Taxon Description (Wikipedia)
Merostomata is a class of marine Chelicerata which includes 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 Metastomata (Weygoldt & Paulus 1979) or Cryptopneustida (Boudreaux, 1979). This change has not been incorporated into most textbooks, which typically use the traditional "Merostomata" without reference to the Eurypterida.
The Merostomata are the only chelicerates to possess compound eyes; the arachnids have lost the ancestral...
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