IX THE COLOURS AND PIGMENTS OF MOLLUSCA 189 



coloured by turbobrunin, a red pigment easily con- 

 verted into biliverdin. Krukenberg regards this as 

 evidence that the bile pigments can arise inde- 

 pendently in Molluscs, without the intervention of 

 haemoglobin, but the whole subject is still obscure. 



As colouring agents of shells the ubiquitous 

 lipochromes also occur ; they have been found in 

 such shells as the gaily coloured Pectens and in 

 Littorina ; it is probable that here, as in Crustacea, 

 they form compounds with the lime of the shells, and 

 are most stable in those cases where the shell con- 

 tains the largest amount of lime. Numerous other 

 pigments have been described as colouring the shells 

 of Molluscs, but as most of them are very imperfectly 

 known, it is unnecessary here to mention the names 

 which have been given them. The relation between 

 the pigments of the mantle and the shell, a subject 

 of great interest, does not appear to have been 

 investigated at all. Although in some cases the two 

 structures are similarly coloured, yet in others there 

 is no apparent relation. It is quite probable that this 

 may often be due to the fact that the pigments of 

 the shell are compounds or oxidised products, but 

 as yet nothing seems known on the subject. 



In connection with this subject we may mention 

 the peculiarly vivid pigment of the tortoise-shell 

 limpet. This pigment does not seem to have been 

 described, in spite of its striking tint. It is entirely 

 confined to the cells of the ciliated epithelium which 

 lines the shell, is turned in to cover the dorsal 

 surface of the foot, and also covers the mantle-fold. 

 During life the colour is very inconspicuous, being 

 only visible in the mantle-fold and in small specimens 



