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Show 146 CRUSTACEA. FAMILY II. SIPHONOSTOMA. The Siphonostomre have no kind of jaws whatever. A, sucker or siphon, sometimes external and in the form of an acute inarticulated rostrum(l ), and at others concealed or but slightly visible, fulfils the functions of a mouth. There are never more than fourteen feet. The shell is very thin and composed of a single piece. They are all parasitical. We will divide this family into two tribes. The first-CALIGIDEs, Lat.-is characterized by the pre. sence of a shell resembling an oval or semi-lunar shield; by the number of visible feet, which is always twelve,-or four. teen, if we include those which Leach considers as such and which I call inferior antennre; by the form and size of the tenth pairs which are sometimes multifid, pinnate or ter· minated in a fin, and well adapted at all times and in the adult, for the purposes of natation, and sometimes foliaceous or broad and membranous. The sides of the thorax are neve; furnished with wing-like expansions directed backwards and enclosing the body posteriorly. Here, the body, exhibiting several segments above, ~ el?ng~ted and narrowed posteriorly, terminating in a kind of tad With tw? threads or as many other salient appendages at the e~d; this extrem_ity is not covered by a segment of the superior tegumet~ts m the form of a large rounded scale, deeply notched m the posterior margin. The shell is at (1) The composition of this rostrum or beak is not well known. It is eviden~ from the fig~re ~f the .!lrgulus foliaceus, given by Jurine, Jun., tha.t it containsa suckde r~ ; but 1s th1s the case with the others • and of ho w many pt·e ces 1· s 1't com· ~ose f I cannot nnswer the question. I presume, however, that this 11iphon con· ststs 0 the labrum, mandibles and the ligula which forms the sheath of the sucker In the preceding Entomostraca, the four anterior feet whose form is very differ: ent from that of the following ones, would correspond'to the four jaws of the De· capoda. P JECILOPODA. 147 least half the length of the body. This subdivision will comprise two genera of Muller. ARGULus, Mull. This genus was at first designated under the name of Ozolus, and but very imperfectly described. Jurine, Jun. has since studied its type with the most scrupulous attention, followed it throughout all its changes of age, and produced a perfect and complete monograph of it. He has restored to the genus the original name given by Muller. · TheArguli are furnished with an oval shield, posteriorly emargi-nated, covering the body, the posterior extremity of the abdomen excepted, and bearing on a mediate, triangular space distinguished by the name of clypeus, two eyes, four very small, almost cylindrical antennre placed in front, the superior of which, shorter and triarticulated, have a stout, edentated ancl recurved hook at their base; and the inferior quadriarticulated, with a small tooth on the first joint. The siphon is directed forwards. There are twelve feet. The two first terminate in a transversely annulated disk, striated and edentated along the margin, and presenting internally a sort of rosette formed by the muscles, and apparently acting in the manner of a cup or suclter. Those of the second pair are prehensile, the thighs large and spinous, and the tarsi composed of three joints, the last of which is provided with two hooks. The remaining feet are terminated by a fin formed of two elongated pinnul<£, whose edges are fringed with bearded threads: the two first of the latter, or those of the third pair, including the four that precede them, have an additional but recurved toe. The two last are annexed to that portion of the body which projects posteriorly from the shell, or the tail. The female has but a single oviduct covered by two small feet situated behind the two palettes. The organ which is considered as the penis of the male, is placed at the internal extremity of the preceding joint of the same feet near the origin of the two toes. On the same joint of the two preceding feet, and facing these organs of copulation, is a vesicle presumed to be seminal. The abdomen, by which we mean that part of the body which extends posteriorly from the ambulatory feet, the rostrum, and a tubercle containing the heart, is entirely free, without distinct articulations, and terminates directly after the last feet behind, by a sort of tail, in the form of a rounded lamina, deeply emarginated or bilobate, and without terminal hairs: it is a species of fin. The body is so transparent that the heart may be distinguished through its parietes. It is situated behind the base of the siphon, lodged in a solid tubercle, semi-diaphanous and |
