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Show 142 CRUSTACEA. twelve feet( I), and nearly all the remainder have e'th t t T · . 1 or wen y-two. hen· usual habitat is on aquatic aneimr ten 1 and most commonly on fishes. as, We divide this order into two families(2). FAMILY I. XYPHOSURA. This family is distinguished from the second b ch~racters : there is no siphon; the haunches of they fiseve~al pair of feet are covered with small spines and ~ rst stx office of · h perJ.orm the Ja~s ; t ere are twenty -two feet; the first t . th.e exception of the two anterior ones in the males en, With mmated by a didactyle forceps, and inserted as w 'nare tertwo that follow, under a large semi-lunar shield • ~h as the have the sexual organs attached to them and the fu e /atter leaflets, as in the case with the ten foil ' . h. rm o large chial and inserted under a second shel~wtmeg,. w Icdh bare bran-hard · ~ ' rmmate y a very ' ensiJ.orm and movable stylet Th . mals, and form t~e genus . ey are wandermg ani- LIMULus, Fab. The species are known in commerce b th Crab. The suborbicular sli btl I y e name of the Molucca ed body is divided into' t g y e o.ngated and posteriorly narrow-d wo parts, mvested b I'd pose of two pieces one to h y a so 1 shell com-senting above two iong' t d~ac 1 par~, very hollow beneath, and pre-h ' 1 u ma sulc1 one on e h 'd on t e middle of the back Th fi ' ac S1 e, and a carina h L' • e rst part of the sh 11 I covers t e tore-part of the b d . e , or t 1at which an extensive semi-lunar sh. ol ]y, ~~much larger than the other, forms · h le c' With a reflected d f wit two oval eyes of num f e ge, urnished above each side, exterior to a lo:gr?;~. acel ts, resembling granules, one on 1 u Ina carina; and on the anterior ex- (1) Fourteen in several ace d' two first, howevex·, appea; to m~\~n~e t;wLe.ach;. those which he considers as the seem to be the most favoured sub o l?fer!Or antenn::e. The Arguli, which twelve feet • genus Wltb respect to locomot'I On, ha ve bu t (2) In my Fam. Nat. du Re . gne Allim., they form two orders. P.IECILOPODA. 143 tremity of the middle one, and common to both pieces of the shell, two small, closely approximated, simple eyes( 1 ); these carinre are armed with teeth or acute tubercles. The duplicature of this shell at its anterior ext1·emity, beneath, forms a level border, strongly arcuated, and terminated inferiorly by a double arc, projecting like a tooth towards the centre of union. Immediately under this projection, in the cavity of the shield, is a small inflated labrum, carinated in the middle and terminating in a point, above which are inserted two little antennre, in the form of small didactyle forceps, flexed into an elbow in the middle oftheir length, at the point of union be· tween the first joint and the second, or of the forceps properly so styled. Directly beneath, inserted and approximated by pairs, and 00 two lines, are twelve feet, the ten first of which, the two or four anterior ones of the males excepted, terminate in a didactyle forceps; their radical joint, projecting inwards like a lobe and covered with points, performs the office of a jaw. The size of these feet augments progressively; those of the fifth pair excepted, they are all composed of six joints, the movable toe of the forceps included. The latter have an additional joint, and also differ from the preceding ones by having, at their external base, a hi-articulated appendage, directed backwards, the last joint of which is compressed and obtuse; by their fifth joint being terminated on the inner side by five small, movable, horny, narrow, elongated and pointed leaflets, and by the two toes of the forceps being movable or articulated at base. The two pieces situated between these feet, which M. Savigny considers as the ligula, appear to me to be merely two maxillary lobes of these organs, but detached or free. The pharynx occupies the interval included by all these feet. The males are distinguished from the females by the form of the forceps, which terminate the two or four anterior feet: they are inflated and deprived of the movable toe. The two last feet of this shield are united in the form of a large, membranous, and almost semi-circular leaflet, having the sexual organs on its posterior face, and presenting, in the middle of an emargina· tion of the posterior margin, two small triangular, elongated and pointed divisions, which appear to represent the internal toes of the forceps; the other articulations are indicated by sutures. The se· cond piece of the shell, articulated with the first in the middle of its posterior emargination and filling the interval it forms, is nearly triangular, and is angularly truncated and emarginated at its posterior extremity. Its lateral edges are alternately emarginated and dentated, and in the middle of each of the emarginations, counting (1) One on each side of the tooth that terminates this carina. |