King Crabs

King-Crabs, an order of branchiate Arachnida (q.v.), known as the Xiphosura. They are of great interest, as they are the nearest living representatives of the extinct orders the Trilobita (q.v.) and Eurypterida (q.v.), and their geological affinities have been in the main determined by work upon this group. The body of the Common King-Crab (Limulus polyphemus) consists of three main regions. A large rounded shield protects the anterior end of the body (cephalothorax). This is continued back into two sharp angles, between which is a smaller area formed of the rest of the thorax and the abdomen (more correctly, the mesosoma and metasoma) fused together; this region protects the folded plate like gills or branchiae. The third region consists of a long, straight spine or telson. The appendages around the mouth are very characteristic of this order, as there are six pairs, which all end in claws, while the basal joint (or coxa) of each is modified to act as a jaw. At one time the King-Crabs were placed with the Crustacea, but their true affinities with the Arachnida are now generally recognised. In their development they pass through a stage much like the Trilobite Trinucleus, and there can be no doubt that this order and that of the great Eurypterids (q.v.) of the Silurian and Devonian periods must follow the King-Crabs into the Arachnida. The living forms are marine, and inhabit the Indian and Pacific Oceans and the Caribbean seas.