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 



