Xiphosura (/zɪfoʊˈsjʊərə/;[2] from Ancient Greek ξίφος (xíphos) 'sword' and οὐρά (ourá) 'tail', in reference to its sword-like telson) is an order of arthropods related to arachnids.

The group has hardly changed in appearance in hundreds of millions of years; the modern horseshoe crabs look almost identical to prehistoric genera and are considered to be living fossils.

The most notable difference between ancient and modern forms is that the abdominal segments in present species are fused into a single unit in adults.

[5] Several recent phylogenomic studies place Xiphosura within Arachnida, often as the sister group of Ricinulei; included among them are taxonomically comprehensive analyses of both morphology and genomes, which have recovered Merostomata as a derived clade of arachnids.

The mouth is located on underside of the center of the prosoma, between the bases of the walking legs, and lies behind a lip-like structure called the labrum.

The fifth and final pair of legs, however, has no pincers or spines, instead having structures for cleaning the gills and pushing mud out of the way while burrowing.

Behind the walking legs is a sixth set of appendages, the chilaria, which are greatly reduced in size and covered in hairs and spines.

[10] The opisthosoma terminates in a long caudal spine, commonly referred to as a telson (though this same term is also used for a different structure in crustaceans).

The stomach secretes digestive enzymes, and is attached to an intestine and two large caeca that extend through much of the body, and absorb the nutrients from the food.

[10] Xiphosurans have well-developed circulatory systems, with numerous arteries that send blood from the long tubular heart to the body tissues, and then to two longitudinal sinuses next to the gills.

[15][16] Through a series of successive moults, the larva develops additional gills, increases the length of its caudal spine, and gradually assumes the adult form.

[1] Carcinoscorpius rotundicauda (Mangrove horseshoe crab) Tachypleus gigas (Indo-Pacific horseshoe crab) †Tachypleus decheni †Tachypleus syriacus Tachypleus tridentatus (tri-spine horseshoe crab) †Heterolimulus gadeai †Volanalimulus madagascarensis Limulus polyphemus (Atlantic horseshoe crab) †Limulus coffini †Crenatolimulus paluxyensis †Crenatolimulus darwini †Keuperlimulus vicensis †Casterolimulus kletti †Victalimulus mcqueeni †Allolimulus woodwardi †Mesolimulus crespelli †Mesolimulus walchi †Mesolimulus tafraoutensis †Mesolimulus sibiricus †Tarracolimulus rieki †Yunnanolimulus henkeli †Yunnanolimulus luopingensis †Boeotiaspis longispinus †Shpineviolimulus jakovlevi †Panduralimulus babcocki †Tasmaniolimulus patersoni †Limulitella bronni †Limulitella tejraensis †Psammolimulus gottingensis †Batracholimulus fuchsbergensis †Vaderlimulus tricki †Austrolimulus fletcheri †Dubbolimulus peetae †Valloisella lievinensis †Norilimulus woodae †Xaniopyramis linseyi †Moravurus rehori †Paleolimulus kunguricus †Paleolimulus signatus †Rolfeia fouldenensis †Bellinuroopsis rossicus †Prestwichianella anthrax †Prestwichianella mariae †Prestwichianella rotundatus †Liomesaspis laevis †Anacontium carpenteri †Pringlia birtwelli †Stilpnocephalus pontebbanus †Alanops magnifica †Andersoniella sp.