NERVOUS AND EEPRODUCTWE SYSTEMS 183 



b. Nerves arising by multiple roots. These apparently 

 correspond to the ventral or motor ro'ots 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 

 nearly equal to the interval between two suc- 

 cessive roots. The several rootlets do not unite, 

 but pass separately through the connective-tissue 

 sheath of the cord, and end in the myotomes. 



I. The Sense Organs. 



The sense organs of Amphioxus are extremely simple in 

 structure, and differ markedly from those of the higher verte- 

 brates in beiag unpaired. 



1. The pit-like depression of the skin that places the central 



canal of the nervoas system in communication with 

 the exterior has been described as an olfactory 

 organ. 



2. The ' eye ' is a rounded pigment-spot in the anterior wall 



of the ventricle, i.e. at the extreme anterior end of 

 the central nervous system. 



K. The Reproductive Organs. 



The sexes are distinct, but the males and females are 

 alike except as regards the microscopic structure 

 of their reproductive organs. There are no special 

 genital ducts. 



1. The female. 



The ovaries are a series of saccular organs of a 

 horseshoe shape, arranged in a row along the inner 

 surface of the atrial fold on each side of the pharynx, 

 and extending back as far as the atrial pore. They 

 lie in cavities, which are extensions of the coelom into 



