CRUSTACEANS. 479 



segments being twenty-one. Each ring 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 from the tergum, 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 Geoffrey Saint Hilaire, like the molluscs, live inside 

 and not outside the bony column. The analogue of the Crustacea 

 amongst vertebrata is to be found amongst Sturgeonidse, whose hard, 

 immovable bony case encloses a softer skeleton ; agreeing in its 

 characters with that of the higher divisions of vertebrata, although 

 not possessing 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 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 are 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, contri- 

 bute colour to the sea red, purple, or scarlet : such are Grimothea 

 D' Urvillei and G. gregarea. 



Before the year 1823, it was not generally supposed that this class 

 of animals was subject to change of shape from the larva condition, 

 and during its progressive development ; but about this time, and for 

 some years following, certain able microscopic experiments clearly 

 demonstrated that a minute nondescript kind of animal called the 

 Zoea Taurus, was nothing more nor less than the young of a kind 



