MOLLUSCA—THE NERVOUS SYSTEM 



145 



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

 siphons, and is furtlier 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 tyj)ical 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 Molluscs (c/. p. 

 36). The true morphological position 

 will be given in brackets after the con- 

 ventionally accepted position. 



1. Tetrabpanehia (Figs. 125, 126). 



In the complex of ganglia 

 which in Nojutilus surrounds the 

 oesophagus 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 oesophagus, and from them run 

 two ganglionic cords, one anterior 



(lower) and one posterior (upper), ^ ,„. „ 



^ , . 1 . ^ 1 \ /I 1 • 1W1 Fig. 125.— Nervous system of Nautilus (after 



which pass just below (behmd) the JHering). l, Buccal ganglion; 2, pharyngeal 



ganglia ; 3, pedal coniniissure ; 4, infundibular 

 nerve ; 5, nerve in the female for the tentacles of 

 the posterior and inner lobes ; this nerve soon 

 swells to form a ganglion ((/. Fig. 126); 6, nerves 

 for the other tentacles; 7, pedal cord ( = pedal 

 ganglia); 8, auditory organ ; 0, olfactory nerve; 10, 

 optic ganglion ; 11, nerve of the optic tentacles ; 

 12, connective to the pliaryngeal ganglion ; 13, 

 labial nerves'; 14, cerebral cord (= cerebral 

 ganglia) ; 15, pleurovisceral cord. 



cesophagus. The anterior (3) re- 

 presents the pedal, and the posterior 

 (15) the combined pleural and 

 visceral ganglia. 



The cerebral cord gives rise 

 laterally to the large optic nerves 

 (each of which at once swells into 

 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 



