198 
COLOUR IN NATURE 
CHAP. 
structures to the pattern of the whole is so common 
among higher organisms. 
Leeches .—Among worms, as we have already seen, 
the leeches are distinguished par excellence by their 
markings and patterns. There are few facts more 
striking to one interested in colour than the change 
seen in passing from the Nemerteans and the marine 
Annelids, as one finds them on the shore, with their 
bright, uniform, and fugitive colours, to the leeches, 
with their dark persistent tints and their beautiful 
markings. That the colours of, for example, the 
medicinal leech show a considerable amount of 
variation, no one accustomed to handling specimens 
can deny, but at the same time there is sufficient 
constancy to admit of the dark spots being employed 
as a ready means of counting the segments. The 
occurrence of a constant and elaborate scheme of 
colour-markings at such a low grade in evolution is 
very interesting. 
We may notice also that the development of 
elaborate patterns in the leeches, as compared with 
the marine worms, is associated with the develop¬ 
ment of a large amount of dark pigments. This is 
interesting because it is perhaps universally true that 
elaborate patterns are dependent, at least in part, 
upon dark pigments, while the bright pigments tend 
as a rule to be more uniformly distributed. It is 
difficult to avoid coming to the conclusion that the 
fact is associated with the insolubility of the dark 
pigments, which will render them on the whole less 
readily diffused than the more soluble bright-coloured 
pigments. Lipochromes, for example, dissolve readily 
in solutions containing albumen or in fat, which 
