ARACH'NIDA, or ARACIINIDES (from the Gr. arachne, a spider), a class of articulated animals, commonly regarded as intermediate between insects and crustacca. They were included by Linnaeus amongst insects, and placed in the order aptera. Like the crustacea, they have the head and thorax united into one piece, but they differ from them and from insects in having simple eyes, and in the absence of proper antenna, instead of which many of them are provided with a sort of autennal claws called chelicerce. These and other organs connected with a complex mouth, disappear, however, in some of the lower kinds, which have merely a sort of proboscis for suction. Some of them breathe by means of pulmonary cavities; others, by trachew, like insects; and upon this differ ence is founded the primary division of the class into two orders-utinnnaria and trachearia. Spiders and scorpions belong to the first of these orders, and mites, ticks,
etc. (awn), to the second. Some of the A. inhabit water, but their mode of respiration lis that of terrestrial animals; and they seem to carrair with them by means of the hair which covers their bodies. The sexes are distinct. They are oviparous. They have two or more eyes, very frequently eight; and the relative position of these affords marks for distinction of genera. They have generally eight legs, but some have only six. With the exception of the acari, they arc solitary in their mode of life, and most of them plrcy upon insects, of which, however, in general, they only suck the blood. Some of the lower kinds are parhsitia upon insects, and it few live on decaying animal and vegetable substances. See ACARUS, MITE, SCORPION, SPIDER, and TICE.