HISTOLOGICAL BASIS OF ADAPTIVE COLORS IN PARALICHTHYS ALBIGUTTUS. 25 



SUMMARY. 



1. Chromatophores in the skin of Paralichthys alhiguttus are of two types, viz, 

 melanophores and xanthophores. The former contain melanin granules which are 

 dark brown or black, the latter contain xanthine granules which vary in color from 

 yellow to orange. Changes in shade or color are due primarily to changes in the dis- 

 tribution of the pigment in the chromatophores and changes in the relationships of the 

 guanophores with these bodies. 



2. Many melanophores are eliminated from the skin through the epidermis. Under 

 certain conditions the number of melanophores in the layer just beneath the epidermis 

 becomes materially reduced. 



3. Evidence is advanced in support of the theory that changes in the distribution 

 of the pigment in the chromatophores are brought about by movements of the pig- 

 ment granules toward the periphery and in turn back toward the central area while the 

 form of the chromatophores remains essentially unchanged. 



4. The evidence available indicates that the chromatophores are under the direct 

 control of the sympathetic nervous system. 



5. Shade is due primarily to the degree of expansion of the melanophores in the 

 superficial layers of the skin. Very light shades maintained for a prolonged period 

 involve a reduction in the number of melanophores in the superficial layer. 



6. The initial response to a yellow background involves a moderate degree of con- 

 traction of the melanophores and a marked degree of expansion of the xanthophores 

 in the superficial layers of the skin. Thorough adaptation to a yellow background 

 involves a moderate reduction in the number of melanophores in these layers. 



7. Adaptation to a green background involves approximately the same degree of 

 expansion of the melanophores and a somewhat greater degree of expansion of the 

 xanthophores in the superficial layers of the skin than adaptation to a moderately dark- 

 gray background. The resultant yellowish-green tone probably depends upon the 

 ratio of the distribution of the xanthine to the distribution of the melanin pigment 

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

 guanophores. 



8. Adaptation to a dark-red background involves almost maximum expansion of 

 the melanophores and the xanthophores containing orange-colored pigment and a 

 marked degree of contraction of the xanthophores containing yellow pigment in the 

 superficial layers of the skin. The resulting reddish-brown color is due largely to the 

 wide distribution of orange pigment, the effect of which is probably modified to some 

 extent by the blending of orange and black and the optical effects produced by the 

 guanophores. The dark shade is due to the wide distribution of melanin pigment. 



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

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

 ficial layers of the skin. Many of the guanophores also become arranged with reference 

 to the melanophores and closely associated with them. The xanthine pigment prob- 

 ably has little effect in the resultant color. The dark shade is due primarily to the 

 wide distribution of the melanin pigment. Doubtless, the greenish-blue tone depends 

 largely upon the optical effects produced by those guanophores which are closely asso- 

 ciated with melanophores. 



