igS 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 i&-vf 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 



