NERVES AND GANGLIA. i5f) 



tral nervous system. The remaining p(jrti«jn, which consists 

 mainly of nerve-fibres, tliongli it may also contain many nerve- 

 cells and small sporadic ganglia, is known as the peripheral 

 Tiervoiis system. 



Gener'ol Anatomy of the Nervous System. In the earth- 

 v^^orm the central system consists of a long series of douhk* gangUa, 

 metamerically repeated, and connected by nL;rvL*-c(jrds known a« 

 commissures. The most anterior pair of ganglia, known as the 

 supra-msophageal or cerebral ganglia, lie on the dorsal ii,si)ect of 

 the pharynx, a short distance behind the anterior extremity 

 (Figs. 24, 29), From each of them a slender cord, the circum- 

 msophageal commissui'e^ passes dow^n at the side of the pharynx 

 to end in the sub-oesophageal or first ventral ganglion on the 

 lower side, forming with its fellow a complete ring or pharyn- 

 geal collar around the alimentary canal. From the sub-(jesoi)lia- 

 geal ganglion a long double ventral nerve-cord proceeds back weirds 

 in the middle ventral line. The ventral cord consists of a series 

 of double ganglia, one to each somite, connected by conunissures 

 and giving off lateral nerves.^ 



Internally the cerebral ganglia and the ventral cord (com- 

 missures as well as ganglia) consist of both nerve-cells and nerve- 

 fibres as described on p. U-i. 



Peripheral Nervous System. To and from the central sys- 

 tem just described run the nerves wdiich constitute the peripheral 

 system. These are as follows : 



1. A pair of nerves running out on either side of each ven- 

 tral ffano^lion and lost to \dew amons^ the muscles of the ])ody- 

 wall. 



2. A single nerve proceeding from the ventral commissures 

 on each side immediately behind the dissepiment to which it is 

 mainly distributed. 



8. A pair of nerves from the sub-03sophageal ganglion. 



4. A nerve from each half of the pharyngeal collar just 

 beyond its divergence from its fellow. (Origin incorrectly 

 shown.) 



5. Two large cerebral nerves, which run forwards from the 



* So closely are tlie two halves of the ventral cord uniteil that its double 

 nature can scarcelv be made out without sections. 



