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 lanthina 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 hsemocyanin or haemoglobin. The latter, rare 

 in the Crustacea, is distributed in an exceedingly 

 capricious way throughout the Mollusca, sometimes 

 occurring in the hsemolymph, sometimes only in 



