504 THE OCEAN WORLD. 
salt water, and establishes itself accordingly under some moist cliff 
overhanging the sea, where it can enjoy both. 
One of the necessary consequences of the condition of these 
animals enclosed in a hard shell is their power of throwing it off. 
The solidity of their calcareous carapace would effectually prevent 
their growth, but at certain determinate periods Nature despoils the 
warrior of his cuirass, the creature moults, and the calcareous crust 
falls off, and leaves it with a thin, pale, and delicate skin. In this 
state the Crustacean is no longer worthy of its name—its skin has 
become as vulnerable as that of the softest mollusc ; but it has the 
instinct of weakness, it retires into lonely places, and hides its 
weakness in some obscure crevice, until another vestment, more 
suitable for resistance, and adapted to its increased size, has been 
given to it. 
The body of a Crustacean consists of a great number of distinct 
pieces, connected together by means of portions of the epidermis 
which have not yet become hardened, somewhat as the bones in the 
skeleton of the vertebrata are connected by cartilages, the ossification 
of which only takes place in old age. The body of the Crustacean 
consists of a series of segments varying in number, the normal 
number of the body-segments being twenty-one. Each segment is 
divisible into two arcs—one upper or dorsal, the other lower or 
ventral ; and each arc may present four elementary pieces, two of 
which are united in the mesial line forming the /ezgwm, or back ; the 
lower arc is a counterpart of this, while the others form the two side 
or epimeral pieces. The skin, therefore, performs the functions of a 
skeleton, so that the Crustaceans—as was said by Geoffroy Saint 
Hilaire—like the molluscs, live inside and not outside their bony 
column. The analogue of the Crustacea amongst Vertebrata is to be 
found amongst Sturgeons, whose hard outer immovable bony case 
encloses a softer skeleton ; these latter agree, however, in all their 
other characters with the higher divisions of the vertebrata, although 
their internal skeleton does not possess the solidity of bone. 
The Crustaceans vary greatly in colour ; some are of a dark iron 
grey with a dash of steel-blue, like metal weapons forged for combat ; 
a 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 an epidermis, with pigmentary matter, which 
colours the former. 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 
