Xiphosura, a group of Arthropods (q.v.), the members of which have a shield-shaped carapace or shield covering the head and thorax (or cephalo-thorax); the body terminates behind in a long abdominal spine or telson. The abdominal region is not marked off into two distinct regions, as in many allied animals. On the anterior part of it, which must correspond to the mesosoma, there are six pairs of limbs. The first pair of these form an operculum which protects the reproductive organs. The remaining five pairs are flat plates with a series of folds on their hinder surfaces. These act as the respiratory organs. The first pair of the appendages round the mouth are clawed. The order at the present day is represented only by the Limulus, or King Crab, species of which occur in the seas of Malaysia and on the East Coast of America and Asia. They are of considerable geological interest, especially in the Paleozoic period.