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 

 ccerulea 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 Goi^onians display brilliantly iridescent 

 colours. Agassiz ( Voyage of the Blake, vol. ii. pp. 

 144, 14s) describes the s^&c\t.soi 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 



