22 BULLETIN OF THE BUREAU OP FISHERIES. 



were nearly or quite maximally expanded; in others they were expanded only to a 

 moderate degree. The majority of the yellow xanthophores were contracted to such a 

 degree that the major part of the pigment was concentrated in the central area. Not 

 infrequently xanthophores of this type were observed in which all the pigment was 

 concentrated in the central area. Many of the xanthophores which were most strongly 

 contracted were largely obscured by associated guanophores. Those which were 

 expanded to a greater or lesser degree were not largely obscured by other elements 

 except where they were in part overlain by melanophores. The ratio of the surface 

 area occupied by xanthine pigment to that occupied by melanin pigment was greater 

 than in specimens adapted to other dark-colored backgrounds, but appreciably less 

 than in those adapted to a yellow background. 



In general, the guanophores in the superficial layer were more intimately associated 

 with the xanthophores than with the melanophores, and appeared to be arranged 

 with respect to the former rather than the latter; consequently, they occupied a rela- 

 tively large part of the surface area not occupied by melanophores. Inasmuch as a 

 relatively large percentage of the guanophores lay in proximity with xanthophores, 

 the greenish-yellow tone of the former, due to their proximity ^\•ith xanthine pigment, 

 was widespread and very apparent. The red background had no direct effect either 

 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 and orange-colored xanthophores were well expanded, while the 

 yellow xanthophores were expanded only to a moderate degree. The peripheral pig- 

 ment-free portions of many of the xanthophores, as well as the guanophores l^ing in 

 proximity with xanthine pigment, assumed the same greenish-yellow tone as the guano- 

 phores l}'ing in proximity with xanthine pigment in the superficial layer. 



Figure 7, 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-red background for a period of 21 days. 

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

 adapted to a dark-red background does not differ essentially from the vertical distri- 

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



Adaptation to a dark-red background involves a response on the part of the 

 melanophores similar to the response of these organs to a moderately dark-gray back- 

 ground. The xanthophores whose color approximates the color of the background 

 most closely, i. e., those containing orange-colored pigment, become greatly expanded. 

 Consequently, the pigment which simulates the color of the background most closely 

 becomes almost maximally effective. Furthermore, a relatively large percentage of 

 the guanophores lie in proximity wdth xanthophores. Doubtless, the resulting reddish- 

 brown color is due to a mingUng of the colors of the xanthine and the melanin pig- 

 ments plus the optical effects due primarily to those guanophores which lie in prox- 

 imity with xanthophores. The dark shade is due to the degree of expansion of the 

 melanophores. 



Specimens newly taken which were placed in a red aquarium responded very 

 promptly to the red background. The initial response involves a marked degree of 

 expansion of the melanophores and orange-colored xanthophores and a moderate 

 degree of contraction of the yellow xanthophores in the superficial layers of the skin. 



