THE EYES OF ROUND WORMS 87 



mention the arrow worm, Sagitta hexaptera, which has two eyes placed 

 laterally on the dorsal surface of the head. Each eye is rounded in form 

 and contains three biconvex lenses ; these are separated by a tri-radiate 

 pigment zone and are surrounded by converging rod-like sensory cells. 

 The latter end in nerve fibres which are grouped together into a pair of 

 optic nerves which end in the cerebral ganglion (Fig. 55). This tri- 

 radiate arrangement of the pigment zone, and of the refractile elements 

 and sensory cells of the paired eyes of Sagitta hexaptera, is very similar 

 to that which is seen in the median triplacodal eyes of arthropods (Figs. 

 248, 249, 250), and like these it appears to have originated by the fusion 

 of 3 separate ocelli, see pp. 360, 361, 362. ! 



1 Hertwig, O., 1880, Die Chaetognathen. Gustav Fischer, Jena. 



