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Merostomata is a class of chelicerate arthropods that contains the extinct Eurypterida (sea scorpions) and the extant Xiphosura (horseshoe crabs). The term was originally used by James Dwight Dana to refer to Xiphosura only, but was emended by Henry Woodward to cover both groups.
Xiphosura were historically placed in the class Merostomata, although this term was intended to encompass also the eurypterids, whence it denoted what is now thought to be an unnatural (paraphyletic) group (although this is a grouping recovered in some recent cladistic analyses). [3]
Some authors, such as John Sterling Kingsley in 1894, classified the Merostomata as a sister group to the Arachnida under the class "Acerata" within a subphylum "Branchiata". Others, such as Ray Lankester in 1909, went further and classified the Merostomata as a subclass within the Arachnida, raised to the rank of class. [95]
Horseshoe crabs are arthropods of the family Limulidae and the only surviving xiphosurans.Despite their name, they are not true crabs or even crustaceans; they are chelicerates, more closely related to arachnids like spiders, ticks, and scorpions.
The subphylum Chelicerata (from Neo-Latin, from French chélicère, from Ancient Greek χηλή (khēlḗ) 'claw, chela' and κέρας (kéras) 'horn') [1] constitutes one of the major subdivisions of the phylum Arthropoda.
Until recently, eurypterids were thought to belong to the class Merostomata along with order Xiphosura. It is now believed that eurypterids are a sister group to Arachnida, closer to scorpions and spiders than to horseshoe crabs. [20] [27] [28] Eurypterus was the first recognized taxon of eurypterids and is the most common.
Limulus is a genus of horseshoe crab, with one extant species, the Atlantic horseshoe crab (Limulus polyphemus).One fossil species is currently assigned to the genus though several other species have been named, which have since been assigned to other genera.
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