IV 



VERVES NERVOUS SYSTEM 



223 



o'J- 



out sharp limitation, so the posterior end of the ventral chord in most Oligochccta 

 and Pol ill-tint, i passes without sharp limitation into the hypodermis of the anal 

 segment. In many Chcetopoda of the most varied divisions, indeed in single genera of 

 di H't-rent families, the nervous system even in adult animals shows an embryonic con- 

 dition, in so far as it lies outside the body musculature in the deeper part of the 

 epidermis. This position of the central nervous system is thus far from being 

 characteristic of the so-called Archiannelida, and we 

 can in general give it no special systematic signifi- 

 cance. We find besides in various divisions all 

 transition stages, from the position of the ventral 

 chord in the body cavity to its hypodermal position, 

 since it can lie in the body musculature between the 

 hypodermis and the body cavity. Moreover, in 

 < 'I'/n'fclhf, anteriorly, the ventral chord lies in the body 

 cavity, then passes in between the musculature, and 

 finally, posteriorly, comes to lie in the hypodermis. 



The symmetrical halves of the ventral chord may 

 separate, in some cases, and the ventral chord can 

 assume the form of a ladder nervous system 

 146 and 147) (Hcrmella, many Serpultiln , 

 and in the pharyngeal region in the 

 Chcetogastridee). In the Aplmnoncura (dEolosomn) 

 the ventral chord is said to be entirely wanting, or 

 it lies in a cjuite embryonic condition imbedded in 

 tin- hypodermis. 



thus 

 (Figs. 



A 



gan- 





The nervous system of the 

 (Fig. 148) deserves special attention, 

 distinctly marked supra - cesophageal 

 glion or brain is wanting. The cesophageal 

 ring is very much elongated, in keeping 

 with the great length of the prostomium. 

 Its two limbs (cesophageal commissures), 

 which run laterally in the prostomium (pro- 

 boscis), arid which coalesce at its anterior 

 end, give off numerous branches towards 

 both the exterior and interior ; those 

 branches which run dorsally inwards may 

 represent transverse commissures between the 

 limbs of the cesophageal ring. Below and be- FIG. 148.- Nervous system of 

 hind the mouth the limbs of the cesophageal Echiurus, diagrammatic. s ., The 



i f i , two limbs of the a-sophageal ring, 



ring coalesce to form an unpaired ventral wMc h join at the anterior end of 



Strand, which in adult animals has no gan- the prostomium and enter the ex- 



glion swellings, but is supplied with gang-Home treme anterior end of ie ven t ral 



chord (Inn) behind the mouth ; nr, 

 nerve rings ; vb, anterior hooked 



cells 



throughout 



its whole course ; this 



strand runs in the ventral middle line on setEe ; 7l& . the two posterior circles 

 the body wall to the posterior end of the of setfe ; MI> n ' 2 > a P ertures of the 



nephridia ; a, anus. 



body, and gives off to the right and left 



at short intervals lateral branches which correspond with the rings 



of the outer integument. The corresponding right and left lateral 



