There are only 3 genera and 5 species of Xiphosura left alive today, but they were much more numerous and diverse during the Palaeozoic era.  The surviving horseshoe crabs (Limulus) are 'living fossils', barely changed in some 250 million years (since early Triassic time).  Members of this class have a large shield that covers the cephalothorax; the carapace is hinged between the cephalothorax and abdomen.  The exoskeleton generally consists of three parts, the large, semicircular cephalothorax, or prosoma, the usually smaller, subtriangular and in earlier forms "trilobite"-like opisthosoma, and the long stout tail-spine or telson (which is actually the end part of the opisthosoma).

The prosoma contains both head and visceral organs. The compound eyes are small (and absent in some early forms), and there are six pairs of legs (in the living Limulus) but no antennae.  The second pair of appendages, the pedipalps, resemble walking legs. Respiration is via 5 pairs of book gills, the flaps of which beat in a metachronal rhythm to produce a vigorous current.  Recent xiphosurids (Horseshoe crabs) feed on worms and other small invertebrates.  They are often used as laboratory animals by physiologists.  It has been argued that because of their unique status as prehistoric "living fossils" they deserve special conservation status.