u6 COLOUR IN NATURE chap, v 
Krukenberg in considering that it is nutritive in 
function, apparently regarding it as of importance 
in the nutrition of the young individuals. This case 
is of some interest, because it is so common to find 
coexisting with the so-called excretory pigments 
others whose function is unknown or doubtful, but 
which are apparently not excretory. 
It is also of interest to notice that the so-called 
respiratory pigments, those which are capable of re¬ 
peated oxidation and deoxidation, frequently occur 
in organisms where it is exceedingly doubtful whether 
they have really respiratory significance. 
