350 



COMPARATIVE ANATOMY. 



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are also found in the Cephalopoda ; but they are more closely 

 approximated in consequence of the shortening of their com- 

 missures. They form a ring around the oesophagus. In the 

 Dibranchiata this ring is enclosed by the cephalic cartilage in 

 such a way that the peripheral nerves pass out through foramina 

 in it. 



In Nautilus, the upper part of the oesophageal ring is formed 

 by a transversely disposed nervous mass (Fig. 186, aa); the nerves 



of the higher sensory organs are given off 

 at the sides. It corresponds to the cerebral 

 ganglia, but these also extend some distance 

 ventrally (bb), and give off, in that region, 

 nerves which pass to the tentacles (ft). 

 The ventral segment only of this ring is 

 homologous with the pedal ganglion, as it 

 gives off the nerves for the funnel. A 

 second ventral mass (cc) is attached to the 

 lower nervous mass, which, as we have 

 already said, is partly formed from 

 the cerebral ganglia ; this corresponds to 

 the visceral ganglia, and gives off nerves 

 for the mantle (m) } in addition to two small 

 trunks which accompany the vena cava, and 

 pass to the gills and vascular system. Each 

 of these two nerves forms a ganglion (d), 

 which, again, supplies the generative appa- 

 ratus. 



In the Dibranchiata the nervous system 

 is much more concentrated. The cerebral 

 ganglionic masses are placed much more 

 to the sides and ventral surface, so that 

 they unite in the middle line in front of 

 the part which corresponds to the pedal 

 ganglia. The visceral are completely fused 

 with the pedal ganglia, and the only rem- 

 nant of the primitive independence of these 

 ganglia is a narrow point through which 

 the pedal artery passes ; the separate character of these ganglia 

 is much more distinct in the Tetrabranchiata. That portion of 

 the cerebral ganglionic masses which still remains above the oeso- 

 phagus is differentiated into several portions. The part which 

 has reached the ventral surface gives off the nerves for the arms, 

 and these unite to form a ganglion at their base. The pallia! 

 nerves are given off from the visceral masses, and each of these 

 nerves forms a large ganglion (ganglion stellatum). 



Compared with the rest of the Mollusca, the Cephalopoda have 

 much more highly differentiated central organs from a histological 

 point of view ; this is due to their greater size. In all parts of the 

 nerve-centres it is possible to distinguish an outer gray layer, formed 



Fig. 18G. Nervous system 

 of Nautilus pompilius. 

 a Superior, h Inferior gan- 

 glionic masses of the oeso- 

 phageal ring. c Visceral 

 ganglia, d Genital ganglia, 

 m Pallial nerves, t if Ten- 

 tacular nerves (after Owen). 



