Arachnids are a class (Arachnida) of joint-leggedinvertebrateanimals in the subphylumChelicerata. 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" (Oxford English Dictionary).
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 (Cracraft & Donoghue, 2004).
"Arachnid". Oxford English Dictionary (2nd ed.). 1989.
Joel Cracraft & Michael Donoghue, ed. (2004). Assembling the Tree of Life. Oxford University Press. p. 297.
Barcode of Life Data Systems (BOLD) Stats Specimen Records:187081 Specimens with Sequences:134737 Specimens with Barcodes:127132 Species:8825 Species With Barcodes:7757 Public Records:118792 Public Species:4327 Public BINs:14447