NERVOUS SYSTEM. 191 



acid for three days, then washing thoroughly and leaving in 

 ivater for a day. By this treatment the connective tissue is 

 so softened and loosened that on shaking the bottle the nervous 

 system is readily and completely separated from all the other 

 parts. It may then be transferred to alcohol, stained, and 

 mounted in the usual manner. 



1. The central nervous system or spinal cord is of nearly 



uniform diameter along the greater part of its length : 

 it diminishes slightly in size towards the anterior 

 end, and much more markedly near the posterior 

 end, where it is very slender. 



The central canal lies much nearer the ventral 

 than the dorsal surface : it is very small except at 

 the extreme front end, where it expands to form the 

 ventricle. From the anterior end of this a small 

 diverticulum arises which in early life communicates 

 hy a pore with a ciliated pit on the dorsal surface 

 and left side of the anterior end of the body. 

 Through this pit the central canal of the nervous 

 system at first opens directly to the exterior, but the 

 pore closes during development. 



At intervals along the spinal cord are spots of 

 black pigment, contained in cells in the floor of the 

 central canal. 



2. The peripheral nervous system consists of nerves of two 



kinds : (1) nerves which arise by single roots, and, 

 except in the case of the first pair, from the dorsal 

 surface of the spinal cord ; (2) nerves arising by 

 multiple roots, and from the ventral surface of the 

 cord. 



a. Nerves arising by single roots. These apparently 

 correspond to the dorsal or sensory roots of the 

 spinal nerves of other vertebrates. Only the 

 first two pairs h^ive ganglionic enlargements. The 

 first and second are purely sensory nerves, and 

 the remainder are chiefly sensory. 



