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, ahd 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 Cmstacean 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 fe?'gum, 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 



