396 



FISHES 



CHAP. XIV 



depressions, but, as a rule, when one of them is present the 

 other is absent. It is probable that both these structures 

 were associated with sensory organs, of which one may have 



Fig. 228. — Vertical section through the parietal eye and the pineal vesicle of Petro- 

 myzon mariniKS. c.t. Connective tissue ; p, pellucida ; p.o, pineal organ ; pt,Oj 

 parietal eye ; /•, retina ; iii i>, third ventricle. (From Wiedersheim, after Stud- 

 nicka. ) 



been a parietal eye and the other a pineal eye. Some Teleosts 

 (e.g. many deep-sea Scopelidae) have a transparent, convex, 

 cornea-like prominence on the upper surface of the head which 

 may be related to one of these singular organs.^ 



^ Chun, loc. cit. p. 536, 



