208 
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 
