44 
COLOUR IN NATURE 
CHAP. 
In Crustacea there is probably only one series of 
blue colouring-matters, the so-called soluble blues, 
which will be more fully described in the chapter on 
the group. 
Among Mollusca we have notably the blue pig¬ 
ment of Ianthina and the purple one of Aplysia , the 
undescribed colouring matters of the shells of mussels, 
of the mantle of Tridacna , and of others. 
Among Vertebrates blue pigments do not appear 
to have been as yet described, except in the case of 
the blue pigments of the eggs of many birds. These 
pigments are apparently derivatives of haemoglobin. 
A green colour is of course almost universally 
distributed among plants, where it is due to the 
important pigment chlorophyll. Among animals 
green like blue is rare as a pigmental colour, except 
in simple organisms. The questions connected with 
the green colours of animals have been greatly com¬ 
plicated by the common habit of hastily assuming 
that any pure green colour in animals is likely to be 
due to chlorophyll. Chlorophyll occurs apparently 
in many Protozoa, in Hydra viridis , in the fresh-water 
sponge Spongilla , and in some Turbellarian worms. 
Its occurrence in numerous other animals, especially 
worms and insects, has been often asserted, probably 
in some cases because the gut contained unaltered 
chlorophyll which gave the characteristic spectrum. 
“ Modified chlorophyll ” is said by Mr. Poulton to 
colour the tissues of many caterpillars. A group of 
pigments which resemble chlorophyll, and which 
have been often mistaken for it, are apparently 
widely spread among Invertebrates. They usually 
occur in connection with internal organs, and are 
