igo COLOUR IN NATURE chap. 



as a faint green line down the centre of the foot, 

 produced by the coloured dorsal epithelium shining 

 through. In the cells the pigment is present in 

 small granules, varying in colour from light green 

 to greenish-blue or even light blue. At times it 

 has a black or brownish tint, the extreme edge of 

 the mantle-fold being always brown. The pigment 

 is soluble in water, or better in alcohol, but in solution 

 the colour rapidly disappears. Alkalies destroy the 

 colour at once, but it returns upon acidification ; dilute 

 acids give the pigment a blue tint. The shell of 

 this little limpet is a warm brown colour, and in view 

 of the tendency of the green pigment to become 

 brown, it is not unreasonable to suppose that during 

 the process of shell -formation the green pigment 

 becomes converted into the brown. 



This green pigment is of some interest ; it varies, 

 as already seen, from blue to green, and is perhaps 

 somewhat widely distributed among Invertebrates. 

 The one just described certainly resembles closely 

 that found in the eggs of Eulalia viridis, and in 

 Thalassema. The origin of the pigment is quite 

 unknown, but it is not impossible that it arises from 

 the " enterochlorophyll " of the digestive gland. 



As to the glandular secretions which are so 

 common in the Mollusca, we find that their pigments 

 exhibit very diverse characters. The ink of Cephalo- 

 pods, for example, contains a black or brown pig- 

 ment, which is nitrogenous and is said to have a 

 chemical composition almost identical with that of 

 the black pigment of crows' feathers. According to 

 the older analyses of Girod it is free from ash, but 

 Dr. Emil Andrd considers it identical with melaine, 



