OCR Text |
Show 116 CRUSTACEA. a ted leaflets. The pal pi of the mandibles are always wanting. They all have two eyes, situated in some at the extremity of two movable pedicles; their antenme, but two in numbe1· in several, are generally small and not fitted for natation. We will divide the Lophyropa into three principal and verynatu. ral groups, the two fit·st of which approach the Crustacea of our three first orders in their mandibles, each of which is furnished with a palpus, and in some other characters. 1. Those-CAROINOIDA, Lat.-whose more or less ovoid shell is not doubled like that of a bivalve, and leaves the inferior portion of the body exposed. They never have antenn::e resembling ramified arms. They have ten feet, more ot· less cylindrical or setaceous, The ova, in those females whose gestation has been observed, are contained in two external sacs situated at the base of their tail. Some of them have eyes. 2. Those-OsTRAoonA, Lat.; OsTRAPonA, Straus-whose shell is formed of two pieces or valves resembling those of a muscle, united by a hinge, and closing while the body is quiescent. They have but six feet( I), neither of which terminates in a digitated fin, nor is accompanied by a branchial lamina. Their antenn::e are simple, fiJi. form or setaceous. They never have more than one eye. Their mandibles and superior jaws ue furnished with a branchial leaf. The ova are placed under the back. 3. The last-CLAnooERA, Lat.; DAPIINIDEs, Straus-have but one eye, and the shell doubled but without a hinge (Jurine), termi· nating posteriorly in a point, and leaving the head, which is coyer· ed by a kind of shield like a rostrum, exposed. They have two, usually very large, antenn::e, resembling arms, divided into two or three branches directly above the peduncle, which are furnished with threads, always pt·ojecting and serving as oars. Their ten feet(2) are terminated by a digitated or pectinated fin accompanied, the two first excepted, by a branchial lamina(S). Their ova are also placed under the back; their body always terminates posteriorly in the mannet· of a tail, with two or three threads at the end. The anterior extremity of the body is sometimes pro· . (1) According to Straus, the first pair of feet; but although these parts by serv. mg as oars perform their functions, I nevertheless consider them as analogous to the lateral n~tenna: of the superior Crustacea and to the two superior ones of a Cyclops, wh1c~ here ~lso concur with the feet in producing locomotion. (2} Muller g1ves e1ght to the Cytherea:; reasoning from analogy, we may pre· sume that he was mistaken. (3 ~ T.h~s .character applies especially to Daphnia, the most numerous subgenus ofth1s diVIsiOn, and by analogy, to Polyphemus and Lynceus. llRANClliOPOnA. 117 d . to a kind of rostrum, and at others forms a kind of head, Ionge 10 t entit·ely occupied by a large eye. aln108 The first d1' v .1s.to n of the Lophyropa Branch. iopoda-that of thef Carc. mo. tda.-may be divided into two accordmg to the number o the eyes. S e of them have two. Hom the shell completely invests the thorax; the eyes are large d ere distinct and the intermediate antennre are terminated by an very ' two threads. ZoEA, Bose. Very large globular eyes completely exposed, and horn-like pro-jections on the thorax. . Zoea pelagica, Bose., lllst. Nat. Crust. II, xv, 3, 4. The b d semi-diaphanous; four autenn::e inserted under the eyes, t:e ~xternal ones bent into an elbow and bifid; a kind of long ros t rum o n the· forepart of the thorax and bet.w een the eyes, d a long pointed prominence on the postenor part of the ban k The feet are very short and hardly visible, the two last eaxcc e·p ted, which are elongated or terminate m· a fi ?·. Th ehta1"I · 1 1s as long as the thorax, curved, and formed of five JOints, t e ast being large, crescent-shaped and spinous. It was discovered by Bose in the Atlantic Ocean. The Monoculus taurus, Slabber, Microsc. V, and the Cancer germanus, L., appear to be allied to it(l). N EBALIA, Leach. Triangular, flattened eyes, partly covered by a triangular and arched scale. The feet are forked, and the terminal appendages of the tail seta-ceous( 2). (1) See the Hist. Nat. des Crust. et des Insect., of Latreille, and the wo~k of Desmarest on the Crustacea. This genus has not yet been completely descr1bed, and we have not been able to procure a single specimen of it. . (2) Nebalia Herbstii, Leach, Zool. Miscell., XLV; Desmar., Cons1d., XL, 5; Rand, Monoc. I, 8? The Nlbalie ventrue Risso Journ. de Phys., Octob. 1822, probably forms a peculi~ r subgenus in the' sectio;1 of the Schizopoda. In the Cyclo!'s eziliem, V.iviani, the thorax is divided into several segments, a circumstance wh1ch excludes 1t fr~m the Nebaliz. It also forms a new subgenus intermediate between the precedmg and following one. N.B. A new species of this genus, theN. Geoff. Saint-Hi!., I?., XV, 1, h~s been very minutely described by Milne Edwards. The head is termmated anter10rly by |