Horseshoe Crab

Classification
Horseshoe crabs resemble crustaceans, but belong to a separate subphylum, Chelicerata, and are therefore more closely related to spiders and scorpions. The earliest horseshoe crab fossils are found in strata from the late Ordovician period, roughly 450 million years ago.
Limulidae is the only recent family of the order Xiphosurida and contains all four living species of horseshoe crabs: