168 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 lght 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.’ 
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 
? Cunningham and MacMunn, op. cit. pp. 768 and 771. 
