VI COLORATION 169 
of black and green; the sides gleam with the most brilliant 
iridescence, changing from silver to yellow or red gold, according 
to the angle at which the Fish is viewed. The black bands of 
the back are produced by the crowding together of black chromato- 
phores and the diminished number of yellow; the green bands by 
an equal blending of yellow and black. Over the dorsal surface 
and sides of the Fish, where the coloured bands extend, there 
is also a reflecting layer external to the chromatophores, and 
to this layer is due the silvery reflection and iridescence. On 
the belly the disappearance of the chromatophores and the 
greater thickness and opacity of the argenteum account for the 
lighter colour and the diminished iridescence and silvery glitter 
of this part of the skin. 
Many Fishes are known to have the power of changing their 
colours, and in some the change is rapid. Such changes are due 
to incident light reflected from surrounding surfaces, acting 
through the visual organs and the nervous system on the 
differently coloured chromatophores. The latter are capable of 
contraction and expansion. Expansion of any particular kind of 
chromatophores is accompanied by a diffusion of their pigment— 
black, red, orange, or other colour as the case may be—and, 
according to the number and distribution of the chromatophores 
affected, the prevailing tint or tints of the whole body will be 
intensified, or only spots, bands, patches, or flushes of colour will 
be produced. Conversely, when chromatophores contract, they 
may shrink up to mere dots and bring about a diminution in 
the vividness of their respective colours, or even an alteration of 
colour, seeing that yellow chromatophores become orange when 
contracted, while orange or red appear brown or black. Colour 
changes of this kind may be artificially brought about. Experi- 
ments with Sticklebacks (Gastrosteus)', kept in glass dishes with 
a bottom of black or white tiles, have shown that the Fishes 
over the white tiles became partially bleached, while others with 
a background of black tiles retained their original coloration. 
Bleached Fishes exposed to the white tiles for a relatively short 
period (three to ten days) tend to regain their original colour 
when subsequently removed to a background of black tiles, but 
prolonged exposure to the former conditions (five to six weeks) 
seems to render the acquired light colour more or less permanent. 
1A. Agassiz, Bull. Mus. Comp. Zool., Camb. U.S.A., xxiii. 1892, p. 189. 
