94 
COLOUR IN NATURE 
CHAP. 
tissues, and the colour has a characteristic permanence. 
The varying translucency and delicacy of the tissues 
in the Ccelentera further determines the apparent 
colour of the pigments, so that we may almost 
describe these as simple forms of optical colours. It 
is likely that the frequent variation of colour in simple 
organisms is to be accounted for in a similar fashion. 
Among the more special colour-phenomena of 
the group, we may notice the comparative rarity of 
the lipochromes. They are said to occur in the red 
coral of commerce, in the skeleton and soft parts of 
some of the Gorgonidae, probably in the Dendrophyllia 
mentioned above, and in a few others. 
The colouring-matter of the blue coral Heliopora 
carulea also deserves mention. It is very different 
from any other blue pigment known, and is very 
insoluble. If, however, the coral be decalcified with 
hydrochloric acid, the pigment is set free, and may 
be dissolved in alcohol. The solution gives no bands 
and turns green with alkali. It seems not improbable 
that the colouring-matter is metallic in origin, perhaps 
a salt of some metal (cf the green colour of the bones 
of Belone , etc.). 
From the simplicity of the structure of the 
Ccelentera it is naturally to be expected that true 
optical colours will be absent. These are not, 
however, quite unknown, for the calcareous axes of 
some of the Gorgonians display brilliantly iridescent 
colours. Agassiz ( Voyage of the Blake , vol. ii. pp. 
144, 145 ) describes the species of Iridogorgia as having 
axes of a bright emerald-green or of burnished gold, 
while others have a lustre like mother-of-pearl. All 
the iridescent forms inhabit deep water, and the 
