SUBDIVISIONS OF CRUSTACEA. 245 



velopment takes place ; so that whilst the young exhibit all the 

 characters of the group, the mature animal appears to be any- 

 thing rather than a crustacean. The typical forms are covered 

 with a shell or carapace of a horny or leathery texture, formed of 

 two pieces; they have jointed feet, fringed with hairs; and their 

 branchiae, when present, are attached to the feet or organs of 

 mastication. This Section includes six Orders, forming three 

 well-marked groups : 



a. BRANCHIOPODA, with a masticating mouth and numerous 

 leaf-like branchiae attached to the feet, including the Orders 



VII. PHTLLOPODA, with the body shell-less, or enclosed in a 

 simple carapace, and furnished with numerous appendages. 



VIII. CLADOCERA, with a carapace enclosing the whole body, 

 like a bivalve shell, and not more than six pairs of appendages. 



b. LOPHYROPODA, with a masticating mouth, to the organs of 

 which the branchiae are attached ; including the Orders 



IX. OSTRACODA, which has the entire body enclosed in a sort 

 of shield resembling a bivalve shell. 



X. COPEPODA, in which there is no such envelope. 



c. PCECILOPODA, with a suctorial mouth, including the Orders 



XI. SIPHONOSTOMA, in which the thorax is composed of dis- 

 tinct rings, and bears three or four pairs of swimming feet. 



XII. LERN^EIDA, with the thorax undivided, and the feet rudi- 

 mentary. 



E. Thejifth Section, CIRRHOPODA, includes numerous animals, 

 which, when mature, depart widely from the type of the Class. 

 They are attached to marine bodies, and enclosed in a shell com- 

 posed of several calcareous valves, from an opening in which 

 they protrude a bundle of cirri. They form a single Order 



XIII. CIRRHOPODA. 



F. The sixth and last Section, ARANEIFORMIA, or the Spider- 

 like Crustacea, includes a small number of species furnished with 

 a suctorial mouth, and with four pairs of long jointed legs, adapted 

 solely for walking. They form but a single Order 



XIV. ARANEIFORMIA. 



It is in the Order Isopoda that we find, in the equality of 

 the segments and of their appendages, as well as in the aerial 



VOL. II. T 



