re and
the action of the peculiar pigmented cells known as "chromatophores"
(which W. Garstang defines as pigmented cells specialized for the
discharge of the chromatic function), and is much better marked when
these have for their background a "reflecting layer" such as is provided
by guanin, a substance closely related to uric acid. Such a mechanism is
seen to greatest advantage in fishes. Among these, guanin may be present
in a finely granular form, causing the light falling on it to be
scattered, thus producing a white effect; or it may be present in a
peculiar crystalline form, the crystals being known as "iridocytes"; or
in a layer of closely apposed needles forming a silvery sheet or mirror.
In the iris of some fishes the golden red colour is produced by the
light reflected from such a layer of guanin needles having to pass
through a thin layer of a reddish pigment, known as a "lipochrome."
Again, in some lepidopterous insects a white or a yellow appearance is
produced by the deposition of uric acid or a nearly allied substance on
the surface of the wings. In many animals, but especially among
invertebrates, colouring matters or pigments play an important role in
surface coloration; in some cases such coloration may be of benefit to
the animal, but in others the integument simply serves as an organ for
the excretion of waste pigmentary substances. Pigments (1) may be of
direct physiological importance; (2) they may be excretory; or (3) they
may be introduced into the body of the animal with the food.

Of the many pigments which have been described up to the present time,
very few have been subjected to elementary chemical analysis, owing to
the great difficulties attending their isolation. An extremely small
amount of pigment will give rise to a great amount of coloration, and
the pigments are generally accompanied by impurities of various kinds
which cling to them with great tenacity, so that when one has been
thoroughly cleansed very little of it remains for ultimate analysis.
Most of these substances have been detected by means of the
spectroscope, their absorption bands serving for their recognition, but
mere identity of spectrum does not necessarily mean chemical identity,
and a few chemical tests have also to be applied before a conclusion can
be drawn. The absorption bands are referred to certain definite parts of
the spectrum, such as the Fraunhofer lines, or they may be given in
wave-lengths. For this pu