(also Arachnida), a class of invertebrates of the phylum Arthropoda. The class embraces approximately 35,000 species and includes the orders Scorpionida, Palpigradi, Pseudoscorpionoidea, Solifugae (Phalangida), Opiliones, Araneida, and Ricinulei; also assigned to the Arachnoidea are the groups Pedipalpi and Acarina.

The arachnids are from 0.1 mm to 17 cm long. The body is usually divided into a nonsegmented cephalothorax and an abdomen, which is only rarely fused. The cephalothorax bears six pairs of appendages: one pair of chelicerae (usually chelate), one pair of pedipalps, and four pairs of walking legs. Many arachnids (Scorpionida, Solifugae, Pseudoscorpionoidea, some Araneida, and some Pedipalpi) have a segmented abdomen. Most Araneida and many Acarina have an unsegmented abdomen, which is sometimes equipped with special appendages—modified abdominal legs (spinnerets in Araneida and pectines in Scorpionida).

Respiration is tracheal or pulmonary; only Dipneumonas and Araneomorphae have both lungs and tracheae. Some arachnids breathe through the surface of their body. Excretion is accomplished by the coxal glands and the Malpighian tubules. The nervous system in most arachnids consists of the supra-esophageal ganglia and the subesophageal nerve complex formed by the fusion of parts of the abdominal ganglia and by its unfused ganglia. In Araneida all abdominal ganglia fuse to form a common mass; in Acarina the abdominal ganglia fuse with the supra-esophageal ganglia. The sense organs consist of eyes and sensory hairs; some arachnids, such as Araneida, react to sounds and odors. Many arachnids, including Scorpionida, Araneida, and some Pseudoscorpionoidea, have poison glands. The sexes are separate. Most species deposit eggs, while some are viviparous.

Arachnids are distributed on all continents. They usually inhabit dry land; only a few species have adopted aquatic life (water spider, water mites). Most arachnids are predators, feeding on other arthropods, worms, mollusks, and small reptiles. The invertebrates are ancient terrestrial arthropods. They are related to Eurypterida; they too trace their ancestry from trilo-bites. The oldest arachnids are known from the Upper Silurian.