Merostomata

Class Merostomata Large marine chelicerates with book gills on the underside of the opisthosoma; prosoma covered by a dorsal carapace; opisthosoma bears a long terminal spine; 2 orders, Xiphosura (horseshoe crabs, 4 species) and Eurypterida (Gigantostraca), which is extinct and includes 200 fossil species from the…

…at all but modern terrestrial merostomes, the horseshoe crab (Limulus) being their closest living relative. In this view the Merostomata, including scorpions, are a group distinct from the arachnids. Whatever their exact taxonomic relationship, it is clear that scorpions form a distinct group consistently separated by taxonomists from other arachnids.

…small classes of chelicerates, the Merostomata, containing the horseshoe crabs, and the Pycnogonida, containing the sea spiders, are entirely marine. The merostomes are an ancient group and probably gave rise to the arachnids. Indeed, the earliest known fossil scorpions were aquatic.