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Show 160 AliACUNIDES. the Arachnides, where the mouth has the form of a siphon or sucker, by two pointed blades which act as lancets( I). A kind of lip-labium, Fab.---or rather ligula, produced by a pectoral prolongation; two jaws formed by the radical joint of the first segment of two small legs or palpi(~), or by an ap· }lendage or lobe of that same joint; a part concealed under the mandibles, called langue sternale by Savigny-descrip· tion and figure of the Plwlangium copticum-and composed of a projection in the form of a rostrum, produced by the union of a very small clypeus terminated by an extremely small tri· angular labrum, and of an inferior longitudinal carina, usually very hairy, are the parts, which, with the pieces termed mandibles, constitute with some modifications the mouth of most of the Arachnides. The pharynx(3) is placed before a sternal projection which has been considered as a lip, but which, from being placed directly behind the pharynx and having no palpi, is rather a ligula. The legs, like those of ( 1) Chelicerce, or forcep&-antennce: the evident result of the comparison between them and the intermediate antenna: of various Crustacea, those of the Pzcilopoda particularly. It cannot then be said, strictly speaking, that the Arachnides are deprived of antenna:, a negative character, which, previous to us, had been ex· elusively attributed to them. (2) They only differ from legs properly so called, by their tarsi, which are composed of a single joint, and are usually terminated by a small hook, resem· bling, in a word, the ordinary feet of the Crustacea. See our general observa· tions on the first order. These jaws and palpi appear to correspond to the pal· pigerous mandibles of the Decapoda and to the two anterior feet of the Limuli. In Phalangium, the four following legs have a maxillary appendage at their origin, so that these four appendages are analogous to the four jaws of the preceding animals. 1 had described these parts, long before the publication of Savigny's memoirs on the invertebrate animals, in a monograph of the species of this genus proper to France. From these and the preceding observations it is evi· dent that the composition of these animals is easily reduced to the same general type which characterizes all articulated animals with articulated feet. The Aracb· nides are not then a sort of aceph:1lous Crustacea, as stated by this savant, usually so exact in his anatomical observations, of which, unfortunately for the sciences, he has become the victim. ( 3) Alt.hough Savigny admits of two orifices, neither Straus nor myself can find but .one; ~t must have been the effect of an optical illusion, arising from the fact of h1s havmg only perceived the lateral extremities of the fissure, its middle being concealed by the tongue with which its ant«:'rim· face is thickened in its medial~ portion. A RACHNIDES. ' 161 insects, are commonly terminated by two hooks, and even sometimes by one more, and are all annexed to the thorax, or rather cepha1o·thorax, which, except in a small number, is only formed of a single segment and is frequently intimately united to the abdomen. This latter part of the body is soft, or but slightly defended, in most of them. With respect to their nervous system, the Arachnides are greatly removed from the Crustacea and Insects; for if we except the Scorpions, which from the knots or joints forming their tail have some additional ganglions, the number of these enlargements of the two nervous cords is never more than three, and even in the latter, all counted, it never extends beyond seven. Most of' the Arachnides feed on Insects which they either seize alive, or to which they adhere, abstracting their fluids by suction. Others are parasitical, and live on vertebrated animals. Some of them however are only found in flour, on cheese, and even on various vegetables. Those which live on other animals frequently multiply there to a great extent. Two of the legs, in some species, are only developed by a change of the tegument, and in general it is not until the fourth or fifth change of skin that these animals are capable of propagation(!). Division of the .IJ.rachnides into orders. Some have pulmonary sacs(2), a heart with very distinct vessels, and six or eight simple eyes. They compose our first order, or that of the PuLMONARIJE. The others respire by trachere, and have no organs of cir· (1) \Ve have seen, according to the observations of Jm·ine, Jun., that they only aeq~ire this faculty after their sixth change. This fact is also applicable to the Lepidoptera, and probably to other inse<(ts that frequently cast their skin, for ca:erpil~ars .usually change it four times before they enter into the state of a chryaalls Which 1s a fifth. The insect does not become perfect until after another, so that it changes its skin six times. ~2) Sacs containing air-branchiro, or fulfilling the functions of lungs, nnd clistingwshed by me from the latter by the name of pn~Umo-branc!Lice. VoL. lll.-V |
