108 COLOUR IN NATURE chap. 
not appear that any good end is served by the over- 
hasty application of the term waste product to all 
pigments occurring in the skin, even when such 
pigments are periodically eliminated by means of a 
moult. In human physiology the term waste pro¬ 
duct has a perfectly definite meaning, and it is surely 
desirable in the interests of scientific nomenclature 
that a term implying a certain chemical composition 
should not be loosely applied to unknown substances. 
Then again the occurrence of a pigment in a struc¬ 
ture which is periodically cast and renewed is not 
absolute proof that the substance is useless or noxious. 
Thus the cuticle of the Crustacea certainly contains 
proteid, and, according to Krakow, also glycogen, and 
glycogen is certainly used up in its formation, but 
proteid and glycogen are not waste products. 
The Coloration of Leeches and the Origin 
of Markings 
With regard both to their pigments and to their 
coloration, the leeches, in the case of the more 
specialised forms at any rate, are sharply contrasted 
with marine worms. The cuticle compared with that 
of many worms is unspecialised, bristles are absent, 
as are also structural colours. The smaller forms 
may exhibit little pigment, but the differentiated 
forms, like Hirudo, are characterised by the develop¬ 
ment of a large amount. This pigment is not 
uniformly distributed, but is arranged, especially on 
the upper surface, in definite lines and spots, which 
give rise to a beautiful and complex style of colora¬ 
tion. Any one who doubts the propriety of the word 
