CHAPTER XI 

 THE EYES OF FISHES 



No book on the comparative anatomy and physiology of the eye would be 

 complete withovit a tribute to victor julius franz (1883-1950) (Fig. 286). 

 The son of a famous astronomer in Konigsberg, he worked successively in Ziirich, 

 Breslau, Halle, Frankfurt, Leipzig and Jena where he occupied the Chair of 

 Phylogeny at the Ernst -Haeckel-Havis until after the World War when, in 1946, 

 he was relieved of his post owing to his political associations with the Nazi 

 party. From the time he gained his doctorate thesis on the anatomy, histology 

 and function of the eyes of Selachians in 1905, his scientifle output was con- 

 tinuous until 1944, and included such subjects as the anatomy of the eyes of 

 Invertebrates and Vertebrates, particularly Acrania and Fishes, the anatomy 

 and function of the brain of Fishes, the structure and function of jDigment cells, 

 investigations into ocvilar functions such as phototaxis, accommodation and the 

 light sense of a vast number of species, and a wide range of other kindred subjects. 

 His systematic writings were also prolific, on comparative anatomy, evolutionary 

 processes and, above all, on the structure and function of the organs of sight 

 in the animal world. 



Compared with Cyclostomes, true fishes show many and con- 

 siderable advances not only in their general structure as in the presence, 

 among other things, of jaws, limbs (fins) and an exo-skeleton of scales 

 from which teeth are derived, but also in their eyes which are more 

 fully differentiated. 



The general co7ifignration of the eyes of Fishes exhibits structural 

 characteristics which might at first sight appear to be peculiarities but 

 most of them depend on the requirements of vision in water : it is to 

 be remembered that the vertebrate eye initially evolved as an under- 

 water visual organ (Figs. 287 to 291). 



In general, the globe is large, its size tending to vary with the 

 depth at which the animal lives ; as a rule deep-sea fishes are provided 

 with large eyes to receive as much light as possible in these dim 

 regions — until, indeed, the absence of light in benthonic depths leads to 

 the degeneration of the entire organ. ^ When a change of habitat 

 occurs during development the size of the globe may vary accordingly ; 

 thus the sunfish, Banzariia truncata. spends its larval life at great 

 depths at which stage the eyes occupy one-quarter of the area of the 

 body, but when the adults come to spend their lives near the surface 

 their eyes become relatively quite small in maturity. A corresponding 

 change occurs in the eye of the eel, Ang^iilla. which grows to a relatively 

 enormous size before it migrates from its river habitat to breed and 

 die in the Atlantic ocean. 



1 p. 722. 



S.O.— VOL. I. 273 18 



