Chelicerata

The subphylum (or phylum) Chelicerata (/kəˌlɪsəˈreɪtə/ or /kəˌlɪsəˈrɑːtə/; New Latin, from French chélicère, from chél-; chela + -cère from the Greek keras, meaning "horn") constitutes one of the major subdivisions of the phylum (or superphylum) Arthropoda, and includes horseshoe crabs, scorpions, spiders and mites. They originated as marine animals, possibly in the Cambrian period, but the first confirmed chelicerate fossils, eurypterids, date from 445 million years ago in the Late Ordovician period. The surviving marine species include the four species of xiphosurans (horseshoe crabs), and possibly the 1,300 species of pycnogonids (sea spiders), if the latter are chelicerates. On the other hand, there are over 77,000 well-identified species of air-breathing chelicerates, and there may be about 500,000 unidentified species.