MEDIAN EYES IN EXTINCT VERTEBRATES 



329 



may be conceived that the eyes of the prototype animal, from which by 

 gradual differentiation the median and lateral eyes of both invertebrate 

 and vertebrate animals have been evolved, resembled the simple paired 



B 



Fig. 229. — Dorsal Aspect of the Parietal Plates of : A — Pholidosteus ; 

 B — Rhinosteus, showing Paired Pineal Pits. (After Erik-a-son Stensio.) 



In Rhinosteus the left pit is completely preserved as an impression, whereas 

 the impression of the right pit has been broken off. ( X 5 diameters.) 



eyes of the type found in living Planaria and Turbellaria or in the Annelid 

 Hcemopis sanguisuga (Fig. 16, Chap. 3, p. 21). In these animals there is a 

 bilaterally disposed nervous system ; the eyes lie on the dorsal aspect of 



