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 
