IV 



VERMES— NERVOUS SYSTEM 



219 



it divides at the most posterior end of the body into two strong 

 branches which run into the caudal bifurcation. The peripharyngeal 

 ganglion is connected on each side with the hypodermis, ais is the 

 ventral strand at the extreme posterior end of the body, in the caudal 

 bifurcation, and indeed along its whole length by means of numerous 

 unpaired median nerves. Nerve fibres radiate forwards from the 

 peripharyngeal ganglion. A medio -dorsal nerve is wanting. It is 

 probable that the peripharyngeal ganglion and the ventral strand of 

 the Gordiidce are homologous with the oesophageal ring and ventral 

 chord of the Annulafa. 



Annulata. — ^Here we can establish a general scheme for the central 

 nervous system. It consists (1) of a brain which lies over the anterior 

 portion of the enteric canal, and (2) of the segmented ventral chord, 

 running through the body in the ventral middle line. These two are 



Fig. 144. — CEsopbageal ring with diverging nerves of Hirudo, enlarged (after Hermann). 

 [l, Brain; sc, cesopliageal commissure ; bgi, first ganglion of the ventral chord (infra - oesophageal 

 ganglion) ; bgo, second ventral ganglion. 



connected by 2 commissures, the oesophageal commissures, which 

 embrace the fore-gut between them. The anterior end of the intestine 

 is thus surrounded by a nerve ring, the so-called oesophageal ring, 

 which enters the brain dorsally, and the first ganglion of the ventral 

 chord ventrally (Figs. 144, 146). The brain (supra-oesophageal gan- 

 glion), whose composition out of 2 lateral halves connected by trans- 

 verse commissures can generally be clearly seen, lies originally in the 

 first, the cephalic or oral segment. The anterior ganglion, or rather 

 double ganglion, of the ventral chord (infra- oesophageal ganglion) 

 probably originally belonged to the second segment of the body. 

 The remaining double ganglia of the ventral chord follow the infra- 



