Arachnida, a class of invertebrate animals belonging to the articulata, and including spiders, mites, and scorpions. The arachnida differ from insects in having no antennae; in the number of eyes being in most species 8, and, even when only 2, in never being placed laterally on the head; in the legs being usually 8, although in some species 6, and in others 10; and in their respiratory apparatus consisting of radiated tracheae Most arachnida are carnivorous. Some parasitic species, such as the minute parasite mites, are furnished with a sucker, in some respects constructed like that of the gadfly. In other species there is a pair of upper jaws and a pair of under jaws carrying jointed feelers, and between them a sort of tongue formed by a projection from the breast. At the back of the mouth there is a piece of horny texture, termed by Savigny, Latreille, and Audouin the pharynx, forming the entrance into the gullet. The gullet, the stomach, and the intestines run in a direct line from the pharynx to the vent. In most arach-nida there is a complete circulatory system of arteries and receiving veins, returning blood.
The respiratory organs have two peculiarities, on which Latreille established his two great divisions of araclmida, pulmonaria and trache-aria. The pulmonaria, which Straus-Durck-heim and Leon Dufour place in the first or chief division, comprises the numerous species of spiders and the scorpions. Their respiratory apparatus consists of small cavities formed by the union of triangular laminae of extreme thinness. The division furnished with air pipes (trachearia) similar to those of insects, comprises the harvest or shepherd spiders, mites, and other genera. "The presence of tracheae, or air pipes," says Latreille, "excludes all complete circulation; that is, the distribution of blood to different parts, and its return from the respiratory organs to the heart.'1 The eyes of the arachnida are all simple. In most species of spiders they are 8 in number, but in some they are 6 and in others only 2. Nothing is known of the organs of hearing in arachnida, although it has been well ascertained that these animals do hear. Male spiders are alway s much smaller than the females. The palpi or feelers of the male are furnished with organs of various forms, usually bulging at the tip; the feelers of the female gradually taper to a point.
The eggs of spiders, not having a hard shell, are soft and compressible. Before being laid, they lie in the egg bag, squeezed together and flattened, within the spider's body, but assume the globular form after being laid. The female spider, in preparing a nest for her eggs, uses her own body as a bird uses its body to give form and proper size to its nest. The eggs are excluded from a cavity just behind the breast. The hatching of the eggs of one species (tho'epeira diadema) has been traced with care, and the successive evolution of the embryo depicted with skill, by Moritz Herold of Marburg. - Latreille arranges the arachnida into two great orders, pulmonaria and trachearia. He subdivides the first order, A. pulmonaria, into two families, under the names of araneidœ and pedipalpi. The araneidœ include our common spiders, having palpi simple, pedi-form; mandibles armed with a movable and perforated claw, emitting a poisonous liquid; abdomen inarticulate, terminating by spinnerets. The pedipalpi, including the scorpion? and their allies, have the abdomen articulate, without spinnerets; palpi produced, cheliform (chela, claw), or shaped like pincers; mandibles with a movable digit.
The second order, trachearia, includes various forms of shepherd spiders and sea spiders, mites, and ticks; true mites, garden mites, spider mites, wood mites; true ticks, plant ticks, water ticks, harvest ticks; false scorpions, book scorpions; shepherd spiders, sea spiders, and parasitic sea spiders.