BRANCHIOPODA. 115 



which, when there are several, are always anterior, charac- 

 terize this order or the sixth of the class. 



These Crustacea are always wandering and are generally 

 protected by a shell resembling that of a bivalve, and fur- 

 nished with four or two antennae. Their feet, with a few 

 exceptions, are wholly natatory. Their number varies, be- 

 ing but six in some, while in others it amounts to twenty, for- 

 ty-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 characters we have employed that of the presence or 

 absence of the palpi of the mandibles with respect to them, 

 presents almost insuperable difficulties(l). The form and 

 number of the feet, that of the eyes, the shell, the antennas, 

 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 Lin- 

 naeus, a single species excepted M. polyphemus, contained 

 but the single genus 



Monoculus, Lin. (2) 



Which we will divide into two principal sections. 



The first, that of the Lophyropa is distinguished by the number 

 of feet, which never extends beyond ten; their joints are also more 

 or less cylindrical or conical, and never entirely lamelliform or foli- 

 aceous; the branchiae are but few in number, and most of them have 

 but one eye. Several, besides, have mandibles provided with a 

 palpus(3); there are, almost always, four antennae which serve for 

 locomotion. 



In the second section that of the Phyllopa the number of feet 

 is increased to at least twenty, and in some amounts to many more; 

 their joints, or at least the last ones, are flattened and resemble cili- 



(1) We will begin, however, with those Uranchiopoda whose mandibles are 

 furnished with palpi; they constitute the two first divisions of the Lophyropa. 



(2) And that of Binocle in the system of Geoffroi. 



(3) M. Straus appears to attribute this character exclusively to Cypris and Cy- 

 therea, which compose his order of the Ostrapoda; but from the observations of 

 Jurine, Sen., and Handohr, it seems that it also belongs to Cyclops. 



