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 
