Horseshoe crabs belong to class
Horseshoe crabs family Limulidae traditionally placed in class Merostomata along with extinct eurypterids, characterized by horseshoe-shaped prosomal carapace, compound lateral eyes, median ocelli, opisthosoma bearing book gills for aquatic respiration, and long telson spine for righting. Despite common name crab they are not crustaceans but marine chelicerates more closely allied to arachnids, molecular data nesting them within Arachnida as Xiphosura. Arachnida includes spiders and scorpions with book lungs, Insecta hexapods, Crustacea biramous appendages. Merostomate status historically reflects marine chelicerate identity with gill respiration distinguishing from terrestrial arachnids.
Ref: Brusca & Brusca, Invertebrates, 3rd ed., Merostomata Limulidae; NCBI Bookshelf, Horseshoe Crab Classification