SYSTEMATIC ARRANGEMENT. 11 



Lamarck, in his 'Hist. Auim. sans Verteb.,' 1818, 

 divides the class Crustacea into two orders, the Hetero- 

 branches and Homobranches. The first of these is 

 divided into four sections, Branchiopodes, Isopodes, 

 Ampliipodes, and Stomapodes. The second is divided 

 into two, the Macrom-es and Brachyures. The first 

 section of the first order, viz. the Branchiopoda, includes 

 all Miiller's Entomostraca, and all the genera known up 

 to that time, except the genus Nebalia of Leach, which 

 is placed in the first section of the second order, the 

 Macroures. 



Latreille, in his last Method, in the ' Regne Animal' 

 of Cuvier, vol. iv, adopts a new arrangement, using for 

 his principal subdivisions the organization of the mouth. 

 He divides the Entomostraca into two orders, the 

 Branchiopoda and Psecilopoda. The order Branchiopoda 

 contains those genera which have organs proper for mas- 

 tication, are possessed of branchiae attached to the feet 

 or jaws, and are for the most part inclosed within a 

 testaceous covering, either in the form of a buckler or 

 that of a bivalve shell. The second order, Psecilopoda, 

 embraces those which are not provided with organs proper 

 for mastication, and are almost all parasitical, living upon 

 fishes and other aquatic animals. The first order, the 

 Branchiopoda, is divided into two principal sections, the 

 Lophyropa and Phyllopa. The first of these again is 

 subivided into three very natural groups or families, the 

 Carcinoida, Ostracoda, and Cladocera ; while the second 

 is subdivided into two, the Ceratophthalma and the 

 Aspidiphora. The Paecilopoda, on the other hand, is 

 composed of rather heterogeneous materials, and is divided 

 into two famihes, the Xiphosura and Siphonostoma.* 



* Latreille appears to have been struck with the reseniblauce which the 

 Leruese bear to some of the geuera of the Siphouostoma ; but as he was thou 

 not aware, of what has since been discovered, that the young of the Lernese 

 undergo a metamorpliosis like that of the Cyclopidte and Cahgida;, and that 

 the adults liave the faculty of cliaugiug tlicir skin or moulting, he considered 

 that the absence of these marks established a positive line of demarcation 

 between them and tlic Entomostraca. 



