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. 



