight nerves to the eight arms. The brachial nerve-centres 

 extend, together with the arms which they innervate, round either 

 side of the oesophagus, and in the Octopoda they meet dorsally and 



