IV THE COLOURS AND PIGMENTS OF SPONGES 79 



are all common. The green colour of the fresh- 

 water Spongilla is usually asserted to be due to 

 chlorophyll, but green tints occur also in marine 

 forms. Sponges whose colours vary from greenish- 

 yellow to red almost all contain lipochromes. The 

 pure red lipochrome known as tetronerythrin is 

 widely spread, and is often associated in sponges with 

 a peculiarly penetrating odour like that of ozone. 

 Krukenberg was of opinion that in these sponges the 

 pigment played some part analogous to that of 

 chlorophyll in plants, being of importance in a pro- 

 cess of assimilation. It does not appear that his 

 observations have been repeated or confirmed. 



In Hircinia variabilis and some other sponges 

 there is a red pigment which, according to Kruken- 

 berg, is very similar to the pigment of the Red 

 Algae, and which is readily decolorised by reducing 

 agents. Some of the Aplysinidse contain pigments 

 of the type described by Krukenberg as uranidines, 

 which are of yellow colour, but tend to rapidly undergo 

 oxidative changes which turn them black. In other 

 forms, e.g. Chondrosia, dark pigments occur — a fact 

 of some interest, because these are rare in simple 

 organisms. 



The Colours of Sea-Anemones, Corals, 



jELLY-FlSH AND THEIR ALLIES (COELENTERA) 



The group of animals whose colours we have next 

 to consider includes some of the most beautiful of 

 existing organisms. Beautiful as are the changing tints 

 of birds and butterflies, they lack for many people 

 the subtle fascination possessed by the delicately 



