- a large marine arthropod, Limulus polyphemus, of shallow coastal waters of eastern North America and eastern Asia, having both compound and simple eyes, book gills, a stiff tail, and a brown carapace curved like a horseshoe: a living fossil related to the woodlouse.
- any marine chelicerate arthropod of the genus Limulus, of North America and Asia, having a rounded heavily armoured body with a long pointed tail: class Merostomata
