NERVOUS SYSTEM 181 



H. The Nervous System. 



The nervous system of Ainphioxixs consists of: — (1) the 

 central portion, which, as in other vertebrates, is a tube of 

 nervous matter lying in a sheath of connective tissue imme- 

 diately above the notochord, and extending almost the entire 

 length of the body ; (2) the peripheral portion, or nerves proper, 

 which connect the central portion with the various parts of 

 the body. 



The nervous system can be well seen in transverse sections 

 of adults, or by examination of young specimens mounted 

 whole. If fresh specimens can be obtained, the entire nervous 

 system can be isolated by placing them in 20 per cent, nitric 

 acid for three days, then washing thoroughly and leaving in 

 water 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 should then be transferred to alcohol, stained, and 

 mounted in the usual manner. 



i. 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 at the posterior end, which 

 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 which a small 

 diverticulum arises which communicates by 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 opens directly 

 to the exterior. 



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, 



