n8 COLOUR IN NATURE chap. 
characters of the Crustacea, but it may be well to 
mention some points which are of importance in 
reference to the coloration. In the first place, we 
may note that the Crustacea, like all other Arthro¬ 
pods, have a firm cuticle of chitin. This chitinous 
coat is not infrequently impregnated with lime salts, 
and is in most cases pigmented. The subjacent 
epidermis almost always contains pigment, either in 
special contractile chromatophores, or in solution in 
the general cells, or more usually in both. In some 
cases, as in the adult lobster, the epidermis is so 
completely concealed during life beneath the thick 
cuticle that it is of practically no importance as 
regards coloration, while in other cases the cuticle 
is thin and translucent, and it is the epidermis which 
is important in the production of surface coloration. 
The degree of calcification is the most important 
factor in the production of opacity in the cuticle, so 
that in general terms we may say that the shell 
or cuticle tends to be transparent in small forms, in 
those inhabiting fresh water, and in abyssal forms. 
Colour-variation in Crustacea 
As a class the Crustacea are remarkable for the 
brilliancy of their colours, blue, green, shades of 
orange, pink, red, and brown being all common 
tints. Nor are the bright colours confined to the 
higher forms ; the tiny Daphnia forms vivid scarlet 
patches on the surface of the sea, Diaptomus bacillifer 
forms red patches in fresh-water lakes, and so on. 
As a general rule, however, the colours harmonise 
with the surroundings, this being especially true of 
