COLORATION 1 69 



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 (Gastrosteusf , 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. 



1 A. Agassiz, Bull. Mus. Comp. Zool., Camb. U.S.A., xxiii. 1892, p. 189. 



