taxon_rank rhenopterus
An arthropod ( from Greek " arthro -", joint + " podos ", foot ) is an invertebrate animal having an exoskeleton ( external skeleton ), a segmented body , and jointed appendages ( paired appendages ). Arthropods ___MASK99___ the ___MASK96___ Arthropoda , which includes the insects , arachnids , myriapods , and crustaceans . Arthropods are characterized by their jointed limbs and cuticle made of chitin , often mineralised with calcium carbonate . The arthropod body plan consists of segments , each with a pair of appendages . The rigid cuticle inhibits growth , so arthropods replace it periodically by moulting . Their versatility has enabled them to become the most species - rich members of all ecological guilds in most environments . They have over a million described species , making up more than 80 % of all described living animal species , some of which , unlike most animals , are very successful in dry environments . Arachnids are a ___MASK28___ ( Arachnida ) of joint - legged invertebrate animals ( arthropods ), in the ___MASK1___ 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 (" aráchn "), meaning " spider ". Spiders are the largest order in the ___MASK28___ . Jaekelopterus (" Otto Jaekel ' s wing ) is an extinct ___MASK90___ of sea scorpion . " Jaekelopterus " lived approximately 390 million years ago . At an estimated length of , it is the largest known arthropod ever discovered , surpassing the " Pterygotus " and even the millipede - like " Arthropleura ". There are two species , one being the type species , " J . rhenaniae ", from freshwater strata in the Rhineland , and " J . howelli " from estuarine strata in Wyoming . Eurypterids ( sea scorpions ) are an extinct ___MASK50___ of arthropods related to arachnids that include the largest known arthropods to have ever lived . They are members of the extinct order Eurypterida ( Chelicerata ); which is the most diverse Paleozoic chelicerate order in terms of ___MASK8___ . The name Eurypterida comes from the Greek words ' ( meaning " broad " or " wide ") and ' ( meaning " wing "). This name was chosen due to the pair of wide swimming appendages on the first fossil eurypterids discovered . The largest , such as " Jaekelopterus ", reached in length , but most ___MASK8___ were less than . They were formidable predators that thrived in warm shallow water , in both seas and lakes , from the mid Ordovician to late Permian . Rhenopterus is an extinct prehistoric eurypterid . Rhenopterus lived in the Lower Devonian period in Europe and possibly North America . The Ordovician is a geologic period and system , the second of six periods of the Paleozoic Era . The Ordovician spans 41 . 2 million years from the end of the Cambrian Period million years ago ( Mya ) to the start of the Silurian Period Mya . The Ordovician , named after the Celtic ___MASK55___ of the Ordovices , was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison , who were placing the same rock beds in northern Wales into the Cambrian and Silurian periods , respectively . Lapworth , recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian periods , and placed them in a period of their own . It received international sanction in 1960 , when it was adopted as an official period of the Paleozoic Era by the International Geological Congress . Scorpions are predatory arachnids of the ___MASK52___ Scorpiones . They have eight legs and are easily recognized by the pair of grasping pedipalps and the narrow , segmented tail , often carried in a characteristic forward curve over the back , ending with a venomous stinger . Scorpions range in size from 9 mm / 0 . 3 in . (" Typhlochactas mitchelli ") to 23 cm / 9 in . (" Heterometrus swammerdami "). The ___MASK1___ Chelicerata ( or ; New Latin , from French " chélicère ", from Greek " khl " " claw , chela " and " kéras " " horn ") constitutes one of the major subdivisions of the phylum Arthropoda . It contains the horseshoe crabs , sea spiders , and arachnids ( including scorpions and spiders ).