1911 Encyclopædia Britannica/King-Crab

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KING-CRAB, the name given to an Arachnid, belonging to the order Xiphosurae, of the grade Delobranchia or Hydropneustea. King-crabs, of which four, possibly five, existing species are known, were formerly referred to the genus Limulus, a name still applied to them in all zoological textbooks. It has recently been shown, however, that the structural differences between some of the species are sufficiently numerous and important to warrant the recognition of three genera—Xiphosura, of which Limulus is a synonym, Tachypleus and Carcinoscorpius. In Xiphosura the genital operculum structurally resembles the gill-bearing appendages in that the inner branches consist of three distinct segments, the distal of which is lobate and projects freely beyond the margin of the adjacent distal segment of the outer branch; the entosternite (see Arachnida) has two pairs of antero-lateral processes, and in the male only the ambulatory appendages of the second pair are modified as claspers. In Tachypleus and Carcinoscorpius, on the other hand, the genital operculum differs from the gill-bearing appendages in that the inner branches consist of two segments, the distal of which are apically pointed, partially or completely fused in the middle line, and do not project beyond the distal segments of the outer branches; the entosternite has only one pair of antero-lateral processes, and in the male the second and third pairs of ambulatory limbs are modified as claspers. Tachypleus differs from Carcinoscorpius in possessing a long movable spur upon the fourth segment of the sixth ambulatory limb, in having the postanal spine triangular in section instead of round, and the claspers in the male hemichelate, owing to the suppression of the immovable finger, which is well developed in Carcinoscorpius. At the present time king-crabs have a wide but discontinuous distribution. Xiphosura, of which there is but one species, X. polyphemus, ranges along the eastern side of North America from the coast of Maine to Yucatan. Carcinoscorpius, which is also represented by a single species, C. rotundicauda, extends from the Bay of Bengal to the coast of the Moluccas and the Philippines, while of the two better-known species of Tachypleus, T. gigas (= moluccanus) ranges from Singapore to Torres Straits, and T. tridentatus from Borneo to southern Japan. A third species, T. hoeveni, has been recorded from the Moluccas. But although Xiphosura is now so widely sundered geographically from Tachypleus and Carcinoscorpius, the occurrence of the remains of extinct species of king-crabs in Europe, both in Tertiary deposits and in Triassic, Jurassic and Cretaceous strata, suggests that there was formerly a continuous coast-line, with tropical or temperate conditions, extending from Europe westward to America, and eastward to southern Asia. There are, however, no grounds for the assumption that the supposed coast-line between America and Europe synchronized with that between Europe and south Asia. King-crabs do not appear to differ from each other in habits. Except in the breeding season they live in water ranging in depth from about two to six fathoms, and creep about the bottom or bury themselves in the sand. Their food consists for the most part of soft marine worms, which are picked up in the nippers, thrust into the mouth, and masticated by the basal segments of the appendages between which the mouth lies. At the approach of the breeding season, which in the case of Xiphosura polyphemus is in May, June and July, king-crabs advance in pairs into very shallow water at the time of the high tides, the male holding securely to the back of the female by means of his clasping nippers. No actual union between the sexes takes place, the spawn of the female being fertilized by the male at the time of being laid in the sand or soon afterwards. This act accomplished, the two retreat again into deeper water. Deposited in the mud or sand near high-water mark, the eggs are eventually hatched by the heat of the sun, to which they are exposed every day for a considerable time. The newly hatched young is minute and subcircular in shape, but bears a close resemblance to its parents except in the absence of the caudal spine and in the presence of a fringe of stiff bristles round the margin of the body. During growth it undergoes a succession of moults, making its exit from the old integument through a wide split running round the edge of the carapace. Moulting is effected in exactly the same way in scorpions, Pedipalpi, and normally in spiders. The caudal spine appears at the second moult and gradually increases in length with successive changes of the skin. This organ is of considerable importance, since it enables the king-crab to right itself when overturned by rough water or other causes. Without it the animal would remain helpless like an upturned turtle, because it is unable to reach the ground with its legs when lying on its back. Before the tail is sufficiently developed to be used for that purpose, the young king-crab succeeds in regaining the normal position by flapping its flattened abdominal appendages and rising in the water by that means. The king-crab fishery

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Fig. 1.

1. Limulus polyphemus, adult (dorsal aspect).
2. Limulus polyphemus, young (dorsal aspect).
3. Prestwichia rotundata, Coal M., Shropshire.
4. Prestwichia Birtwelli, Coal M., Lancashire.
5. Neolimulus falcatus, U. Silurian, Lanark.
6. Hemiaspis limuloides, L. Ludlow, Leintwardine, Shropshire.
7. Pseudoniscus aculeatus, U. Silurian, Russia.

is an industry of some importance in the United States, and in the East Indies the natives eat the animal and tip their lances and arrows with the caudal spine. They also use the hollow empty shell as a water-ladle or pan—hence the name “pan-fish” or “saucepan-crab” by which the animal is sometimes known. Fossil king-crabs have been recorded from strata of the Tertiary and Secondary epochs, and related but less specialized types of the same order are found in rocks of Palaeozoic age. Of these the most important are Belinurus of the Carboniferous, Protolimulus of the Devonian, and Hemiaspis of the Silurian periods. These ancient forms differ principally from true king-crabs in having the segments of the opisthosoma or hinder half of the body distinctly defined instead of welded into a hexagonal shield.  (R. I. P.)