2o8 COLOUR IN NATURE chap. 



pigments. The blue and violet colouring-matters 

 are very unstable, turning brown with most reagents, 

 including water and dilute alkalies. Curiously- 

 enough acid restores the blue colour. Certain of the 

 yellow pigments are not lipochromes, but singularly 

 unstable substances of quite different characters. 

 Thus the lymph of Ascidia fumigata is coloured by 

 a yellow pigment which is very soluble in alcohol 

 and ether, and slightly soluble in water. On stand- 

 ing in air, however, the solutions rapidly become 

 dark coloured, and ultimately black. This is, accord- 

 ing to Krukenberg, due to ferment action ; the in- 

 terest of it is, however, that the tunic of this species 

 is marked with black, the pigment being probably 

 the same as that which arises from the alteration 

 of the yellow pigment. It will be remembered that 

 in many lepidopterous larvae the blood contains a 

 green pigment which darkens in a similar way on 

 exposure to the air, and that this is possibly the 

 origin of the dark pigments both of larvae and adults. 

 These curious pigments — uranidines of Krukenberg 

 — are worthy of further investigation. 



M'Munn (1889) confirms Krukenberg's state- 

 ment that most of the pigments of Ascidians are of 

 lipochrome nature. 



General Characteristics of Vertebrate 

 Coloration 



Before passing on to consider separately the 

 divisions of the higher Vertebrates, it may be well to 

 mention some of the peculiarities of the colours as a 

 whole. In the first place, in spite of the varieties of tint 



