[ 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 Capitellidse, 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 Bonellia 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 faeces ; 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 : 
