Metazoa A mphioxus. 259 



sists of a hollow rod of nerve-fibres and cells surrounded by 

 connective tissue. 



The spinal cord, as the rod may be termed, is not so 

 long as the notochord, ending a short distance behind its 

 anterior termination. When we come to examine the frog 

 in the next section we shall find that this relation of the 

 spinal cord to the notochord is of great importance. From 

 the spinal cord are given off many nerves in two series ; 

 a dorsal series consisting of nerves which spring by only one 

 root from the cord, and a ventral series, each of which has 

 many roots. The nerves of the dorsal series alternate with 

 those of the ventral series. The fibres are distributed to 

 various organs and other parts of the body, and apparently 

 differ in function, the dorsal nerves being concerned in 

 carrying nervous impressions to the spinal cord from the 

 exterior (sensory nerves), the ventral nerves corresponding 

 to what are known as the motor and secretory nerves of 

 higher animals i.e. whose duty it is to convey nerve im- 

 pulses to the muscles, glands, and other organs of the 

 body (p. 318). 



The sense-organs are very rudimentary. The sense of 

 touch is, of course, fully developed, and the sensations are 

 conveyed to the central nervous system by means of the 

 sensory spinal nerves just referred to. A minute sac-like 

 hollow in the skin, lined by ciliated epithelium, situated 

 close to the anterior end of the spinal cord, is believed to 

 represent the olfactory organ, whilst a pigment spot placed 

 on the anterior extremity of the cord itself may do duty for 

 an eye. 



The details of the nervous system in the Chordata, and 

 how it performs its functions, will be more fully dealt with in 

 the two succeeding sections. 



The reproductive system. Lastly we have to discuss 

 the reproductive organs, which, like most of the organs in 

 AmphioxitS) are very simple. The animals are male and 

 female. In both sexes the reproductive glands are situated 



S 2 



