134 
COLOUR IN NATURE 
CHAP. 
2. As to the pigments of the Ophiuroids there is 
even less to be said ; they are probably for the most 
part lipochromes. Points of interest about both 
groups are first, as we have already noticed, the 
tendency for the colours to be more intense in 
deep-sea than in shallow-water forms, and second 
the greater instability of these bright pigments. 
There is reason to believe that this is due to the 
same cause as the similar phenomenon in the 
Crustacea, namely, that diminished power of secret¬ 
ing lime which is characteristic of abyssal organisms. 
3. The pigments of the sea-urchins are more 
numerous and more difficult. Lipochromes probably 
occur, but apparently do not markedly predominate 
in the production of the surface coloration. The 
pigment enterochlorophyll is apparently common 
and present in considerable amount. Other pigments 
have been described, but are somewhat imperfectly 
known. As an example of the colour phenomena 
to be seen in the group, we may take the common 
Echinus esculentus. This is usually of a purplish 
colour with green spines tipped with violet. The 
perivisceral fluid and blood are a deep claret-colour, 
and apparently contain both “ enterochlorophyll ” 
and a brown pigment called echinochrome. If this 
claret-coloured fluid be exposed to the air it turns 
bright green — the colour of the spines. The 
change has been supposed to indicate the presence 
of a respiratory pigment, but there is as yet no 
certainty. 
4. Among the Crinoids there is, as in the sea- 
urchins, a marked tendency for the lipochromes to 
be masked or replaced by more complex pigments. 
