180 AMPHIOXUS 



in the fact that some of their branches are dis- 

 tributed to muscles. 



i. The first pair of nerves arise from the ventral 

 surface of the anterior end of the central 

 nervous system, and supply the anterior fin. 



ii. The second pair of nerves are much larger. 

 They arise from the dorsal surface of the 

 central nervous system immediately behind 

 the first pair. Each divides into three main 

 branches, supplying the anterior fin. 



iii. The third pair of nerves are much smaller than 

 the second, and arise immediately behind 

 them. They supply the parts about the 

 mouth. 



iv. The fourth pair of nerves arise from the dorsal 

 surface of the spinal cord, some distance 

 behind the third pair ; the nerve of the right 

 side being a short distance behind that of the 

 left side. 



v. The fifth pair of nerves are still more oblique, 

 the right nerve arising some distance behind 

 the left. The succeeding nerves, along the 

 whole length of the cord, arise alternately on 

 the two sides. These nerves run outwards 

 along the connective-tissue septa separating 

 the myotomes, and divide into branches sup- 

 plying the skin and muscles. 



b. Nerves arising by multiple roots. These apparently 

 correspond to the ventral or motor roots of the 

 spinal nerves of other vertebrates. 



They arise from the ventral surface of the 

 spinal cord along nearly its whole length, alter- 

 nating with the dorsal nerves ; a ventral nerve on 

 one side being opposite a dorsal nerve on the other. 

 Each arises by a large number, thirty or more, 

 of very slender rootlets, placed one in front of 

 another, the entire length, of a ventral root being 



