Arachnids are a class of joint-legged invertebrate animals. 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 aráchnē, meaning "spider".
Almost all extant arachnids are terrestrial. However, some inhabit freshwater and marine environments marine environments as well. They comprise over 100,000 named species, including spiders, scorpions, harvestmen, ticks, mites and Solifugae.