What is "xiphosura"

Xiphura \Xi*phu"ra\, n. pl. [NL., from Gr. xi`fos sword + ? tail.] (Zo["o]l.) Same as Limuloidea. Called also Xiphosura.

Xiphosura \Xiph`o*su"ra\, n. pl. See Xiphura.

Limuloidea \Lim`u*loi"de*a\ (l[i^]m`[-u]*loi"d[-e]*[.a]), n. pl. [NL. See Limulus, and -oid.] (Zo["o]l.) An order of Merostomata, including among living animals the genus Limulus, with various allied fossil genera, mostly of the Carboniferous period. Called also {Xiphosura}.

Note: There are six pairs of leglike organs, surrounding the mouth, most of which terminate in claws; those of the first pair (probably mandibles) are the smallest; the others have the basal joints thickened and spinose, to serve as jaws, while the terminal joints serve as legs. This group is intermediate, in some characteristics, between crustaceans and certain arachnids (scorpions), but the respiration is by means of lamellate gills borne upon the five posterior abdominal appendages, which are flat and united in pairs by their inner edges, and are protected by the lidlike anterior pair, which also bear the genital orifices.

The Xiphosura are an order of marine chelicerates that includes a large number of extinct lineages and only four extant species in the family Limulidae, which include the horseshoe crabs. The group has hardly changed in millions of years; the modern horseshoe crabs look almost identical to prehistoric genera such as the Jurassic Mesolimulus, 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.

Xiphosura were traditionally placed in the class Merostomata, although this term was intended to encompass also the eurypterids, whence it denoted what is now known to be an unnatural ( paraphyletic) group (although this is a grouping recovered in some recent cladistic analyses). Although the name Merostomata is still seen in textbooks, without reference to the Eurypterida, some have urged that this usage should be discouraged. The Merostomata label originally did not include Eurypterida, although they were added in as a better understanding of the extinct group evolved (see Merostomata for additional details). However, as the similarities, which were once thought to tie the group together, are now understood to be symplesiomorphies, it seems that abandoning the term altogether to avoid any confusion arising from these historical usages of the term is the best strategy.