i88 COLOUR IN NATURE chap. 



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 



