HISTOLOGICAL BASIS OF ADAPTIVE COLORS IN PARALICHTHYS ALBIGUTTUS. 21 



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

 both melanophores and xanthophores showed, approximately, the same degree of 

 expansion as the corresponding chromatophores in the skin of specimens adapted to a 

 moderately dark-gray background. Relatively few guanophores occurred in these 

 layers. 



Figure 6, plate ii, is a camera lucida drawing of chromatophores and guanophores 

 in the layer just beneath the epidermis in the skin of a specimen 19 cm. in length which 

 had been kept on a green background continuously for a period of 28 days. The ver- 

 tical distribution of chromatophores and guanophores in the skin of specimens adapted to 

 a green background does not differ essentially from the vertical distribution of these 

 elements in the skin of any moderately dark-colored specimen. 



As a fish becomes adapted to a green background the melanophores in the super- 

 ficial layer of the skin become expanded to approximately the same degree as in one 

 adapted to a moderately dark-gray background. The xanthophores become expanded 

 to a somewhat greater degree than the corresponding bodies in the skin of a speci- 

 men adapted to a gray backgroimd. That the guanophores in the superficial 

 layer may assume somewhat different spacial relationships with the chromatophores 

 is quite probable. However, no conclusive evidence on this point could be obtained. 

 Greenish tones can not be observed microscopically except in the guanophores which 

 lie in close proximity with xanthine pigment and in the almost pigment-free peripheral 

 portions of the radial processes of some of the xanthophores. However, these greenish 

 tones may be observed quite as well in the skin of specimens which have not been on 

 a green background. 



A microscopic study of the color-producing elements in the skin of these specimens 

 affords no very definite data upon which to base an explanation of the cause of the 

 greenish color which they assume. The most definite finding is the ratio of the surface 

 area occupied by xanthine pigment to that occupied by melanin pigment. Doubtless, 

 the greenish color is due to a mingling of the colors of xanthine and melanin pigment 

 plus the optical effects due to the diffraction of light by the guanin crystals in the 

 guanophores. The dark shade is due primarily to the degree of expansion of the 

 melanophores. 



FISH ON RED BACKGROUND. 



Some of the specimens used in this study were kept for an interval of 2 1 days, others 

 for a shorter interval, in a wooden aquarium, the bottom and sides of which were painted 

 dark-carmine red. When thoroughly adapted to this background these specimens were 

 reddish brown in color and of a shade approximately as dark as that of specimens 

 adapted to a moderately dark-gray background. 



The melanophores in the layer just beneath the epidermis in the skin of these 

 specimens were all well expanded. In the darker areas the majority of them were nearly 

 or quite maximally expanded ; in the lighter areas they were expanded only to a moder- 

 ate degree. In general, the melanophores in the superficial layer of the skin were 

 expanded to a somewhat greater degree than in the majority of the specimens newly 

 taken. 



Among the xanthophores in the layer just beneath the epidermis, those which con- 

 tain orange-colored pigment were well expanded, while those which contain yellow 

 pigment were strongly contracted. In many areas the orange-colored xanthophores 



