106 COLOUR IN NATURE chap. 
The other pigments of the bristle-bearing worms 
are not well known ; it is possible that some of the 
pink pigments are lipochromes. 
As to the colours of Polychaetes in general, the 
striking features are the presence of optical colours, 
the number of green forms, and the absence of those 
beautiful and elaborate markings which are so char¬ 
acteristic of the leeches. The plan of the colora¬ 
tion is throughout much simpler; when brilliant, 
it seems to be usually dependent upon coloured in¬ 
ternal tissues shining through the skin. A great 
number of forms, moreover, especially those inhabit¬ 
ing tubes, are colourless and transparent. 
The colours of the Oligochaetes, such as the earth¬ 
worm, hardly merit separate notice : they are mostly 
dull or inconspicuous, but in some cases show a 
tendency to develop simple patterns. Of this tend¬ 
ency our own brandling ('Lumbricus fcetidus ) affords 
a convenient example. 
The Pigments of the Capitellidae 
The pigments of the small group of Polychaetes 
known as the Capitellidae are perhaps worthy of more 
detailed notice. The Capitellidae are the subject of 
one of the large Naples monographs, and the author, 
Professor Eisig, makes some observations on their 
colours which have been very widely quoted. The 
Capitellidae are remarkable among Annelids in 
having no closed blood-vascular system, the blood 
being contained in the general body cavity. Ac¬ 
cording to Eisig, it always contains haemoglobin, and 
is of a bright red colour which directly affects the 
