192 AMPHIOXUS. 



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. 

 Their branches have ganglionic swellings near 

 their distal ends. 



ii. The second pair of nerves are much larger. 

 They arise from the dorsal surface of the 

 spinal cord behind the first pair. Each 

 divides into three main branches, supplying 

 the anterior fin. The finer branches have 

 ganglionic enlargements near their distal 

 ends. 



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 separating the 

 myotomes, and divide into branches sup- 

 plying the skin and, to a limited extent, 

 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- 



