COLOUR IN NATURE 
CHAP. 
190 
as a faint green line down the centre of the foot, 
produced by the coloured dorsal epithelium shining 
through. In the cells the pigment is present in 
small granules, varying in colour from light green 
to greenish-blue or even light blue. At times it 
has a black or brownish tint, the extreme edge of 
the mantle-fold being always brown. The pigment 
is soluble in water, or better in alcohol, but in solution 
the colour rapidly disappears. Alkalies destroy the 
colour at once, but it returns upon acidification ; dilute 
acids give the pigment a blue tint. The shell of 
this little limpet is a warm brown colour, and in view 
of the tendency of the green pigment to become 
brown, it is not unreasonable to suppose that during 
the process of shell-formation the green pigment 
becomes converted into the brown. 
This green pigment is of some interest; it varies, 
as already seen, from blue to green, and is perhaps 
somewhat widely distributed among Invertebrates. 
The one just described certainly resembles closely 
that found in the eggs of Eulalia viridis , and in 
Thalassema. The origin of the pigment is quite 
unknown, but it is not impossible that it arises from 
the “ enterochlorophyll ” of the digestive gland. 
As to the glandular secretions which are so 
common in the Mollusca, we find that their pigments 
exhibit very diverse characters. The ink of Cephalo- 
pods, for example, contains a black or brown pig¬ 
ment, which is nitrogenous and is said to have a 
chemical composition almost identical with that of 
the black pigment of crows’ feathers. According to 
the older analyses of Girod it is free from ash, but 
Dr. Emil Andrd considers it identical with melaine, 
