222 COLOUR IN NATURE chap. 



some cases at least, for while the outer layer may- 

 exhibit brilliant iridescence, the inner "presents 

 either a chalk-white opaque surface or an evenly 

 bright silvery surface." 



The pigments contained in the chromatophores, 

 so far as they are at present investigated, appear 

 to be always either lipochromes or dark melanins. 

 The lipochromes are apparently very widely dis- 

 tributed and exhibit many different shades of colour. 

 They are chiefly known by their spectroscopic char- 

 acters. In some cases, as in many of the flat-fishes, 

 the yellow and black pigment cells produce a direct 

 effect upon the coloration — that is, the combination of 

 the two pigments produces a brownish colour tend- 

 ing either towards yellow or black, according as the 

 one or the other pigment is most abundant. Simi- 

 larly in the gurnards some species like Trigla lyra 

 are red, and others like T. gurnardus are gray, 

 the difference being, according to Cunningham and 

 M'Munn, merely due to the relative development 

 of black and red pigment. There are, however, 

 other cases of greater difficulty in which the colour 

 is not obviously due to the combination of two 

 pigments. Thus the beautiful green colour of the 

 mackerel is produced not by a green pigment, but 

 apparently by a blending of black and yellow 

 chromatophores. In the black bands the black 

 chromatophores are much more numerous than the 

 yellow, while in the green bands the two kinds are 

 equally abundant. The statement made above, that 

 the green is the result of the " blending " of the two 

 colours, is due to Mr. Cunningham and Dr. M'Munn, 

 but it is a little difficult to believe that there may 



