20 BULLETIN OF THE BUREAU OF FISHERIES. 



phores and guanophores in the layer just beneath the epidermis in the skin of a speci- 

 men, 17 cm. in length, which had been kept on a yellow background for a period of 26 

 days. 



All the specimens placed in a yellow aquarium became adapted to the yellow 

 background almost as rapidly as those which were placed in a white aquarium did to 

 the white background. The initial response of a dark-colored fish to a yellow back- 

 ground consists in a moderate degree of contraction of the melanophores and a marked 

 degree of expansion of the xanthophores in the superficial layers of the skin. This 

 initial response of a specimen which has not previously been adapted to a yellow back- 

 ground occurs somewhat less promptly than the initial response of a specimen newly 

 taken to a white background. It results in a distinctly grayish-yellow color of a light 

 shade. The production of a maximally yellow color requires no longer time than the 

 production of a maximally white color. The complete adaptation to a yellow back- 

 ground involves some rearrangement of the guanophores and a moderate reduction in 

 the number of melanophores in the layer just beneath the epidermis. The reduction 

 in the number of melanophores is less marked than in the skin of specimens adapted 

 to a white background. The averages of actual counts of the number of melanophores 

 in a given unit area on detached scales taken from various parts of the body of speci- 

 mens newly taken and of specimens thoroughly adapted to a yellow background, indi- 

 cate a reduction in the number of melanophores in the layer just beneath the epidermis 

 in the latter of approximately 16 per cent. No change in the number of xanthophores 

 could be determined. 



FISH ON GREEN BACKGROUND. 



The specimens used in this study were kept, for intervals varying from 5 to 28 

 days, in a wooden aquarium the bottom and sides of which were painted moderately 

 dark green. When thoroughly adapted to this background the general color of these 

 specimens was a little lighter and more uniform than the color of the majority of the 

 specimens newly taken and showed a distinct yeUowish-green tone. 



Many of the melanophores in the layer just beneath the epidermis in the skin of 

 these specimens were nearly or quite maximally expanded while others were expanded 

 only to a moderate degree. The condition of the melanophores in this layer did not 

 differ materially from the condition of the corresponding bodies in the skin of many 

 of the specimens newly taken in which the general color was most uniform. 



The xanthophores in the layer just beneath the epidermis were expanded to a 

 moderate degree. Many of them were in part overlain by guanophores, but were not 

 largely obscured by the latter. The percentage of the total surface area occupied by 

 xanthine pigment is appreciably greater in these specimens than in those adapted to 

 either a black or a white background, but materially less than in those adapted to a 

 yellow background. 



The guanophores in the superficial layer of the skin occupied, approximately, the 

 same percentage of the surface area as they did in the majority of the specimens newly 

 taken. Guanophores which lay above or in immediate proximity with xanthine pig- 

 ment assumed a greenish-yellow tinge. However, this was no more apparent in these 

 specimens than in those adapted to a yellow background. The green background had 

 no direct effect on either the shade or the color of the guanophores. 



