ENTOMOSTRACA. 317 



in the form of barbs, combs or tufts, constitute a part of those feet 

 or of a certain number of them, and sometimes of the upper mandi- 

 bles. 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 dia- 

 phanous, or at least with a large anterior thoracic segment, fre- 

 quently confounded with the head, which appears to replace the 

 shell. The teguments are usually rather horny than calcareous, 

 thereby approximating thiese animals to thelnsectaand Arachnides. 



ORDER I. 



BRANCHIOPODA(l). 



A mouth composed of alabrum, two mandibles, aligula, and one 

 or two pairs of jaws, and branchias, the first of which, when there 

 are several, are always anterior, characterize this order or the sixth 

 of the class. 



These Crustacea are always errant and are generally protected 

 by a shell resembling that of a bivalve, and furnished with four or 

 two antennae. Their feet, with a few exceptions, are wholly nata- 

 tory. Their number varies, being but six in some, while in others 

 it amounts to twenty, forty-two, or more than a hundred. Many of 

 them have but one eye. 



Most of these animals, as we have already stated, being nearly 

 microscopical, it is evident that the application of one of the char- 

 acters we have employed — that of the presence or absence of the 

 palpi of the mandibles — with respect to them, presents almost insu- 

 perable difliculties. The form and number of the feet, that of the 

 eyes, the shell, the antennae, furnish us with more visible marks, 

 and such as are within the observation of every one. 



This order in the systems of De Geer, Fabricius and Linnseus, a 

 single species excepted — M. polyphemuSf — contained but the single 

 genus 



(1) Gill-footed 



