104 
COLOUR IN NATURE 
CHAP. 
the muscles ; and some earthworms owe their very 
dark colour to pigment occurring in this situation, 
or present in considerable quantity in the cells 
of the peritoneal epithelium. Accompanying the 
differentiation which gives size and opacity to the 
body, there is frequently in Annelids, as already 
noticed, a differentiation of the cuticle which gives 
rise to structural colour. It is curious to note that, 
contrary to the usual rule that bright colours in 
worms when not due to structure are the result of 
the shining through of coloured internal structures, 
we find that the bright colours of the little Oligo- 
chsetes belonging to the genus AEolosoma are said 
to be due to coloured oil-globules contained in the 
skin (see Beddard, Ann. and Mag. Nat. Hist. vol. ix. 
pp. 12-19). The oil-globules are in the different 
species blue-green, yellow-greeen, or orange-red, and 
may be of lipochrome nature. 
As to the pigments themselves, if we bear in mind 
the statement just made that, when bright, the colours 
of worms are due in the general case to coloured in¬ 
ternal tissues, it must be obvious that the pigments 
are usually those which by hypothesis are of direct 
physiological importance. Haemoglobin, which is 
widely and irregularly distributed in the group, is 
one of these. It is often exceedingly conspicuous 
in the more delicate forms ; but even in the larger 
and more opaque worms it may shine through the 
thin-walled gills or tentacles. The green pigment 
which occurs in the blood or coelomic fluid of 
Sabella, Branchiomma, Spirographis, and Siphono- 
stoma , is another example. This pigment has been 
accredited with important respiratory functions. It 
