COLOR CHANGES IN TELEOSTS 533 



same conditions as the evanescent physiological changes, their causational 

 chain of events is not yet wholly clear. They appear to be usually under 

 the ultimate control of the eye, though when flounders are illuminated 

 from below and proceed (after many months) to acquire active chrom- 

 atophores of all sorts on their erstwhile snow-white undersides, it is some- 

 times hard to see how the eyes could have been responsible. Not all flat- 

 fish species have their eyes raised on any sort of 'turrets', so that they 

 could possibly see the substrate. It is difficult to imagine how year-long 

 streams of nerve impulses can evoke chromatophores from the meso- 

 dermal nowhere, or cause them to vanish entirely. And, the increase in 

 the number of melanophores of an illuminated, eyeless, fish is as much 

 of a mystery as is the physiological darkening of such fishes by light. 



The commonest morphological changes in teleosts occur outside of 

 laboratories. Aquarists have long fretted over the fact that some of their 

 most gorgeous prizes soon become drab in captivity. The loss of glamor 

 can often be forestalled by careful attention to the diet; for a goodly part 

 of dermal matching-of-environment is really quite automatic, due to the 

 fact that the fish acquires many of its pigments directly by eating the 

 flora and fauna of his immediate environment. To a certain extent, the 

 fish can't very well help taking in some of the very colors which surround 

 him! 



When a fish is kept for a long time on a dark or black background, 

 the actual number of melanophores increases and the total amount of 

 melanin extractible from the fish (and, perhaps, per melanophore) also 

 increases. Concomitantly the guanophores decrease — at least, the amount 

 of extractible guanin is reduced. Kept for weeks on a white ground, the 

 fish will increase its guanin coating and will decrease the number of mel- 

 anophores. Just what happens to these we do not know, though Ogneff 

 thought they were phagocytized, eaten up by wandering tissue cells. 



Blinded teleosts, and amphibians too, usually lose melanophores when 

 kept in the dark. This fact has been used to account for the absence of 

 dermal pigment in (permanent) cave-dwelling vertebrates, all of which 

 belong to these two classes. Eyed animals of course also become depig- 

 mented in darkness, but eyeless individuals do not remain pallid when 

 brought into the light. Their melanophores not only quickly expand, 

 but soon begin to increase in numbers — not, however, if the pituitary 

 gland is removed along with the eyes. Minus its pituitary, an eyed or 

 blinded fish proceeds to lose melanin. Hilton found that adult Typh- 

 logobius, which are normally eyeless and unpigmented (p. 388), would 



