98 
COLOUR IN NATURE 
CHAP. 
worms. The richest purples appear on velvety skins 
of deep brown or black, each of the soft and mobile 
folds giving shades that vary in intensity and lustre. 
Bright yellow contrasts with dark brown, white with 
vermilion brown and dull pink, while individual 
uniformity is characterised by such hues as rose- 
pink, white, green, yellow and olive, the gradation of 
colour in the various parts of a single specimen 
being so subtle that enthusiasm as well as skill is 
necessary in the artist who sets himself to the task 
of faithful delineation ” (Ray Soc. vol. xxxiv. p. 2). 
This description is based on the British forms only, 
and the tropical are said even to surpass these in 
splendour. In addition to the beauty of colour, 
we find that simple forms of marking, such as 
longitudinal and transverse stripes, are common. 
Professor MTntosh speaks also of the “silver sheen,” 
and the “ ever-changing iridescence of the active 
cilia,” and considers that in point of beauty and 
variety the Nemerteans even surpass the Annelids. 
Light seems to have some influence on colour, but 
the exact extent of the influence is apparently not 
determined. Thus we are told that some of the 
most brightly coloured live in crevices and dark 
corners, but that under natural conditions the colour 
varies according to the amount of exposure to light. 
In captivity Linens mannus turned pale, especially 
in the anterior region, but others, e.g. Amphiporus 
lactifloreus , developed more pigment so that the skin 
became opaque and of a deeper colour. Some are 
transparent, and the food contained in the gut 
(Algae, etc.) shines through and produces a marked 
and peculiar coloration. There is virtually no sexual 
