XIV ORIGIN OF PIGMENTS 295 



coloration. It may be that worms are an exception 

 to this rule. It is commonly stated that in the 

 earthworm carmine introduced into the gut is re- 

 moved by the yellow cells, which then go free and 

 pass out with their burden by the nephridia. In 

 the Capitellidae, and in Tubifex, on the other hand, 

 the pigment is not wholly eliminated, but is in part 

 stored in the skin. These facts may show that in 

 worms the products of the metabolism of "liver" cells 

 are not readily eliminated by the gut itself, and so 

 may in some cases be important in coloration. It 

 seems possible that in BonelUa also the colour of the 

 skin is due to a modification of a pigment occurring 

 in the gut (?), or in the cells lining the body cavity. 

 Further, we have seen (p. 191) that in Mollusca the 

 peculiar pigment enterochlorophyll, at least in some 

 cases, colours the fsces ; that it occurs both in the 

 cells of the gut and in the digestive gland ; and 

 finally, that it is possible that in some cases it may 

 give rise to the pigments colouring the mantle, and 

 ultimately to those of the shell. Unfortunately this 

 is as yet uncertain. 



When we come to the fate of pigments intro- 

 duced into the body cavity, and their natural 

 analogues, the matter is much more difficult and 

 complicated. In the following table the natural 

 analogues column must be almost left blank : — 



