l
muscles of the mollusca--it occurs alone. Although closely related to
haemoglobin or its derivative haemochromogen, the histohaematins are yet
totally distinct, and they are found in animals where not a trace of
haemoglobin can be detected. Another interesting pigment is turacin,
which contains about 7% of nitrogen, found by Professor A. H. Church in
the feathers of the Cape lory and other plantain-eaters, from which it
can be extracted by water containing a trace of ammonia. It has been
isolated, purified and analysed by Professor Church. From it may be
obtained turacoporphyrin, which is identical with haematoporphyrin, and
gives the band in the ultra-violet which J. L. Soret and subsequently A.
Gamgee have found to be characteristic of haemoglobin and its compounds.
Turacin itself gives a peculiar two-banded spectrum, and contains about
7% of copper in its molecule. Another copper-containing pigment is
haemocyanin, which in the oxidized state gives a blue colour to the
blood of various Mollusca and Arthropoda. Like haemoglobin, it acts as
an oxygen-carrier in respiration, but it takes no part in surface
coloration.

A class of pigments widely distributed among plants and animals are the
lipochromes. As their name denotes, they are allied to fat and generally
accompany it, being soluble in fat solvents. They play an important part
in surface coloration, and may be greenish, yellow or red in colour.
They contain no nitrogen. As an example of a lipochrome which has been
isolated, crystallized and purified, we may mention carotin, which has
recently been found in green leaves. Chlorophyll, which is so often
associated with a lipochrome, has been found in some Infusoria, and in
_Hydra_ and _Spongilla_, &c. In some cases it is probably formed by the
animal; in other cases it may be due to symbiotic algae, while in the
gastric gland of many Mollusca, Crustacea and Echinodermata it is
derived from food-chlorophyll. Here it is known as entero-chlorophyll.
The black pigments which occur among both vertebrate and invertebrate
animals often have only one attribute in common, viz. blackness, for
among the discordant results of analysis one thing is certain, viz. that
the melanins from vertebrate animals are not identical with those from
invertebrate animals. The melanosis or blackening of insect blood, for
instance, is due to the oxidation of a chromogen, the pigment produced
being known as a uranidine. In some sponges a somewha