Merostomata is a class of marine Chelicerata which inclues horseshoe crabs and eurypterids. It includes only four living species but dozens of fossil species mainly from the Paleozoic. Some favor the abandonment of the term class Merostomata, in favour of splitting the class into two classes, Xiphosura and Eurypterida. This is because Merostomata is thought to be paraphyletic, with the Eurypterida more closely related to the Arachnida, forming the group Cryptopneustida (Boudreaux, 1979). This change has not been incorporated into most textbooks, which typically use the traditional "Merostomata" without reference to the Eurypterida.