182 AMPHIOXUS 



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

 surface of the central nervous system ; (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 : they differ 

 from these, however, in having no ganglia, and 

 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 be- 

 hind them. They supply the parts about the 

 mouth. 



iv. The fourth pair of nerves arise from the dorsal 

 surface of the spinal cord, a short 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 separatiag 

 the myotomes, and divide into branches sup- 

 plying the skin and muscles 



