King Crab, Or Horse-Shoe Crab, a common name for the limuloid group of the entomos-tracan order of crustaceans, from their large size and peculiar form. This order is the lowest of the class, as the segments and feet are fewer than in the other orders. In the genus limulus (xiphosura, Milne-Edwards), the tail is reduced to a mere spine, and the bases of the first six pairs of legs, being rough with points, perform the functions of jaws, their free extremities ending in nipping claws. The whole upper surface is protected by a kind of buckler, made up of an anterior semicircular shield, and a posterior hexagonal plate, to the hinder margin of which is jointed the long sharp spine of the tail; the branchial appendages are on the under surface of the posterior plate. The Molucca king crab attains a size of 2 to 3 ft., and both eggs and flesh are eaten by the Malays; the spine attached to a spear makes a formidable weapon. Our common species (limulus polyphenols) also grows very large on the Atlantic coast of the middle states, and is of a blackish brown color; its flesh is sometimes given to pigs and poultry, but, while it fattens them, imparts a bad flavor to their meat; on the New England coast the size is small, and their delicate yellowish cast-off shells are frequently thrown upon the beaches.
The legs are feeble, and the use of the tail seems to be to enable it to turn over by a kind of spring, should its wide flat body be thrown by the waves upon its back; the anterior limbs in the male are short, stout, and swelled, with nippers for holding the female. The eggs, fertilized in summer as they are extruded, are placed in a hole excavated in the sand on the edge of high tide, the sand at once covering them; their hatching is thus aided by the heat of the sun until the tide rises again. The eggs of the king crabs are very tenacious of life. The extinct eurypteridce are closely allied, and some, as pterygotus, attained a length of 6 ft. Dr. Packard, in the " Memoirs and Proceedings of the Boston Society of Natural History," 1870-'72, and "Proceedings of the American Association for the Advancement of Science," 1870, has shown that the larva, which hatches in about six weeks, bears a striking resemblance to the trilobites; he therefore regards the pceciloptera, or the king crab and its allies, as a subdivision of the branchiopods.
Prof. Van Beneden, on the contrary (1872), thinks that the king crabs are not crustaceans, having none of their characteristic phases of development, but show the closest resemblance to scorpions and other arachnids; and that trilobites, eurypteridce and pcecilopoda, must form with the arachnids a distinct division. Considering the palaeozoic trilobites as the lowest, the next series in time and in rank would be such forms as eurypterus and pterygotus, the limulus or king crab being the highest, and beginning to appear as the trilobites were dying out; some forms of trilobites had a spiny tail like limulus.
King Crab - 1. Lower surface.
2. Upper surface.