78 
COLOUR IN NATURE 
CHAP. 
it is in solution. The colouring-matter is very 
susceptible to the action of reagents, being destroyed 
by very dilute alkalies and acids, iodine or alcohol. 
It also disappears after death or encystment. This 
extreme instability is interesting, for, as we have seen, 
it is characteristic of so many blue or violet pigments. 
In general the pigments of Protozoa seem to be 
usually chlorophyll, lipochromes, or blue or violet 
pigments of unknown relations. The notes given 
above are obviously incomplete, but the pigments 
have been little investigated. The notes may, how¬ 
ever, be sufficient to emphasise the points of import¬ 
ance about the pigments of the group. These are 
briefly as follows : in spite of the fact that the colours 
show great variability, we find every now and again 
in the group complex and unstable pigments of vivid 
and beautiful tint; these may in some cases, as in 
that of chlorophyll, have an important function, but 
such functions are entirely unknown—it would seem 
that in some instances the pigments are merely 
introduced with the food ; there is also considerable 
evidence to show that the pigments vary in tint in 
harmony with the varying physiological conditions of 
the organism. In all these respects the pigments of 
the Protozoa afford an interesting commentary on 
those of higher animals. 
The Colours of Sponges 
About the colours of sponges it is not at present 
possible to say much. It is a familar fact of observa¬ 
tion that they are exceedingly variable and often 
bright ; red, orange, yellow, green, and dull colours 
