COLORATION OF THE SKINS OF FLAT-FISHES. 117 
placed in the interior of thin glass beads to make artificial pearls. 
The coloured chromatophores of fishes are always of some shade of 
yellow or orange, deepening to red ; in some the colour is a distinct 
red, as in the gurnards and red mullet. Green fishes occur, e. g. the 
green pipe-fish (Siphonostoma typhle) and the mackerel, but in such 
cases the colour is not due to green chromatophores. The coloured 
chromatophores in such fish are of a lemon-yellow colour by trans- 
mitted light, though it approximates to green when viewed at certain 
angles by reflected hght. The green colour exhibited by the fish is 
due to the mixture of this yellow colour with the black of the black 
chromatophores, just as a mixture of gamboge and black among artists’ 
pigments produces a green. No blue pigment either occurs in any 
fishes that I have examined, blue colours being due to the reflections 
of iridocytes, modified by black chromatophores. 
As to the histological nature of these elements, it has generally 
been held that they are modified connective-tissue cells. This may 
be true of the chromatophores, but probably is not true of the irido- 
cytes and reflecting tissue. 
From a chemical point of view the reflecting substance is composed 
of a definite organic compound in an almost pure state, and the 
opacity and reflecting properties of the reflecting tissues or elements 
are those of this compound, varying according to the state of aggre- 
gation in which it exists. This substance, whose formula is C;H,N,O, 
is connected with the uric acid series, but its chemical relations are 
not well understood. It is found in small quantities in the excretions 
of the excretory organs of certain Invertebrates, but has never been 
found associated with the kidneys of Vertebrates. It was first 
recognised as an abundant constituent of guano, derived in that 
substance from the skins of the fish devoured by the sea-birds, whose 
excrement formed the guano. The pigment of the black chroma- 
tophores is known as melanin, an organic compound which is 
extremely insoluble and indestructible, and, with slight variations, 
occurring almost universally in the animal kingdom. The pigments 
of the coloured chromatophores all belong to a well-characterised 
class of pigments known as lipochromes or fat-pigments, being of an 
oily nature, soluble in alcohol, ether, and other fat solvents, giving 
absorption bands in the spectroscope, and easily bleached under the 
action of light when removed from the living body. 
To return finally to the effect of the action of light on the lower 
side of the flounder. Analysis shows that the result is a development 
of black and yellow chromatophores exactly similar to those of the 
upper side. At the same time there is a gradual diminution in the 
amount of the reflecting substance in the argenteum, while a change 
in the superficial iridocytes has not been observed. Whence do 
