<Ul> <Li> Acari </Li> <Li> Amblypygi </Li> <Li> Araneae </Li> <Li> † Haptopoda </Li> <Li> Opiliones </Li> <Li> Palpigradi </Li> <Li> † Phalangiotarbi </Li> <Li> Pseudoscorpiones </Li> <Li> Ricinulei </Li> <Li> Schizomida </Li> <Li> Scorpiones </Li> <Li> Solifugae </Li> <Li> Thelyphonida </Li> <Li> † Trigonotarbida </Li> <Li> † Uraraneida </Li> </Ul> <P> Arachnids are a class (Arachnida) of joint - legged invertebrate animals (arthropods), in the subphylum Chelicerata . All arachnids have eight legs, although the front pair of legs in some species has converted to a sensory function, while in other species, different appendages can grow large enough to take on the appearance of extra pairs of legs . The term is derived from the Greek word ἀράχνη (aráchnē), from the myth of the hubristic human weaver Arachne who was turned into a spider . Spiders are the largest order in the class, which also includes scorpions, ticks, mites, harvestmen, and solifuges . </P> <P> Almost all extant arachnids are terrestrial, living mainly on land . However, some inhabit freshwater environments and, with the exception of the pelagic zone, marine environments as well . They comprise over 100,000 named species . </P> <P> Almost all adult arachnids have eight legs, and arachnids may be easily distinguished from insects by this fact, since insects have six legs . However, arachnids also have two further pairs of appendages that have become adapted for feeding, defense, and sensory perception . The first pair, the chelicerae, serve in feeding and defense . The next pair of appendages, the pedipalps, have been adapted for feeding, locomotion, and / or reproductive functions . In Solifugae, the palps are quite leg - like, so that these animals appear to have ten legs . The larvae of mites and Ricinulei have only six legs; a fourth pair usually appears when they moult into nymphs . However, mites are variable: as well as eight, there are adult mites with six or even four legs . </P>

How many different species of arachnids are there