Horseshoe Crab

Horseshoe crabs are arthropods that live primarily in shallow ocean waters on soft sandy or muddy bottoms. They will occasionally come on shore for mating. They are commonly used as bait and in fertilizer, and in recent years there has been a decline in number of individuals, as a consequence of coastal habitat destruction in Japan and overharvesting along the east coast of North America. Tetrodotoxin may be present in the roe of species inhabiting the waters of Thailand. Horseshoe crabs are considered living fossils.
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.