146 



COMPARATIVE ANATOMY 



tentacles come from 



Fig. 12(3.— Nervous system of Nautilus, 

 from the right side. Numbering the same 

 as in Fig. 125. «, Ganglion for the ten- 

 tacles of the posterior and inner lobes in 

 the female. 



a brachial ganglion, which, however, does not 

 supj)ly all the tentacles (Fig. 126, a) ; 

 this is joined to the pedal ring by a 

 brachiopedal connective. 



The pleupoviseeral eord gives oft' 

 numerous pallial nerves (there is no 

 stellate ganglion), and two strong vis- 

 ceral nerves which run near the middle 

 line accompanying the vena cava, inner- 

 vate the gills, the osphradia, and the 

 blood-vessels, and form a genital gan- 

 glion high up in the visceral dome. 



Tlie sympathetic nervous system consists 

 of an infra-cesophageal commissure, which rises 

 from the cerebral ganglion, and passes close under the oesophagus in the musculatui'e 

 of the buccal mass ; two ganglia, a pharyngeal and a buccal ganglion, are found on 

 each side in its course. 



2. Dibranchia (Figs. 127, 128). 



The peri-oesophageal mass of ganglia, comprising the Avhole of the 

 central nervous system, is entirely enclosed in the cephalic cartilage. 

 The large typical ganglia are so crowded together that it is extremely 

 difficult to distinguish them one from another, and the connectives 

 and commissures are not visible externally. The whole complex has 

 a continuous cortical layer of ganglion cells. 



The more or less distinct separation of each pedal ganglion into 

 two, one anterior (lower) and one posterior (upper), is characteristic 

 of the Dibranchia. The former of these is the brachial ganglion, and 

 innervates the arms, which must be considered as parts of the foot ; and 

 the latter is the infundibular ganglion, and innervates the siphon, 

 which may be regarded as the epipodium. This differentiation of the 

 pedal ganglia can be traced to the great development of that part of 

 the foot (viz. the arms) which surrounds the head. In the same way 

 in Natica, where the anterior part of the foot is strongly developed, 

 and is bent back over the head, a propedal ganglion becomes 

 differentiated from the pedal ganglion. The brachial ganglia become 

 joined in the Dibranchia to the cerebral ganglia by cerebrobrachial 

 connectives. In Elcdone and Octopus, they are further connected with 

 one another by a thin supraoesophageal commissure. 



The pleural ganglia lie laterally in the perioesophageal mass, while 

 the ganglia of the visceral connectives, i.e. the parietal and visceral 

 ganglia which lie close together, their connectives having shortened as 

 much as is possible, form the posterior (upper) portion of the infra- 

 ocsophageal mass. 



The following are the connectives which are revealed by sections 

 through the peri-cesophageal mass : — 



