192 
COLOUR IN NATURE 
CHAP. 
digestive gland and its secretion, intermingled with 
the contents of the gut, and finally in unaltered form 
in the faeces. It thus acts like a true bile pigment. 
We have already frequently spoken of the peculiar 
group of pigments to which enterochlorophyll belongs, 
and of the facility with which they can be made to 
yield bright-coloured derivatives. It is evidently 
here a useless substance, and it is very probable that 
in some cases, instead of being eliminated unaltered 
in the faeces, it may undergo modification into other 
pigments which may be deposited in the mantle or 
shell. We have already noticed such a suggestion 
in the case of Acmcea, and it is possible that many 
of the bright pigments of Mollusca arise in this 
way. 
The yellow, orange, or black pigments of naked 
forms such as the slugs, Doris , etc., are probably 
due either to lipochromes or the dark - coloured 
“ melaines.” 
It seems probable that the pigments of Cephalo¬ 
poda are chiefly of the dark-coloured nitrogenous 
type, though they have not been fully investigated. 
The beautiful changing colours are in large part due 
to the movements of the chromatophores, which as 
they expand or contract alter the whole appearance 
of the animal. So long ago as 1852 Briicke watched 
the colour-changes of Sepiola rondeletii , and noting 
how the tints varied in the order of the spectrum 
from blue to green, yellow, and red, came to the 
conclusion that the colours were optical. He thought 
that they were due to the colours of thin plates, but 
according to Krukenberg they are due to fine ridges 
in the surface of the cells. 
