OCR Text |
Show 10 CRUSTACEA. place, is composed of a labrum, tongue, two mandibles (frequently furnished with palpi), and two pairs of maxillre covered by the foot-jaws. In a great number each eye is placed on an articulated and movable pedicle, and the branchire are concealed under the lateral margins of the upper or lower shell ; in the others they are usually placed under the postabdomen. This sectio~ consists of five orders: the DECAPODA, STOMAPODA, LJEMODIPODA, AMPHIPODA, and the lsOPODA. The four first embrace the genus CANCER of Linnreus, and the last his 0NISCUS. The second, the Entomostraca, or "Insects with shells" of Muller, is formed of the genus MoNOCULus, Lin. Here the teguments are horny and very thin, while a shell, resem .. bling a buckler, com posed of from one to two pieces, covers or incloses the body of the greater number. The eyes are almost always sessile, and frequently there is but one. The feet, the number of which varies, are mostly fitted for natation, and without a terminal nail. Some of them, having an ante~ior mo~th com~osed of a labrum, two mandibles-rarely furmshed With palpi, a tongue, and one, or at most two pairs of jaws, of which the external ones are naked or are not covered by the foot-jaws, approximate to the preceding Crustacea. In the other Entomostraca, which seem to approach the Arachnides in several particulars, the organs of manducation ~re ~ometirnes simply fo.rmed by the coxre of the feet, proJCCtmg and arranged hke lobes bristling with small spines round a large central pharynx. At others they either com· pose a little siphon or beak, used for suction as in several ~r~chnides and lns~cts, or they are wholly ( o; nearly so) inVIsible externally, either because the siphon is internal or because the suction is produced in the manner of a cup.' The Entomostraca are thus dentated or edentated. The first will form our order of the BRANCHIOPODA(l ), and the . (~) ln. my work entitled l'amilles Nat. du Regne .llnimal, the Entomostraca are d!v1dcd mto four orders: the LoPRYROPon.A, PuYLt.OPODA, XTPllosun.A, :mel the 81· PHONOSTOJ\U. CRUSTACEA. 11 second that of the P JECILOPODA, which in the first edition of this work were a mere section of the preceding order. The singular fossils called TRILOBITEs, ofwhichM. Brongniart has given an excellent Monograph, being considered by him, as well as by many other naturalists, as Crustacea allied to the Entomostraca, we will briefly speak of thein after we have done with the latter. |