HISTOLOGICAL BASIS OF ADAPTIVE COLORS IN PARALICHTHYS ALBIGUTTUS. ^ 



In a fully contracted melanophore all the pigment is concentrated in the central area. 

 A fully expanded xanthophore (fig. 2, pi. i) may or may not show a pigment-free area 

 at the center. The central pigmented area is usually more or less irregular in outline 

 and from it relatively few irregular processes radiate. The pigment granules show no 

 regular arrangement. Xanthine granules disposed along definite radial lines are rarely 

 observed. In a fully contraced xanthophore the pigment is all concentrated in a more 

 or less irregular mass in the central area. Both melanophores and xanthophores can 

 be observed in any phase intermediate between maximal expansion and maximal con- 

 traction. 



Guanophores are cells or groups of cells which contain guanin in the form of minute 

 crystals. They occur either isolated or in groups and are frequently associated, more 

 or less intimately, with chromatophores. Guanophores are sufficiently abundant in 

 the superficial layers of the skin to occupy a relatively large part of the surface area. 

 They also possess the capacity to contract and expand to a certain degree. When a 

 fish changes from a darker to a lighter shade more or less rapidly, the guanophores 

 occupy less of the surface area and appear somewhat more compact in the latter than 

 in the former condition. The number of guanophores in the superficial layer of the 



Fig. z. — Diagrammatic drawing illustrating the distribution of chromatophores and guano- 

 phores in a longitudinal section of the skin of Paralichthys albiguttus. (i. epidermis; 2, 4, 

 layers of chromatophores and guanophores superficial to scales; 3, s, 7, scales; 6, 8, chroma- 

 tophores and guanophores in the deeper layers of the skin.) 



skin apparently varies somewhat with the shade assumed by the fish if that shade is 

 maintained for a sufficient length of time. In a fish which has for some time assumed 

 a very light shade the guanophores in the superficial layers of the skin are sufficiently 

 abundant to occupy a very large portion of the surface area. In a fish which has 

 for some time assumed a darker shade the guanophores in the superficial layers of the 

 skin are apparently less abundant. Conclusive evidence on this point could not be 

 obtained. Attempts at counting the guanophores in a given unit area are unsatis- 

 factory because individual guanophores can not always be recognized. On the other 

 hand, the degree of expansion of these bodies and their particular spacial relationships 

 with the chromatophores are important factors in determining how large a portion of 

 the surface area shall be occupied by them. Relatively few guanophores occur in the 

 deeper layers of the skin. 



The chromatophores and guanophores in the superficial layers of the skin are 

 located superficial to the scales. The corresponding layers of these bodies, therefore, 

 imbricate with each, other as do the scales (text fig. i). Both chromatophores and 

 guanophores are more closely aggregated over the distal than over the proximal area 

 of the scales. Consequently, they are most numerous over the area of each scale which 

 is intimately covered by epidermis. Furthermore, the chromatophores in these areas 



