chap, xn.j NER vo us S\ '.v TEM OF PERI PA rus. 40 1 



A .' aln 



The ventral nerve cords are widely separated from one 

 another, but are 

 connected together 

 by a large number 

 of commissures (co 1 ), 

 of which there are 

 from nine to ten for 

 each segment of the 

 body. From the 

 outer borders of the 

 cords nerve fibres are 

 given off to all parts 

 of the body, the 

 whole of which is 

 consequently sur- 

 rounded by the ner- 

 vous system ; and we 

 have here, therefore, 

 what is essentially a 

 plexiform arrange- 

 ment, but one which 

 has, so to speak, be- 

 come regulated. A 

 further advance is to 

 be found in the fact, 

 that while the cords 

 are everywhere co- 

 vered by ganglion . 

 cells on their ventral 

 surface, the ganglia 

 are more especially 

 numerous at one 

 point in every seg- 

 ment of the body, 



\\ here they form such an enlargement as that marked 

 fy ] in Fig. 172. 



Proneomenia may be taken as the simplest type of 



A A--16 



Pig. 172. Anterior portion of the central 

 Nervous System of Peripaius, show- 

 ing the Anterior Cerebral Ganglia, 

 with the Lateral Nerve Cords con- 

 nected with one another by numerous 

 commissures (co). (After Balfour;) 



E, Eye ; atn, antennary nerve ; co l, first com- 

 missure : orn, nerves for the mnutb ; ore/, 

 oral ganglion ; pn, pedal nerves; fc/', 

 first ganglionic eulai gement for the 

 pedal nerves. 



