402 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



the Mollusca, but it is impor- 

 tant to note that along this 

 phylum we have persisting a 

 larger number of conditions 

 than are at present, at any 

 rate, known among the Ar- 

 thropoda. A reference to Fig. 

 173 will show that, in Pro- 

 neomenia, there are, on either 

 side, two cords which run 

 down the whole length of the 

 body, and both of which ter- 

 minate in a ganglionic swel- 

 ling ; the two inner cords are 

 seen to be connected with one 

 another by commissural fibres, 

 and each of these with the edge 

 of the cord that lies outside 

 it ; as these latter give off 

 peripheral nerves it follows 

 that here again we have a 

 plexus of nerve fibres distri- 

 buted through the body. In 

 the case of Proneomenia we 

 have ganglion cells not only 

 accompanying the nerve fibres 

 throughout the whole of their 

 length, but they are also, as 

 they are in some of the com- 

 missures of Peripatus, found 

 on the commissures which con- 

 nect these cords with one an- 

 other. Here, then, we have 

 yet another instance of the 

 plexiform disposition of_ 



JPVG - 



Fig. 173. Diagram of the 



Nervous System of Proneo- 

 menia. 



CG, Cerebral ganglion; slg, sub- 

 liminal ganglia ; APG, PPG, PVG, 

 anterior pedal, posterior pedal, 

 posterior lateral (vi.-ceral) 

 ganglia ; si, sublingual con- 

 nectives ; Cpc, cerebropedal 

 connectives ; pe, longitudinal 

 pedal nerve trunks ; la, longi- 

 tudinal lateral nerve trunks. 

 (After Hubrecht.) 



tion of fraiioiioiiic cells in 



