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 
Algm, and which is readily decolorised by reducing 
agents. Some of the Aplysinidae 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-Fish and their Allies (Ccelentera) 
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 
