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Show 140 CRUSTACEA. appearing to form a third to the second and following feet, as far aa the tenth pair. In proportion as these organs diminish in size th 1 fl . . , e ea ets approximate more closely, the forceps IS more clearly defined and less pointed, and the first toe becomes wider, shorter and rounder The two anterior feet, which are much larger and are formed Jik~ oars, resemble ramous antennre, and have been considered as such by some writers( I): they exhibit four multi-articulated setaceous threads, the two last joints, one of them particularly, being much longer than the others which are situated on the internal side or anteriorly. The two at the extremity are evidently analogous to the toes of the forceps, the remaining two also correspond to as many of the .lateral leaflets;. it is easy to convince ourselves of this by com. parmg these parts In young specimens. After their sixth or seventh change of tegument, the two or three following feet of the latter greatly resemble the two anterior ones, and even their antennre are longer in p1·oportion than in the adult, and are terminated by setre or hai~s. The ele;enth pair are very remarkable(2). The first joint, behmd the vesicles, presents two circular valves, laid one on the other, formed by two leaflets, and containing the ova, which resem. ble granules of a bright red colour. Every specimen which has hitherto been examined being always found to possess this kind of feet, they have been considered as hermaphrodites, and are consi· dered capable of self·impregnation. These animals inhabit ditches, pools, stagnant waters, &c. and usually in myriads. Abducted, when thus assembled, by violent winds, they have been seen to descend in rain. They generally make their appearance in sp1·ing, and in the beginning of summer. Their customary food is the Tadpole. They swim well on their back and when they sink into the mud they erect their tail. When firs~ produc~ d they ~ave but one eye and four feet, resembling arms or oars, furmshed With tufts of hairs, the second of which are the largest. Their remaining organs are regularly developed after each change of tegument. M. Valenciennes, an attache of the Mus. d'Hist. Nat. h~s remarked that these Crustacea are f1·equently devoured by the bird vulgarly called the Lavandiere(3). The number of species known being very small, it is unne· (1) They also seem to represent the two first foot-jaws. . (2) s.cha:ffer distinguishes them by the name of uterine feet. The preceding mne pairs, according to his phraseology, form forceps, those of the first oars, or tru~ feet~ finally, those which follow the uterine feet, or the twelfth pair and fol· d lowmg one.s, hra~cltial feet. The vesicular sacs lengthen and lessen just as gra· ually; the1r use 1s unknown. (3) The Motacilla alba, and cinerea, L. .11m. Ed. BRANCHIOPODA. 141 essary to imitate Leach in forming a separate genus-LEPIDU· :us, Leach-for those which have a lamina between the threads of the tail. Such is the .!lpus prolongatus; Monoculus apus, L.; Schreff., Monoc., VI; Limule serricaude, Herm., Jun.; Desmar., Consid. LII, 2. The carina of the shield terminates posteriorly in a small spine, which is not seen in the .!1pua canciformia; Binocle a queue en filet, Geoff., Insect., XXI, 4; Limulua paluatria, MUlL;. Schceff., Monoc. I-V; .!1pua vert, Bose.; Desmar., lb., LI, 1; the latter, besides, has no lamina between the caudal threads; it is the type of the genus APus, Leach, or the Apus properly so called. The same naturalist has figured another species, .!lpus Montagui, Edinb. Encyclop. Suppl. I, XX. ORDER II. P £CILOPODA. The Precilopoda are distinguished from the Branchiopoda by the diversity in the form of their feet, among the anterior of which an indeterminate number are ambulatory, or fitted for prehension ; while the others, lamelliform or pinnate, are branchial and natatory. It is principally, however, by the absence of the usual mandibles and jaws that they are removed from all other Crustacea. Sometimes these parts are replaced by the spinous haunches of the first six pairs of feet ; and sometimes the organs of manducation consist either of an external siphon in the form of an inarticulated rostrum, or of some other apparatus fitted for suction, but concealed or slightly apparent. Their body is almost always, either wholly, or for the greater portion, invested with a shell in the form of a shield, consisting of a single plate in most of them, and of two in others, which always presents two eyes when those organs are distinct. Two of their antennre-Cheliceres, Lat.-form a forceps in several, and fulfil its functions. Most of them have |