96 



THE EYE IN EVOLUTION 



Anguilla 



1935 ; Hogben, 1936). In teleostean fishes, however, which show 

 more elaborate colour changes than any other species, a dual mechanism 

 emerges for a more efficient nervous control has developed, depending 

 on the excitability of specialized retinal areas above and below the level 

 of the optic nerve (Hogben and Landgrebe, 1940) (Fig. 69). In some 

 types, it is true, the hormonal influence remains preponderant ; this is 

 seen in the eel, Anguilla (Neill, 1940 ; Waring, 1940), or in the minnow. 



Fig. 70. — Colour Changes in the Female Clppy, Ljmisr^s kejiculatus 



On the left the noi'mal aninial ; on the right a fish after 25 /Lig. per ml. of 

 LSD (D-lysergic diethvlamide) had been added to the water of the aquarium 

 (Sancloz J. Med. Sci., 1956). 



Figs. 71 and 72. — The Melanophores of the Guppy, Lubistux itmicnLATL 

 {SandozJ. Med. Sci., 1956). 





J^^"^^*- 



.*? 



*'>".<*f«.?j 





Fig. 71. — After adaptation to a light 

 environment. 



Fig. 72.— After LSD (25 /xg. per ml.) 

 had been added to the water of the 

 aquarium. 



Mustelus 



Phoxinus Icevis, in which even section of the sjDuial cord fails to alter 

 the response to the background (Healey, 1951-54 ; Gray, 1956). In 

 most Teleosteans, however, colour changes are dominated by a nervous 

 control which persists in hypophysectomized animals. That the 

 changes in the chromatophores were determined by chemical mediators 

 liberated at the nerve terminals has been shown in a striking series of 

 experiments by Parker (1940-55), who studied the effects of sectioning 

 the radial nerves of the tail-fin. The chromatophores of Selachians 

 possess a single innervation mediated by an adrenalin-like substance, 

 selachine, which is pigment -concentrating (the dogfishes, Mustelus and 



