1893.] 



On the Coloration of the Skins of Fishes, 



385 



tion sous l'lnfhience des Nerfs " (' Journ. Anat. et Phys.,' 1876). He 

 there gives the separate reflecting cells the convenient name " irido- 

 cytes," and refers to the silvery layer of the skin under the name 

 " argenfcure." The anatomical analysis of the structural coloration 

 elements having thus not previously been adequately carried out, we 

 have described these elements as they are found in Pleuronectidee and 

 various other Fishes. In the former family there are two kinds of 

 chromatophores, the black and the coloured, the latter usually of 

 some shade of yellow or orange. The coloured elements in the skin 

 on the upper side are chiefly developed in the more superficial layer 

 immediately beneath the epidermis and for the most part outside the 

 scales, and on the inner side of the skin in the subcutaneous tissue, 

 the rest of the skin being almost destitute of these elements. In the 

 superficial layer the iridocytes are somewhat polygonal plates of 

 irregular shape, distributed uniformly, and separated by small inter- 

 spaces. The chromatophores are much larger, and farther apart, and 

 are superficial to the iridocytes, although sections show that their 

 processes often pass down between adjacent iridocytes. The coloured 

 chromatophores have less definite outlines than the black, and as a 

 rule radiating processes are but indistinctly indicated in them. The 

 external part of the coloured chromatophore consists of diffused 

 yellow pigment, while in the centre the concentration of the pigment 

 produces a deeper colour, varying from orange to red, as in the Plaice 

 and Flounder. On the upper side of the Fish the subcutaneous color- 

 ation elements are quite similar, but not so uniformly distributed ; 

 the iridocytes are larger, and the chromatophores not so symmetrical 

 in shape. 



The lower side of the normal Flounder is uniformly opaque white, 

 like chalk. Here in the more superficial part of the skin there is a 

 uniform layer of iridocytes like those of the upper side, opaque and 

 reflecting, but not very silvery or iridescent. Chromatophores are 

 entirely absent. In the subcutaneous layer there is a continuous 

 deposit of reflecting tissue, to which the whiteness of the skin is due, 

 the superficial iridocytes not being sufficiently thick to make the skin 

 so opaque. We have shown by comparing the adults of different 

 species, and stages of growth of the same species (Flounder), that 

 the subcutaneous deposit is merely a further development of a layer 

 of separate iridocytes which enlarge until they become continuous. 

 In some cases, e.g., Arnoglossus, the subcutaneous layer of the lower 

 side remains permanently as a layer of separate plates or iridocytes. 

 The continuous deposit in all respects corresponds to the subcutaneous 

 tissue to which is due the silvery glitter of the Mackerel or Herring, 

 and we have called it the argenteum. 



We have shown by descriptions of the coloration elements in a 

 number of species of symmetrical Fishes such as Mackerel, Whiting, 



VOL. LIII. 2 E 



