vi NERVOUS SYSTEM 343 



It is to be noted that one division of the central nervous 

 system the brain lies altogether above and in front of the 

 enteric canal, the other division the ventral nerve-cord 

 altogether beneath it ; and that, in virtue of the union of the 

 two divisions by the cesophageal connectives, the enteric 

 canal perforates the nervous system. Both brain and cord 

 are composed of delicate nerve-fibres and nerve-cells, the 

 latter being situated in the ventral and lateral regions 

 of the cord along its whole length, so that there is 

 here hardly any distinction into ganglia and connectives, 

 although the swellings are often spoken of as ganglia. 

 Along the dorsal side of the cord are three transparent 

 tube-like structures, known as giant-fibres, the function of 

 which is not known (Fig. 81). The whole cord is enclosed 

 in a sheath consisting of connective-tissue and muscular 

 fibres. 



The peripheral nervous system consists of a number of 

 nerves, both sensory and motor (p. 162), which arise from 

 the central nervous system and supply the various parts of the 

 body. From the brain a number of nerves are given off 

 to the prostomium, and from each ganglionic enlarge- 

 ment two pairs of nerves can be traced into the body- 

 wall, while between these enlargements one pair is given 

 off which supply mainly the septa. 



Comparing the nervous system of the earthworm with 

 that of a medusa it is important to notice the con- 

 centration of the central nervous system in the higher type, 

 and the special concentration at the anterior end of the 

 body to form a brain. When, again, we compare the central 

 nervous system of the earthworm with that of the frog (pp. 28 

 and 155) several important points of difference are noticeable. 

 In the former it lies freely in the ccelome, and, with the 

 exception of the brain, is situated on the ventral side of the 



