COLOUR IN NATURE 
CHAP. 
188 
certain muscles; but in the Mollusca, as else¬ 
where in Invertebrates, it demands further study. 
Haemocyanin, on the other hand, is very widely 
distributed in both groups ; the question whether it 
can function as a pigment strictly so called is a 
somewhat difficult one. It will be recollected that 
it differs from a pigment like haemoglobin in dis¬ 
playing colour only in the oxidised condition— 
reduced haemocyanin is quite colourless. It is in 
consequence difficult to believe that haemocyanin can 
be of permanent value as a pigment, except in delicate 
organs freely exposed to sea-water. Under such 
conditions, however, there is nothing intrinsically 
improbable in the idea that it may give rise to 
brilliant colour. In dissecting recently killed sea- 
slugs the bright blue colour of the abundant blood is 
very noticeable. It seems quite possible that the 
bright blue tints seen in the transparent papillae of 
many Eolids may be due to the oxidised blood 
shining through. The colours are at least exceedingly 
fugitive after death. 
The fate of haemocyanin in the body is unknown, 
but there is no reason to suppose that it can give 
rise to series of pigments in any way resembling those 
which arise in vertebrates from the breaking down 
of haemoglobin. Derivatives of haemoglobin have 
on the other hand been described in various mol¬ 
luscs. Dr. M'Munn, for example, describes haemato- 
porphyrin in the slugs Limax and Avion , and so on. 
According to Krukenberg the shells of some species of 
Haliotis , Trochus, and Turbo are coloured by biliverdin, 
one of the bile pigments of Vertebrates, while other 
species of the first and last genera have their shells 
