Spider

Arachnids are a class (Arachnida) of joint-legged invertebrate animals in the subphylum Chelicerata. All arachnids have eight legs, although in some species the front pair may convert to a sensory function, while in other species, different appendages may grow large enough so as to appear like an extra pair of legs. The term is derived from the Greek words (aráchne), 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.

http://en.wikipedia.org/wiki/Arachnid