208 



TEXTBOOK OP ZOOLOGY 



The Nervous System 



The "brain" of the earthworm consists of the suprapharyngeal 

 ganglion, two circumpharyngeal connectives, and the suhpharyngeal 

 ganglion. The ventral nerve cord extends posteriorly the length of 

 the body with a ganglion and three pairs of nerves in each segment. 



-prosfomium 



-SvLpharyn^Zo/ Gang/fo/7 



Posterior <Sejfmeti^a/ A/erve 



Bo</y IVa// 

 Sefi/um 



^et-^e Core/ 



Fig. 108. — Anterior portion of the nervous system of an earthworm from dorsal 



view. 



Each ganglion is really the fusion of two, a deviation from the con- 

 dition found in many annelids and arthropods where there are two 

 ganglia in each segment and the nerve cord is double. The supra- 

 pharyngeal ganglion lies dorsal to the pharynx in the third segment 

 and the suhpharyngeal ganglion lies ventral to the pharynx in the 

 fourth segment. Nerves from these two ganglia innervate the first 

 three segments and the prostomium. 



