ENTOMOSTKACA. 113 



constitute a part of those feet or of a certain number of them, 

 and sometimes of the upper mandibles(l). Hence the origin 

 of our term Branchiopoda, affixed to these animals, of which 

 at first we formed but a single order. Nearly all of them 

 are provided with a shell composed of one or two pieces, 

 very thin, and most generally almost membranous and nearly 

 diaphanous, or at least with a large anterior thoracic segment, 

 frequently confounded with the head, which appears to re- 

 place the shell. The teguments are usually rather horny 

 than calcareous, thereby approximating these animals to the 

 Insecta and Arachnides. In those which are provided with 

 ordinary jaws, the inferior or exterior are always exposed, all 

 the foot-jaws performing the office of feet properly so called, 

 and none of them being laid upon the mouth. The second 

 jaws, those of the Pliyllopa at most excepted, resemble these 

 latter organs ; Jurine sometimes distinguishes them by the 

 name of hands. ^ 



These characters distinguish the gnawing Entomostraca 

 from the Malacostraca ; the others, those which constitute 

 our order of the Poecilopoda, cannot be confounded with the 

 Malacostraca, inasmuch as they are deprived of organs of mas- 

 tication, or because the parts which ,sfeem to act as jaws are 

 not united anteriorly nor preceded by a labrum as in the an- 

 tecedent Crustacea and the gnawing Insecta, but are simply 

 formed by the branches of the locomotive organs, which, for 

 that purpose, are furnished with small spines. The Poecilo- 

 poda in this class of animals represent those which in that of 

 Insects are knoAii^by the name of Suctoria or the Suckers. 

 Nearly all of them are parasitical, and they seem to lead to 

 the Lernsese by insensible gradations ; but the presence of 

 eyes, the faculty of changing their skin, or even of undergo- 

 ing a sort of metamorphosis(2), and that of locomotion by 



(1) See Cypris. ■ 



(2) The young of Daphnia, and of some neighbouring- subgenera, and probably 

 also those of Cypris and Cytherea, with the exception of size, scarcely differ, if 

 at all, from their parents on quitting the eg^\ bitt those of Cyclops, the P%llopa, 

 and the Arguli, experience considei'able changes while young, either as respects 



Vol. III.— P 



