CRUSTACEA, ARACHNIDES, INSECTA. 305 



ply their place in toto. Whatever be the modifications of these 

 parts, however, they can always be recognized and referred to a 

 general type. 



CLASS I. 

 CRUSTACEA. 



The Crustacea are articulated animals, with articulated feet, re- 

 spiring by means of branchiae, protected in some by the borders of 

 a shell, and external in others, but which are not inclosed in special 

 cavities of the body, and which receive air from openings in the 

 surface of the skin. Their circulation is double, and analogous to 

 that of the Mollusca. The blood is transmitted from the heart, 

 which is placed on the back, to the different parts of the body, 

 whence it is sent to the branchiae, and thence back again to the 

 heart. These branchiae, sometimes situated at the base of the feet, 

 or even on them, and at others on the inferior appendages of the 

 abdomen, either form pyramids composed of laminae in piles, or bris- 

 tled with setae, or tufted filaments of simple ones, and even appear 

 in some cases to consist wholly of hairs. 



The Crustacea are apterous or deprived of wings, furnished with 

 compound eyes, though rarely with ocelli, and usually with four an- 

 tennas. They have mostly the Paecilopoda excepted three pairs 

 of jaws, the two superior ones, designated by the name of mandibles, 

 included; as many foot-jaws, the last four of which, however, in a 

 great many instances, become true feet; and ten feet properly so 

 called, all terminated by a single small nail. The mouth, as in In- 

 sects, presents a labrum and a ligula, but no lower lip properly so 

 called, or comparable to that of the latter; the third pair of foot-jaws, 

 or the first, closes the mouth externally, and replaces that part. 



Their envelope is usually solid, and more or less calcareous. 



They change their skin several times, and generally preserve their 



primitive form and natural activity. They are mostly carnivorous 



and aquatic, and live several years. They do not attain their adult 



2 O 



