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a class of predominantly fossil marine arthropods, related to the trilobites and arachnids. (Merostomata are joined with the arachnids in the subphylum Chelicerata.) Respiration is by branchial gills. The body is covered with a chitinous carapace and is divided into a cephalothorax and a spiculate abdomen, each bearing six pairs of appendages (the abdominal ones have branchial plates). There are three or four orders. Two or three orders are traced to the Cambrian and Devonian, while one—Xiphosura—appeared in the Silurian and exists now. The extinct order Eurypterida is often assigned to Merostomata.