Chelicerata

The Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, horseshoe crabs, and arachnids (including scorpions, spiders, and mites), as well as a number of extinct groups, such as the eurypterids (sea scorpions) and chasmataspidids. The group originated in marine habitats but almost all modern chelicerates are terrestrial. Two living groups are marine, the horseshoe crabs and the sea spiders.