1 68 FISHES CHAP. 



ance varies in different Fishes, and in different parts of the 

 surface of the same Fish. Where chromatophores are most 

 abundant, usually on the back, the reflecting tissue is relatively 

 scanty, and vice versd. On the sides and belly of a Fish the 

 place of the inner layer of the dorsal surface may be taken by 

 the " argenteum." This layer is devoid of chromatophores, and 

 consists of reflecting tissue in which the iridocytes form a con- 

 tinuous stratum, either in the form of granules, or as a close 

 network of rod-like bodies or of polygonal plates in contact with 

 one another, instead of being less numerous and more scattered 

 as on the back. When iridescence is produced, it is due to 

 the iridocytes of the outer layer of the skin ; the dead whiteness 

 and silvery lustre, on the other hand, have their origin in the 

 different ways in which incident light is reflected from the inner 

 layer or argenteum. 



To the relative abundance of chromatophores, the kind of 

 pigment they contain, and the manner in which they are dis- 

 tributed and blended, combined with the different reflecting 

 properties, or the iridescence, of the iridocytes, are due the extra- 

 ordinary wealth and variety of colour in Fishes. 



The part played by the different coloration elements in the 

 production of the characteristic colours of different Fishes may 

 be illustrated by two examples. 1 



In the common Whiting (Gadus merlangus) the back of the 

 Fish is a dark bluish-grey ; the sides have a beautiful iridescence 

 and silvery glitter, while the belly is very nearly a dead white. 

 Briefly, these appearances are due to the fact that chromato- 

 phores (black and deep yellow) are most abundant on the back, 

 less numerous on the sides, and wanting altogether on the belly ; 

 while the iridescence and silvery appearance of the sides are due 

 to the iridescence of the iridocytes external to the scales, com- 

 bined with the non-iridescent but highly reflective property of a 

 layer of iridocytes internal to the scales ; and the dead white of 

 the belly to the different reflecting power of the argenteum, and 

 the absence of chromatophores in that region. 



In the Mackerel (Scomber scombrus) the distribution of colora- 

 tion elements is different, inasmuch as they are mainly situated 

 in the deeper part of the skin, internal to the deciduous scales. 

 The back is marked by the well-known alternating wavy bands 

 1 Cunningham and MacMunn, op. cit. pp. 768 and 771. 



