LimuloideaLim`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.