OCR Text |
Show 162 ARACHNIDES. culation, or if they have, the circulation is not complete. The trachero are divided near their origin into v~rious branches, and do not, as in Insects, form two trunks whiCh run parallel to each other throughout the whole length of the body and receive air from various points by means of numerous stigmata. Here, but two, at most, are distinctly visible, and they are situated near the base of the abdomen(l ). The · number of si~ple eyes is at most but four. They constitute our second and ]ast order, or that of the TRACHEARI.tE. ORDER I. PULMONARLE(2). we here find a well marked circulating system and pulmonary sacs, always placed under the abdomen, announced ex· tern ally by transverse openings or fissures (stigmata), of which there are sometimes eight, four on each side, and at others four, or even two. The number of simple eyes is from six to eight(3), while in the following order it never exceeds four, and is most generally but two; sometimes they are hardly per· ceptible, or even annihilated. The organ of respiration is formed of little laminre. The heart is a large vessel which ( 1) The Pycnogonides exhibit no stigmata, and seem, in this respect, to approach the last of the Crustacea, such as Dichelestium, Cecrops, and other Siphonostomou1 Entomostraca. Savigny thinks they have a closer affinity to the Lremodipoda, from which, however, they are greatly removed, by the organization of the mouth as well as by their eyes and feet. We still believe, however, from the en· semble of their characters, that they rather belong to the class of Arachnides,and that they approximate particularly to Phalangium with which various autbon have arranged them. We also think that they may respire by the surfaceoftbeir skin. At all events, we must await the results of anatomical investigation, be· fore we can decide. (2) UNOGA.TA, Vab. (3) The Teasaropa of Rafin., according to him, has but four eyes; I presume, however, that the lateral ones escaped his notice. See the subgenus ErCIUI· PULMONARI.IE. 163 extends along the back, and gives off branches on each side and anteriorly(!). There are always eight legs. The head is always confounded with the thorax, and presents at its anterior superior extremity two mandibles-so called by authors, the chelicerm or antenne-pinces, Latr.-terminated by two fingers, one of which is movable, or by a Ringle one resembling a hook or claw that is always movable(2). The mouth is composed of a labrum(3), of two palpi, sometimes resembling arms or claws, of the two or four jaws, formed, when there are but two, by the radical joint of these palpi, and moreover, when there are four, by the same joint of the first pair of feet, and of a ligula consisting of one or two pieces(4). If we base our arrangement on the progressive decrease of the number of pulmonary sacs and stigmata, the Scorpions where it is eight, while in the other Arachnides it amounts to but four or two, should form the first genus of this class, and consequently our family of the Pedipalpi should precede that of the Araneides(5). But the latter Arachnides (1) According to Ma1·sel de Serres, Me moire sur le Vaisseau Dorsale des lnsectes, the blood, in the Araneides and Scorpions, is first directed to the organs of respira· tion, and thence proceeds to various parts of the body through particular vessels. Judging, however, ft•om the affinity of these animals to the Crustacea, the circulation would seem to be effected in the contrary direction. See the Memoir of Treviranus on the Anatomy of Spiders and Sc01·pions. (2) These pat·ts are formed of a first very large and ventricose joint, one of whose superior angles, when the chela: are didactyle, forms the fixed finger, and of a second joint, that which forms the opposite and movable finger or the hook, when there is but one finger. In the latter case, as with several of the Crustacea, I will employ the term claw. (3) See our general observations on the class. · (4) That of the Scorpions appears to be composed of four pieces, forming o.n elongated and pointed triangle, directed forwards; the two lateral ones howe,·er are evidently fot·med by the first joint of the two anterior feet, and may be consider; d as two jaws analogous to the first. We see by My gale, Scorpio, &c. that the palpi are divided into six joints, of whi.ch, in the other Araneides, the first or radical one, is anteriorly and internally dilated to form the maxilliform lobe. Even this lobe, in some species, is articulated at base, and thus becomes a maxillary appendage of this same joint. Excluaive of this joint, the palpus consists of but five, and such is the most usual mode of supputation. In the Scorpions the movable finger of the forceps, as in that of the Crustacea, forms the sixth joint. (5) In my Fam. Nat. du Regne Animal, I begin with the Pedipalpi. M. Leon Dufour also thinks that tha Scorpions should come first. • |