IV 



VERMES— NERVOUS SYSTEM 



223 



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

 and Polychccta passes without sharp limitation into the hypbdermis of the anal 

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

 different 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 musculatui'e between the 

 hypodermis and the body cavity. Moreover, in 

 Cajiitella, 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 

 thus assume the form of a ladder nervous system 

 (Figs. 146 and 147) (Sermella, many Serpulidm, 

 Hpinther, and in the pharyngeal region in the 

 Chcetogastridce). In the Aphccnoneura {JEolosoma) 

 the ventral chord is said to be entirely wanting, or 

 it lies in a quite embryonic condition imbedded in 

 the. hypodermis. 



The nervous sj^stem of the EcMuricke 

 (Fig. 148) deserves special attention. A 

 distinctly marked supra - oesophageal gan- 

 glion or brain is wanting. The oesophageal 

 ring is very much elongated, in keeping 

 with the great length of the prostomium. 

 Its two limbs (oesophageal commissures), 

 which run laterally in the prostomium (pro- 

 boscis), and 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 oesophageal ring. Below and be- fig. 14S.- Nervous system of 

 hind the mouth the limbs of the oesophageal Eohiurus, diagrammatio. ss, The 

 ring coalesce to form an unpaired ventral 



strand, which in adult animals has no gan- 



glion swellings, but is supplied with ganglionic 



cells throughout its whole course ; this 



strand runs in the ventral middle line on 



the body wall to the posterior end of the 



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 



two limbs of the oesophageal ring, 

 which join at the anterior end of 

 the prostomium and enter the ex- 

 treme anterior end of the ventral 

 chord (&ni) behind the mouth ; nr, 

 nerve rings ; vb, anterior hooked 

 set£e ; Jib, the two posterior circles 

 of setfe ; iij, n^, apertures of the 

 nephridia ; a, anus. 



