88 
COLOUR IN NATURE 
CHAP. 
Similarly, D. ramea also displays the same deep tint 
whether it grows in shallow water (7-8 fathoms) or 
at great depths (600 fathoms). 
In connection with the effect of light we may 
mention the prevalence of blue colours among the 
pelagic jelly-fish, which many would regard as directly 
due to the action of light. 
The Pigments of the Ccelentera 
The number, beauty, and great variability of the 
tints of the Ccelentera make the question of the 
nature of their pigments one of great interest, but, as 
is so often the case, the bright pigments are very 
unstable and their examination is a matter of great 
difficulty. In consequence, the observations which 
have been made are, in most cases, very incomplete. 
We shall not attempt to give a detailed account of 
the pigments already described, but shall merely 
describe the characters of the better known of them. 
We have already emphasised the predominance 
of a green tint in the sessile Ccelentera, and the 
frequent tendency for this tint to be in whole or in 
part replaced by another colour, such as blue, pink, 
or brown. The brightness of the tint has suggested 
to many the possibility that the pigment might be 
chlorophyll or some related colouring-matter, impart¬ 
ing to the organism the power of taking carbon from 
the air and evolving free oxygen. Among the more 
recent supporters of this view we have Prof. Hickson, 
who during his stay in N. Celebes remarked on the 
absence of Algae in the neighbourhood of coral-reefs, 
and suggested that their green pigment might enable 
