Limulidae

Limulidae is the only living family of the order Xiphosura within the chelicerate subphylum, the family properly called horseshoe crabs, though despite the name its members are not true crabs or crustaceans at all but chelicerates more closely related to spiders, ticks, and scorpions. Only three genera and four species survive: Limulus polyphemus, the Atlantic horseshoe crab; Carcinoscorpius rotundicauda, the mangrove horseshoe crab; and two species of Tachypleus, T. tridentatus, the tri-spine horseshoe crab, and T. gigas, the Indo-Pacific horseshoe crab. The body is divided into three parts, a horseshoe-shaped cephalothorax carrying most of the sensory organs and limbs, a fused abdominal section, and a mobile, tail-like telson, and the animals breathe through book gills using six pairs of appendages. Limulids are often described as living fossils, having remained relatively unchanged for roughly 250 million years.