ARA'CIINIDA, a class of animals including Spiders, Mites, and Scorpions, all ranked by Linnaeus under Insects, but which are very properly separated from them, on account of external form, structure, and habits. The separation was first made, we believe, by Enbricius, who, looking chiefly to the structure of the mouth, characterised the greater number of the animals now ranked under Arachnida, by the jaws (maxilla) being horny and furnished with a claw (unogata). uL Lamarck afterwards made the A ruehnida a distinct class; but we owe to M. Latreillo and Dr. Leach the establishment of characters Inure precise, and extending to a greater number of genera. .Much has been done in perfecting the knowledge of their structure, manners, and numerous species by Clerck, De Geer, Walckenaer, Trevirnnus, Leon Dufour, Iferold, Straus-Dfirekheirri, Blackwall, and others. Wo shall condense into as short a compass 11.9 we can the most important points investigated by these naturalists.
The Arachnida (Accra, Virey) differ from Insects in having no antenme; in the eyes being in most species 8, and, even when only two in number, never being placed laterally on the head ; in the legs being usually 8, though in some species 6, and in others 10; and in their respiratory apparatus consisting of radiated trachea, communi cating with a sort of gills inclosed in pouches in the lower part of the abdomen.
The skin or crust of the Arachnida is in general more leathery than horny; but whether it be soft, as in most species, or hard, as in a few, it performs a similar office to the bones of larger =finale in giving support to the soft parts and attachment to muscles, the legs being joined upon and radiating from a common breast-pinto (sternum) externally; while, according to Stmus-Diirckheiln, there is also an internal breast-plato of a gristly texture (un sternum cartilogincux inttrieur) in form of a horse-shoo, the two ends of which are directed forwards.
The greater number of the A rachnid4 are carnivorous, and aro furnished with appropriate organs for their predatory life. Some parasitic species, such as the minute Parasite Mites (Leeti, De Geer), which we have observed infesting numerous species of insects, from the largest butterflies to the smallest gnats, are furnished with it sucker, in some respects constructed like that of the Gadfly (Tabanus).
In other species, there may be distinguished a pair of upper jaws (mandibula), a pair of under jaws (ma.rilla), carrying jointed feelers (palpi), and between them a sort of tongue formed by a projeCtion from the breast At the back part of the mouth is placed a piece of horny texture, which Savigny, Latreillo, and Audouin term tiro pharynx, forming the entrance into the gullet The gullet, together with a bulging on the fore part of it, termed the stomach, as well as the intestines, run in a straight line from the pharynx to the vent. Near the upper portion of the gullet are found salivary vessels, whose exterior aperture is in the first joint of the upper jaws. The saliva secreted by those vessels appears to be poisonous. Lower down are the binary vessels, which resemble those of insecta.
In the greater number of Arachnida there is a complete and very distinct circulatory system. The heart, which diMrs materially from the dorsal vessel by some termed the heart in insects, occu pies the abdomen, and its pulsations may be distinguished externally. It is a thick longitudinal vessel, giving origin to a certain number of arteries, and receiving veins by which the blood returns from the respiratory organs in other parts of the body.
The respiratory organs have two striking peculiarities, upon which Latreille founded his two great divisious of Araehnido,.
The division furnished with air-pipes, similar to those of insects, comprises Harvest- or Shepherd-Spiders (Phalangia), Mite.s, and several other genera. " The presence of air-pipes (trach4es)," says M. Latreille, "excludes all complete circulation, that is, the distribu tion of blood to different parts, and its return from the respiratory organs to the heart." The other division of the class comprises the numerous species of Spiders, and the Scorpions, which M. Straus-Diirckheim and Leon Dufour place first. Their respiratory apparatus consists of small cavities formed by the union of a great number of triangular white lamina of extreme thinness. The number of these is usually two, but in some species there are four, and in others eight. -The external apertures of these, termed spiracles, and, as M. Latreille well remarks, objectionably stigmata, are transverse chinks, corresponding in number with the pulmonary pouches.