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Show 16 CRUSTACEA. The inferior face of the brain produces four nerves, which belong to the antennre, and that also give off some twigs to the neighbouring parts. Two nervous and_ ~ery long cor_ds, e~bracing the esophagus laterally and u~ttlng beneath It, ar1s_e from its posterior margin. There, as 111 the Brachyura, this nnion only takes place in the middle of the thorax, the medulla then assuming the form of a ring whose proportions are eight times larger than those of the brain: six nerves on each side arise from this ring ; the anterior ones belong to the parts of the mouth, and the five others to the five feet of the same side. From the posterior margin arises another nerve which runs to the tail, without producing any sensible ganglion, and that apparently represents the ordinary nervous cord. Here, as in the Macroura, each of the two nervous cords, previous to uniting beneath the esophagus, and at about the middle of its length, gives off a thick nerve for the use of the mandibles and their muscles. United, they form a first-sub-cervicalganglion that distributes nerves to the maxillre and the footjaws(!); they afterwards continue approximated throughout their length, presenting eleven successive ganglions, each of the five first furnishing nerves to as many pairs of feet, and the remaining. six those of the tail; that of the Pagurus has some ganglions less, thus appearing to form the passage from the Brachyura to the Macroura. M. Serres thinks that he has recognized in these Decapoda, vestiges of the great sympathetic(~). The lateral margin of the shell is bent under, to cover and protect the branchire, leaving an opening anteriorly for the (1) According to M. Stt'aus, the anterior division of the body of the Limuli, that which is covered by o. semi-lunar buckler, presents, besides the brain, no other ganglion but this, whence we may infer that the inferior organs of locomotion correspond to the parts of the mouth in the Decapoda, Stomapoda, and even in the Arachnides, and that those of the other division of the body, or of the second buckler, are analogous to the feet of the same Decapoda. (2) Messrs Audouin and Edwards have observed in the Maia and in the Palinu· rus a nerve analogous to the one called by Lyonet, in his .llnatomie de la Chenille du Saule, "recurrent." The discovery of the other gastric nerves is also uue to them. DECAPbDA. 17 passage of water. Sometimes,-see Dm·ippe-the _posterior and inferior extremity of the thorax has two pecuhar apertures for that purpose. The branchire are situated at the origin of the last four foot-jaws and feet; the four anterior ones have less extent. The six foot-jaw~ .are all of a different form, are applied to the mouth, and divided into two branches, the exterior of which resembles a sma11 antenna, formed of a pedicle, and a setaceous and pluri-articulate stem-it has been compared to a whip, palpus jlagelliformis(l ). The two anterior feet, and sometimes the two or four fo11owing ones, are in the form of claws. The penultimate joint is dilated, compressed, and in the form of a hand; its inferior extremity is lengthened into a conical point, representing a sort of finger, opposed to another formed by the last joint, or the tarsus proper. This one(2) is movable, and has received the name of thumb-jJollex; the other is fixed, and considered as the index-index. These two fingers are also called mordaces. The last is sometimes very short, and has the form of a simple tooth; in this case the other is bent underneath. The hand with the fingers constitutes om• forceps properly so called. The preceding, or antepenultimate joi~t is termed carpus. The respective proportions and the direction of the organs of locomotion are such, that these animals can walk sideways or backwards. With the exception of the rectum, which opens at the end of the tai1(3), all the viscera are contained in the thorax, so that this portion of the body represents the thorax and the greater part of the abdomen of insects. The stomach, sup- ( 1) There is a long, tendinous and hairy lamina at its base. (2) The hand being placed on its edge, the finger is uppermost. (3) This suite of segments which, in the Crustacea of the first orders, immediately succeed those to which the five last pairs of feet are attached, compose what I have termed the post-abdomen. The appellation of tail usually affixed to it, and wbicl1, in order to accommodate ourselves to common parlance, we have retained, is very improper; it can only apply to the posterior terminal appendages of the body which extend considerably beyond it. See my Fam. Nat. clu Regne Anim., p. 255, et seq. VoL. III.-C |
