Arachnida
Arachnida A class of terrestrial arthropods of the phylum Chelicerata, comprising about 65 000 species and including spiders, scorpions, harvestmen, ticks, and mites. An arachnid's body is divided into an anterior prosoma and a posterior opisthosoma. The prosoma bears chelicerae, pedipalps, and four pairs of walking legs. The opisthosoma may bear various sensory or silk-spinning appendages (see spinneret). Arachnids are generally carnivorous, feeding on the body fluids of their prey or secreting enzymes to digest prey externally. Spiders immobilize their prey with poison injected by the fanglike chelicerae, while scorpions grasp their prey in large clawed pedipalps and may poison it using the posterior stinging organ. Ticks and some mites are parasitic but most arachnids are free-living. They breathe either via tracheae (like insects) or by means of thin highly folded regions of the body wall called lung books. See also Acarina.
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Arachnida
