VII 



MOLLUSCA~THE NERVOUS SYSTEM 



145 



pallial nerve gives off branches to the organs at the edge of the mantle and to the 

 siphons, and is further connected with a rich nerve plexus in the mantle fold, in 

 which certain connecting nerves, further from the edge of the mantle, but running 

 parallel to it, are particularly strongly developed. A varying number of small 

 peripheral ganglia attain development in the pallial plexus and in the siphonal 

 nervous system. 



E. Cephalopoda. 



The symmetrical nervous system of all Cephalopoda is marked by 

 the great concentration of the typical Molluscan ganglia, including 

 those of the visceral connective. 



In the following description of the nervous system, we shall consider the body in 

 its physiological, not in its true morphological position, i.e. we shall imagine the 

 pharynx and cesophagus to be running 

 horizontally as in other JIoUuscs (c/. p. 

 36). The true morphological position 

 will be given in brackets after the con- 

 ventionally accepted position. 



1. Tetrabranehia (Figs. 125, 126). 



In the complex of ganglia 

 which in Nautilus surrounds the 

 cesophagus behind the great buccal 

 mass, and which is not yet com- 

 pletely enclosed in the cephalic 

 cartilage, the ganglia are not very 

 distinct from the commissures and 

 connectives. The cerebral ganglia 

 (14, in Figs.) are represented by a 

 broad band-like nerve cord running 

 over (morphologically in front of) 

 the cesophagus, and from them run 

 two ganglionic cords, one anterior 

 (lower) and one posterior (upper), 



which pass just below (behind) the Jlermg). l, Buccal ganglion; 2, pharyngeal 



CeSOphaaUS. The anterior (3) re- g™glia ; 3, pedal commissure ; 4, infundibular 



, ,-, , , J ii, 4. ■ nerve ; 5, nerve in the female for the tentacles Of 



presents the pedal, and the posterior ^j^^ posterior and inner lobes ; this nerve soon 



(15) the combined pleural and swells to form a ganglion (c/. Fig. 126); 6, nerves 



vi«PPT>nl S-ane-lia ^°'' *'''* °*^'^'' t^'iolfs; 7, pedal cord (=pedal 



visociai s"'"S""" _ ^ ganglia); 8, auditory organ; 9, olfactory nerve; 10, 



The cerebral cord gives rise opti^ ganglion ; ll, nerve of the optic tentacles ; 



laterally to the large optic nerves 12, connective to the pharyngeal ganglion ; 18, 



/ 1 p 1 * T_ i. „ ,,,-^n« 4^+^ labial nerves'; 14, cerebral cord (=cerebral 



(each of which at once swells into g.^glia) ; 15, pleurovisceral cord. 



an optic ganglion), numerous nerves 



to the lips, the nerves for the optic tentacles, the auditory and olfactory 



nerves, and the cerebrobuccal connectives. 



From the pedal cord, nerves run to the tentacles round the mouth 

 and to the funnel. In the female, the nerves for the inner circle of 

 VOL. II L 



Fig. 126.— Nervous system ot Nautilus (after 



