io8 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 



