PHYSIOLOGY OF THE INVERTEBRATA. 341 



In the CepJudopoda, the nervous system consists of a cerebral 

 or supra-cesophageal, pedal and parieto-splanchnic ganglia 

 situated around the oesophagus, and connected by commis- 

 sures. " In addition to these, buccal, visceral, branchial, and 

 pallial ganglia may be developed on the nerves which supply 

 the buccal mass, the alimentary canal, heart, branchiae, and 

 mantle." 



In the Dibranchiata (Fig. 68 0), the cerebral ganglia send 

 off nerves to the eyes, &c., and to the buccal ganglia ; in the 

 Tetrahranchiata, the same ganglia supply nerves to the eyes, 

 &c., and to the buccal mass. The pedal ganglia, in the 

 Dibranchiata, supply the arms, funnel, and they are connected 

 with the auditory nerves. In the Tetrahranchiata, the pedal 

 ganglia supply the branchiae and the funnel. In both sub- 

 orders of the Cephalopoda, the parieto-splanchnic ganglia 

 supply the branchiae, but in the Dibranchiata they also send 

 nerves to the pallium and sexual organs. In the iast-]nen- 

 tioned sub-order, " each parieto-splanchnic ganglion gives off 

 a nerve, which runs along the shell-muscles to the anterior 

 wall of the mantle, and there enters a large ganglion — the 

 ganglion stellatum." The anterior and posterior buccal 

 ganglia give off nerves to the oesophagus and stomach. The 

 nervous system of the Cephalopoda is characterised by its 

 great concentration and high development. 



Notwithstanding the apparent irregularity of its general 

 arrangements, the nervous system of the Mollusca is modelled 

 upon the same plan as that of the Arthropoda. In the 

 Mollusca, we still find the oesophageal ring, giving off from 

 its central portion a ganglionic peripheral nervous system, 

 distributing itself to the various organs, but without sym- 

 metry, as, however, the general conformation of the body 

 demands. The cerebral or super-oesophageal ganglia are very 

 small in the Damellibranchiata ; but are not exceptionally so, 

 as these animals have no head provided with sense-organs. 



The cerebral ganglion is, however, very large in the 

 Cephalopoda, due to the highly developed sense-organs. 



