INTRODUCTORY. 5 



Many of these pigments appear to have no special use in the 

 animal economy, except in so far as they may be utilised iu 

 order to produce a protective resemblance to the surroundings r 

 and in other ways treated of in the succeeding chapters. They 

 are often merely waste products which are temporarily stored 

 up in the skin (see p. 1^6). 



On the other hand, there are certain coloured substances 

 which have been proved to have a functional importance. 



Colours associated with Substances of Physiological Importance 



to the Animal. 



Some animals owe their colour to substances embedded in 

 the skin, or existing in the tissues beneath, which play an 

 important part in the processes of respiration, digestion, and in 

 other physiological functions. The coloration of such animals 

 may be advantageous or disadvantageous as a means of con- 

 cealment ; if disadvantageous, we must assume that the direct 

 advantages of the processes accompanying life which the 

 coloured substances give outweigh the disadvantages in ren- 

 dering the animal conspicuous, etc. If they happen to be 

 advantageous in the latter way, it must be looked upon as a 

 fortunate accident. The substances themselves which give the 

 colour cannot probably be changed without destroying or 

 altering their useful physiological purposes ; nor, in some cases 

 at least, can they be concealed without rendering them useless 

 for their particular purpose. 



Haemoglobin. 



For example, most rivers and lakes abound with minute 

 worms of half an inch to two inches or so in length, which are 

 frequently of a bright red colour. These worms belong to 

 several genera of the Oligochoeta a group which also i 

 the common earthworm. 



