Class which includes the king crabs (horseshoe crabs) and the extinct eurypterids or ‘water scorpions’. Eurypterids were usually 10–20 cm in length, but some were much larger, e.g. Pterygotus which grew up to 2 m long. They ranged from the Ordovician to the end of the Palaeozoic. The horseshoe crabs also appeared in the Lower Palaeozoic and have survived to the present.