HISTOLOGICAL BASIS OF ADAPTIVE COLORS IN PARALICHTHYS ALBIGUTTUS. 1 5 



result of a blending of the colors of xanthine and melanin pigment plus the optical effect 

 of the guanophores and other structural elements. In specimens which show only a 

 very slight or no brownish tone the xanthophores are strongly contracted and so thor- 

 oughly obscured by the guanophores that the xanthine pigment has little or no effect. 



FISH ON BLACK BACKGROUND. 



The specimens used in this study were kept in a wooden aquarium, the bottom 

 and sides of which were painted black. Some of them remained in this aquarium for 

 21 days or longer, others for shorter intervals. In those in which simulation of the 

 background was most nearly perfect the skin was very dark gray with a very slight 

 brownish tone, and the whole surface of the body was more uniform in shade than it 

 was in the specimens newly taken. 



In the layer just beneath the epidermis in these specimens the majority of the 

 melanophores in the darker areas were maximally expanded. These melanophores 

 showed a relatively large unpigmented area at the center, and pigment was present in 

 the radial processes well toward their peripheral extremities. In the lighter areas the 

 melanophores were expanded to a somewhat lesser degree. Many of them showed a 

 relatively small unpigmented area at the center, while others showed no unpigmented 

 central area but were still well expanded. In many areas the radial processes of adjacent 

 melanophores interdigitated with each other. Where this condition obtained, a rela- 

 tively large portion of the surface area was occupied by melanin pigment. Further- 

 more, the melanophores lay immediately beneath the epidermis and were not obscured 

 by other elements. 



The majority of the xanthophores in the layer just beneath the epidermis were 

 strongly contracted. The portion of the surface area occupied by xanthine pigment 

 was relatively small in proportion to that occupied by melanin pigment. Furthermore, 

 the xanthophores lay appreciably deeper than the melanophores and many of them 

 were wholly or in part overlain by guanophores. Occasionally a specimen which had 

 become adapted to a black background was observed which showed scarcely a trace of 

 brownish color. In these specimens the xanthophores in the superficial layer of the 

 skin were contracted to a somewhat greater degree than usual and were more effectively 

 obscured by overlying guanophores. 



Guanophores were present in the superficial layers of the skin in approximately 

 the same abundance as in specimens newly taken. In some instances they lay beneath 

 melanophores. However, the majority of them lay in the areas not occupied by melano- 

 phores. 



The chromatophores lying superficial to the proximal areas of the scales and in 

 the deeper layers of the skin showed approximately the same degree of expansion as 

 the corresponding chromatophores in the skin of the darker specimens newly taken. 



The essential difference in the color-producing elements in the skin of specimens 

 adapted to a black background and specimens newly taken from their natural environ- 

 ment consists in a markedly greater degree of expansion of the melanophores and a 

 somewhat greater degree of contraction of the xanthophores in the layer just beneath 

 the epidermis in the former than in the latter. The chromatic organs in the deeper 

 layers of the skin show approximately identical conditions in both cases. The time 

 required for a specimen taken from its natural environment to become maximally black 



