IX THE COLOURS AND PIGMENTS OF MOLLUSCA 191 
the pigment of Limncea stagncilis, which he states is 
iron-containing. Andre regards melaine as a waste 
product stored up in the tissues. When we consider 
the almost universal presence of pigment in the shell 
of shelled forms, it seems not unnatural to conclude 
that the large amount of pigment in the ink of 
Cephalopods is associated with the suppression of 
the shell, which has thus, as it were, forced the 
organism to get rid of its pigment by some new 
means. 
The beautiful purple colouring-matter of the fluid 
of Ianthina has always attracted much attention, but 
in this case, as in those of the chromogen of Murex 
and the pigment of Aplysia, there have been few 
exact investigations. The colour of the secretion 
of Aplysia has been asserted to be due to natural 
aniline dyes, but this, according to Krukenberg, is 
incorrect. Moseley found it to be soluble in alcohol 
with a purple colour, turning violet with acid ; 
it was very unstable and gave two sets of spectra 
according as it was or was not acidified. The pig¬ 
ment of Ianthina , on the other hand, dissolved in 
alcohol to form a violet or pinkish-blue solution, 
acid turning it a light pale blue. It gave a spectrum 
with three bands. It would be interesting to know 
if these curious pigments occur elsewhere in Mollusca, 
and if they have any connection with the blue pig¬ 
ments of shells like the common mussel. 
In connection with the pigments of the secretions 
we may note the peculiar pigment enterochlorophyll, 
which is so widely spread in the digestive glands of 
the Mollusca. In Patella I find that it occurs in 
the epithelial cells lining the alimentary canal, in the 
