HORSEFOOT CRAB, HORSE-SHOE CRAB, or KING CRAB. This marine animal (Limulus polyphemus) was formerly regarded as a crustacean, and is the sole survivor of an extinct group of arthropods intermediate be tween the trilobites and arachnids. It belongs to the order Xiphosura, class Merostomata, and phylum Pahropoda. By some English authors it is regarded as an arachnid allied to the scor pion. This difference of opinion regarding its affinities is due to the generalized structure of the animal, and to the fact that its nearest allies are extinct.
The body of the horsefoot crab is sometimes two feet in length, and consists of a head and a hind-body or abdomen, the latter ending in a long spine (telson), which is elevated by the creature in defense. The head is covered by a broad carapace, and is in shape somewhat like a horse's hoof, and in burrowing it acts as a shovel, being bent down at nearly right angles to the hind-body. There are a pair of compound and of simple eyes; the mouth is on the under side, nearly surrounded by six pairs of walking legs, while on the hind-body are six pairs of broad swimming legs. There are no antenna,jaws, maxilla or foot-jaws, as in the lobster. The horsefoot crab breathes by means of gills attached to the under side of the last five pairs of abdominal legs, which consist of a pile of about 100 thin broad sacs growing out, one pile on each side, from the base of the legs. The nervous system is peculiar from the nature of the brain, and the cesophageal ring; while the entire system behind the brain is enveloped by the arteries, the latter ending in remarkably fine branches. The heart is large, tubular, the liver very voluminous, and the kidneys are represented by four pairs of excretory red glands, arising from a stolon-like base. The animal is bisexual, the male differing from the female in the second pair of legs ending not in a forceps, but in a sort of hand, with an opposing thumb. The ovaries and testes are
voluminous, and the sexual products, eggs and sperm, pass out through a pair of papilla sit uated on the under side of the first pair of abdominal legs.
The female lays her large round eggs loosely in the sand between high and low water, spawn ing in May and June; in about a month they hatch, and the young, after passing late in embryonic life through a trilobite stage, as sumes the form of the parent, differing in the short rudimentary caudal spine. It molts fre quently, and during the process the front edge of the carapace or head splits open, enabling the animal to draw itself out of the old shell. The recently hatched Limulus is strikingly like a trilobite, but while in the latter new segments are added after birth, in Limulus no new ones are added. The young horsefoot is about four millimeters in length. Specimens an inch long are about a year old, and it probably requires several years to grow to the length of a foot or more.
Limulus polyphemus inhabits the eastern coast of North America from Boothbay, Me., to the West Indies and Honduras, but is most abundant in shallow, retired, sandy or muddy bays on the coast of New Jersey, Virginia and North Carolina. Seieral other species inhabit the seas of the Eastern Archipelago, China, Philippines and southern Japan. In the United States it is used as a fertilizer, while in the Malayan markets the animals are sold as food.
The Limuli date from the Devonian. An allied group, in shape and structure approach scorpions, is the Eurypterida, one of which (Stylonurus laccoanus) of the Devonian of New York and Pennsylvania was about five feet in length, while the British Pterygotus anglicus is estimated to have been about six feet in length and two feet across. It is now thought that the scorpions have descended from some merostome, which became adapted for a terrestrial life. See EURYPTERUS.