IX THE COLOURS AND PIGMENTS OF MOLLUSCA 187 
which is present in the well-known secretion of the 
animal. Among the bright colours of shells shades 
of red, orange, and yellow are perhaps the most 
common, but blue and green also occur. 
3. Coloured Secretions .—The third way in which 
conspicuous colours occur in the Mollusca is in con¬ 
nection with the secretions poured out by many. 
Among these may be mentioned the ink of Cephalo- 
pods, the violet fluids of Ianthina and Aplysia , the 
colourless fluids of species of Murex and Purpura 
which in air undergo a series of changes rendering 
them ultimately violet-coloured, the purple fluid of 
species of Scalaria, and so on. Many of these colour¬ 
ing-matters are, or were, of commercial value, and it 
is somewhat interesting to note that in animals, as in 
plants, it is usually the pigments which are incon¬ 
spicuous during life which can be utilised as dyes. 
Pigments of the Mollusca 
To pass now to the pigments themselves : we find 
that these are exceedingly numerous and varied ; in 
this respect they are sharply contrasted with the 
Crustacea, whose pigments are very uniform through¬ 
out large groups. Many of the pigments are, however, 
little known. The Mollusca agree with the Crustacea 
in their blood pigments, and in the presence of a 
limy exoskeleton, frequently impregnated with lipo- 
chrome pigments. In both groups the blood may 
contain haemocyanin or haemoglobin. The latter, rare 
in the Crustacea, is distributed in an exceedingly 
capricious way throughout the Mollusca, sometimes 
occurring in the haemolymph, sometimes only in 
