terrestrial invertebrate arachnid Arachnida 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. Spiders are the largest order in the class, which also includes scorpions, ticks, mites, harvestmen, and solifuges [https://en.wikipedia.org/wiki/Arachnid] arachnida requires discussion