vi NERVOUS SYSTEM 343 



It is to be noted that one division oi 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 pharyn- 

 geal 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 gan- 

 glionic enlargement two pairs of nerves can be traced 

 into the body-wall, while between these enlarge- 

 ments 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 com- 

 pare 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 body ; while 

 in the frog it is enclosed in a neural canal and is dorsal 



