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Show 100 CRUSTACEA. or is wider than it is thick. The mouth consists of the sam pieces as in the preceding Crustacea; but here, those whic~ correspond to the two superior foot-jaws of the Decapoda ex. hibit an appearance of a lower lip terminated by two ;alpi still more than in the latter. The two mediate antennre ar~ almost obliterated in the last Crustacea of this order, which are all terrestrial and also differ from the others in their re. spiratory apparatus. The male organs of generation are usually announced by linear or filiform appendages and sometimes by hooks, situated at the internal origin of the first sub-caudallaminre. The females carry their ova under th thorax, either between scales, or in a pouch or membranou: sac, which they open in order to allow a passage to th · h . ~ yo~ng, w .1 rh are produced _with th~ form of parts peculiar to their speciec, merely changwg then· skin as they increase · . M m size. ost ~.J them are aquatic. Those which are terrestr' 1 ] . 1 Ia, Ike a_ I other Crustacea which live out of water, still require a certam degree of atmospheric humidity to enable them to breathe, ~nd to preserve their branchire in a proper state for the exercise of that function. This order according to the system of Linnreus embraces the genus ONiscus, Lin., Which we will divide into six sections. The ~rst-.EPIOARIDEs, Latr.-is composed of parasitical Iso-p. oda, w1th ne1the1· eyes nor antennre , the body of w hI' c h , m· t h e rna 1e IS very flat, small and oblong; much larger in the female, and havin~ an oval ~orm narrowed and slightly curved posteriorly, hollow be-neath, wtth a thoracic border divided on each . d . fi branous lobes. The feet are placed on th' b Sld e mtod ve mem· 1 • 1s or er an cannot be usee either· for locomotion or natation The d f f I tail is · d d . h . · un er sur ace o t 1e prov~ e Wlt five palrs of small, ciliated, imbricated leaflets corresponchng to as many ' 1 . . segments and arranged in two longitudi-na series; there IS no append h tremit Th . . age, owever, to the posterior ex-y. e only parts distmctly visible in the mouth are two membranous leaflets laid u 1 • • pon another of the same nature, forming a f arhg eI Iq uadrllateral figure · Th e 1· 01"'e n·o r concavity forming a sort o s a ow basket, is filled . th h ' ways found th · d' 'd WI t e ova. Near their outlet is al· e m IVl ual presumed to be the male. Its extreme ISOPOD A. 101 allness seems to forbid all possibility of copulation; according to ~smarest it is provided with two eyes; its body is straight and al-most linear. These Crustacea form but a single subgenus, that of BoPYRus, Lat., The most common species is the Bopyrus crangorum, Lat., Gener. Crust. et Insect., I, 114; Monoculus crangorum, Fab.; Fouger. de Bondar, Mem. de 1' A cad. Roy. des Sc., 1772, pl. 1; Desmar., Consid. XLIX, 8-13. It lives on the Palremon ser· ratus, and the Pal. squilla, placed directly on the branchire and under the shell; it occasions a tumour on one of its sides, re· sembling a wen. The fishermen of the British channel con· sider them as very young Soles or Plaice. A second species, the B. des paUmons, has been described by Risso, under the female of which he observed eight or nine hundred living young ones(1). The second section-CYMOTHOADA, Lat.-comprises Isopoda with four very apparent setaceous antennre, almost universally terminated by a pluri-articulated stem; having eyes, a mouth composed as usual(2); vesicular branchire arranged longitudinally and in pairs; the tail formed of from four to six segments, with a fin on each side near the end; and the anterior feet usually terminated by a small stout nail or claw. They are all parasitical. The eyes are sometimes placed on tubercles on the top of the head; the tail consists of but four segments. SEROLIS, Leach. But a single spec.ies is known, the Cymothoa paradoxa, Fab. The antennre are placed on two lines, and terminated by a pluriarticulated stem. Under the three first segments of the tail, between the usual appendages, there are three others, transversal and terminated posteriorly in a point(3). Sometimes the eyes are lateral and not placed on tubercles; the tail is composed of five or six segments. Here the organ of sight is not formed of smooth, granular, approximated eyes; the antennre are placed on two lines, and consist of seven joints at least; the six anterior feet are usually terminated by a small, stout nail. (1) See the work of Desmarest, who has completely described this subgenus. (2) See our general observations on the Malacostraca with sessile eyes. (3) For other details .consult Desma.r., Consid., p. 292-294. |
