| OCR Text |
Show 86 CRUSTACEA. ganglions which immediately foiJow the two cephalics, form as many small circular masses situated on the median line of the body; but the cords of communication which serve to connect them, remain isolated and attached to each other. It would appear from these facts that the latter Crustacea m·e higher in the animal scale than the preceding ones, but othe1• considerations seem to us to require a considerable separation between the Talitri and Onisci, and the arrangement of the Cymothom and ldotere in an intermediate rank. The organs of generation are situated inferiorly near the origin of the tail. The two first appendages with which it is furnished beneath, and which are analogous to those presented t~ us by the same part in the preceding Crustacea, bnt more diversified, and always, as it appears, supporting the branchim, differ in this respect, according to the sex. The coitus takes place ]ike that of insects, the male placing himself on the back of his female ; the latter carries her ova under the thorax between scales which form a sort of pouch. There they ar; developed, and the young remain attached to the feet or ot~er parts of the body of their mother, until they have ac· qm~ed the strength requisite for natation, and providing for their wants. All these Crustacea are smaiJ, and mo~tly in· habit the sea-coast or fresh water. Some are terr~strial and others are known which are parasitical. ' They are divided into three orders: those whose mandibles are furnished with a palpus, appear to be natural1y connected with ~he p.receding Crustacea-such are the Amphipoda; those m which these organs are deprived of them will consti· tute the two foiJowing orders-the Lremodipoda and the Jso· ~oda. The Cyami, a genus of the second one, being parasi· tlc~I, naturaJly lead us to the Bopyri and Cymothore, with Which we commence the Isopoda. AMPIIIPODA. 87 ORDER III. AMPHIPODA. The Amphipoda are the only Malacostraca with sessile and immovable eyes, whose mandibles, like those of the preceding Crustacea, are furnished with a palpus, and the only ones whose subcaudal appendages, always very apparent, by their narrow and elongated form, their articulations, bifurcations, and other incisures, as well as by the hairs or cilia with which they are provided, resemble false or natatory feet. In the Malacostraca of the following orders, these appendages have the form of laminm or scales ; here these hairs and cilia appear to constitute the branchire. Many of them, like the Stomapoda and the Lremodipoda, have vesicular bursre either between their feet or at their external base, the use of which is unknown. The first pair of feet, or that which corresponds to the se· cond foot-jaws, is always annexed to a particular segment, the first after the head. The antennre, which, with a single ex· ception-the Phronimre,-are four in number, project, gra( lually taper into a point, and consist, as in the preceding Crustacea, of a peduncle and a single stem, or one furnished at most with a little lateral branch, and usually composed of several joints. The body is generally compressed and curved beneath posteriorly. The terminal appendages of the tail are most frequently styliform and articulated. Most of them swim and leap with facility and always laterally. Some inhabit springs and rivulets, and are often found in couples consisting of the two sexes ; most of them however live in salt water. Their colour is uniform, verging on reddish or greenish. They may all be comprised in a single genus, that of GAMMARUs, Fab., Which we may subdivide, in the first place, into three sections,. frotn the form and number of the feet. |
