HISTOLOGICAL BASIS OP ADAPTIVE COLORS IN PARALICHTHYS ALBIGUTTUS. 23 



It results in a color which approximates very closely the color of specimens which are 

 thoroughly adapted to the same background. None of the specimens used in this 

 study ever showed any color which approximated the red color of the background 

 more closely than does the orange pigment. 



FISH ON BLUE BACKGROUND. 



The specimens used in this study were kept, for intervals varying from 5 to 22 

 days, in a wooden aquarium the bottom and sides of which were painted dark indigo- 

 blue. Adaptation to this background was, perhaps, less perfect than it would have 

 been to a blue background of a lighter shade. Those specimens which simulated the 

 color of the background most closely were dark gray with a greenish-blue tone. 



In these specimens many of the melanophores in the layer just beneath the epidermis 

 were nearly or quite maximally expanded; others were expanded to a somewhat lesser 

 degree. In general, the melanophores in this layer were expanded to a slightly lesser 

 degree than in the skin of specimens adapted to a black background. 



The xanthophores in the layer just beneath the epidermis were strongly contracted. 

 In many areas the majority of them were contracted to a maximum degree. In others 

 they were almost maximally contracted. In general the xanthophores lay somewhat 

 deeper than when they are contracted to a lesser degree and were largely obscured by 

 overlying guanophores. 



The guanophores in the superficial layer were more definitely arranged with refer- 

 ence to the melanophores and were apparently more intimately associated with these 

 bodies than in any of the conditions described above. Not infrequently guanophores 

 were closely aggregated around melanophores and extended well beneath the peripheral 

 portions of the latter. Because so many of the guanophores were associated with 

 melanophores in this manner there were relatively large areas between the melano- 

 phores not occupied by guanophores. Wherever guanophores were in close proximity 

 with xanthine pigment they assumed a greenish-yellow tinge. However, relatively 

 few guanophores were affected in this manner by reason of the relatively great degree 

 of contraction of the xanthophores. On the other hand, many of the guanophores 

 closely associated with melanophores and not in close proximity with xanthine pigment 

 showed a greenish-blue metallic tone. The blue background had no apparent direct 

 effect on the shade or the color of the guanophores. 



Superficial to the proximal areas of the scales and in the deeper layers of the skin 

 the melanophores were nearly or quite maximally expanded, while the xanthophores 

 were strongly contracted. Many of the guanophores which were closely associated 

 with melanophores showed the same greenish-blue metallic tone as was apparent in 

 the superficial layer of the skin. 



Figure 8, plate 11, is a camera lucida drawing of chromatophores and guanophores 

 as they appeared in the layer just beneath the epidermis in the skin of a specimen, 20 

 cm. in length, which had been kept on a dark-blue background for a period of 22 days. 

 The vertical distribution of chromatophores and guanophores in the skin of specimens 

 adapted to a dark-blue background does not differ essentially from the vertical distribu- 

 tion of these elements in the skin of any dark-colored specimen. 



Adaptation to a dark-blue background involves almost maximum expansion of 

 the melanophores and almost maximum contraction of the xanthophores in the super- 



