NERVES AND GANGLIA. 65 



tral nervous system. The remaining portion, which consists 

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

 cells and small sporadic ganglia, is known as the peripheral 

 nervous system. 



General Anatomy of the Nervous System. In the earth- 

 worm the central system consists of a long series of double ganglia, 

 inetamerically repeated, and connected by nerve-cords known as 

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

 supra-cesophageal or cerebral ganglia, lie on the dorsal aspect of 

 the pharynx, a short distance behind the anterior extremity 

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

 cesophageal commissure, passes down at the side of the pharynx 

 to end in the sub-wsophageal 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-cesopha- 

 geal ganglion a long double ventral nerve-cord proceeds backwards 

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

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

 and giving off lateral nerves. 44 



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. 94. 



Peripheral Nervous System. To and from the central sys- 

 tem just described run the nerves which constitute the peripheral 

 system. These are as follows : 



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

 tral ganglion and lost to view among the muscles of the body- 

 wall. 



2. A single nerve proceeding from the ventral commissures 

 on each side immediately behind the dissepiment to which it is 

 mainly distributed. 



3. A pair of nerves from the sub-oesophageal 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 the two halves of the ventral cord united that its double 

 nature can scarceK be made out without sections. 



