536 THE WONDER OF LIFE 



brates, or in the fluid of the blood, as in earthworms ; it 

 may be in the form of coloured spicules or calcareous 

 deposits, as in Alcyonarian corals ; it may be in special 

 cells which often show considerable activity the chroma- 

 tophores (see Fig. 87). 



Primary Significance of Pigments. There have been 

 relatively few important inquiries into the physiological 

 significance of pigments, which is a very difficult problem ; 

 but it may be said that some pigmented substances 

 are of the nature of waste- products, like the green guanin 

 in a lobster's kidney, or the sulphur-yellow in the wings 

 of some butterflies, or the sepia of the cuttlefish ; that 

 others are of the nature of reserve products, like the carmine 

 which accumulates in the body of the female cochineal 

 insect ; that others are simply indifferent by-products of the 

 metabolism. That pigments need not be useful as such 

 is quite plain when we remember that the internal organs 

 of many animals are brightly coloured. Thus the gonads 

 of some starfishes and sea-cucumbers are brilliant. 



Primary Significance of Structural Coloration. 

 The cross bars, the concentric lines, the zoned structure, 

 and the superposition of very thin lamellae produce inter- 

 ference colours, but what is their primary significance ? 

 The answer must be, that they are the ripple-marks of 

 growth ; they are expressions of the fact that growth is 

 rhythmic, not continuous. The familiar concentric lines on 

 the stem of a tree express the difference between the 

 summer and the winter wood ; the lines on the surface of 

 a shell are indices of periods of growth punctuated by 

 times of rest. 



As a further illustration of the idea towards which we are 

 groping that many structural features are just, as it were. 



