342 



THE EARTHWORM 



CHAP. 



-Jbrost 



locomotive medusa and the fixed polype was the presence 

 in the former of a well-developed nervous system (p. 319) 

 consisting of an arrangement of peculiarly modified cells, 

 to which automatic action was seen to be due. It is 

 natural to expect in such an active and otherwise highly- 

 organised animal as the earthworm a nervous system 



of a considerably 

 higher degree of 

 complexity than 

 that of a medusa. 



The central ner- 

 vous system (Figs. 

 82, 83, and 85) con- 

 sists of two parts, 

 the brain and the 

 ventral nerve - cord. 

 The brain consists 

 of a pair of white 

 pear-shaped swell- 

 ings or ganglia situ- 

 ated on the dorsal 

 side of the buccal 

 sac, where it is con- 

 tinued into the 

 pharynx. The ven- 

 tral nerve-cord is a 

 longitudinal band 



consisting of two cords surrounded by a common sheath 

 extending along the whole middle ventral line of the 

 body, internally to the longitudinal muscular layer, from 

 the third to the anal segment, and slightly swollen in 

 each segment. The brain is connected with the anterior 

 end of the ventral nerve-cord by a pair of nervous 

 bands, the pharyngeal connectives which pass respec- 

 tively right and left of the buccal sac and thus form a 

 nerve-collar : the cord is also Drimarilv Daired. 



FIG. 85. Anterior portion of nervous system of 



Lutnbricus. (x about 8.) 

 cer. gang, cerebral ganglia or brain ; com. oeso- 

 phagea! connectives ; ne. co. ventral nerve- 

 cord ; prost. prostomium. (From Parker and 

 HasweH's Zoology, after Leuckart.) 



