SENSORY AND REPRODUCTIVE ORGANS. 201 



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 breadth of a ventral root 

 being nearly equal to the interval between two 

 successive roots. The several rootlets do not 

 unite, but pass separately through the connec- 

 tive tissue sheath of the cord, and end in the 

 myotomes. 

 L. The Sense Organs. 



The sense organs of Amphioxus are extremely simple in 

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

 brates in being unpaired. 



1. The pit-like depression of the skin at the front end 



which in the young places the central canal of the 

 nervous system in communication with the exterior 

 has been described as an olfactory organ. In the 

 adult it is connected by a solid stalk with the roof 

 of the ventricle. 



2. Perception of light is probably enhanced by the black 



pigment cells scattered along the spinal cord, and 

 by an especially large pigment-spot in the anterior 

 wall of the ventricle. 

 If'. 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 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 

 He in cavities, which are extensions of the coelom 

 into the atrial folds : these cavities are easily recog- 

 nisable while the ovaries are young, but become 

 almost obliterated when the ova ripen. 



