The Collaborative International Dictionary of English v.0.48:

Limuloidea \Lim`u*loi"de*a\ (l[i^]m`[-u]*loi"d[-e]*[.a]), n. pl.
   [NL. See Limulus, and -oid.] (Zool.)
   An order of Merostomata, including among living animals the
   genus Limulus, with various allied fossil genera, mostly of
   the Carboniferous period. Called also Xiphosura.
   [1913 Webster]

   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.
         [1913 Webster]