CRUSTACEANS. 
475 
The Crustaceans have a dark-grey iron colour, with a dash of steel- 
blue, like metal weapons forged for combat. Some few of them are red, 
or reddish-brown ; others are of an earthy yellow, or of a livid blue. 
“ The integument,” according to Milne Edwards, “ consists of a 
corium, or true skin, and epidermis, with a pigmentary matter, which 
colours the latter. The corium is a thick, spongy, and vascular mem- 
brane, connected with the serous substance which lines the parietal 
walls of the cavities, as the serous membrane lines the internal 
cavities among the vertebrata.” This pigment is less a membrane 
than an amorphous matter diffused through the outer layer of the 
superficial membrane, which changes to red in the greater number of 
species in alcohol, ether, acids, and water, at 212" Fahr. 
The calcareous crust of the animal is thick, and in the dorsal region 
capable of great resistance ; their members arc also of remarkable hard- 
ness ; but in the smaller species the shell is often thin, and of that 
crystalline transparency which permits of its digestion and circulation 
being observed. Many species, which are quite microscopic, con- 
tribute colour to the sea — red, purple, or scarlet : such are Grimothea 
D’ Urvillei and G. grey area. 
Among the sea-spiders, which have no neck ( Cephalothorax ), 
the head gradually 
disappears in the 
breast, but the belly 
remains distinct ; the 
middle of the body 
is compressed, the 
shape narrow and 
graceful. Among the 
Crustaceans which 
have neither neck 
nor shape, the head, 
the breast, and the 
belly form only one 
mass, often short, 
S( fuat, athletic, and 
difficult to take, as 
m Pisa tetraodon (Fig. 332), the four-horned spider-crab. 
Many of these animals have a powerful tail, consisting of a certain 
