Arachnids are a class (Arachnida) of joint-legged invertebrate animals 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), meaning "spider".
Almost all extant arachnids are terrestrial. However, some inhabit freshwater environments and, with the exception of the pelagic zone, marine environments as well. They comprise over 100,000 named species, including spiders, scorpions, harvestmen, ticks, mites and Solifugae.
Ever since contact was made with a spider like sentient species (that had no distinctive name for themselves other than "People" or "Not Prey" recieved the species designation Archas ) they are generally referred to as Arachnids or Arachnoid (spider like) . (It is not the same as an old Pre Astro Medical Condition of Menigitis called Arachnoid.)