EEPEODUCTIVE ORGANS 181 



nearly equal to the interval between two successive 

 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 being unpaired. 



1. The olfactory organ is a pit-like depression of the skin, 



opposite the anterior end of the central nervous 

 system, and on the left side. It is lined by columnar 

 ciliated cells, and is in connection with the apex of 

 the olfactory lobe. 



2. The ' eye ' is a circular 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 ccelom into 

 the atrial folds : these cavities are readily recognis- 

 able while the ovaries are young, but become almost 

 obliterated when the ova ripen. 



The ova, when mature, are discharged into the 

 atrial cavity by dehiscence of the proper wall of the 

 ovary and of the atrial membrane. The discharged 

 ova, together with the projecting ovaries, form a 

 bulky mass, which causes great distension of the atrial 

 cavity and distortion of the pharynx and other organs. 



