The members of this order are Crustacea, often of gigantic size, in which the mouth is furnished with mandibles and maxillae, the terminations of which become walking or swimming feet and organs of prehension.
This order comprises the recent King-crabs, and the extinct Pterygoti and Eurypteri.
"Crustacea having the anterior segments welded together to form a broad convex buckler, upon the dorsal surface of which are placed the compound eyes and ocelli, the former sub-centrally, the latter in the centre in front. The mouth is furnished with a small labrum, a rudimentary metas-toma and six pairs of appendages. Posterior segments of the body more or less free, and bearing upon their ventral surfaces a series of broad lamellar appendages; the telson, or terminal segment, ensiform " (Henry Woodward).
The Xiphosura include no other recent forms than the Limuli (King-crabs, or Horse-shoe Crabs). They are distinguished by the possession of six pairs of chelate limbs, placed round the mouth, having their bases spinous, and officiating as jaws. The anterior portion of the body is covered by a broad horse-shoe-shaped buckler (fig. 150), the upper surface of which bears a pair of larval and a pair of compound eyes. On the lower surface of the carapace is placed the aperture of the mouth, surrounded by six pairs of limbs, the bases of which are spinous and officiate as jaws, whilst their terminations are converted into chelae or nipping-claws. The first pair of appendages is placed in front of the mouth, and has been generally said to represent the antennae; but according to Milne-Edwards they are not supplied with nerves from the cerebral ganglia, and therefore cannot be of this nature. Behind the cephalic buckler comes a second shield, composed of six amalgamated segments, below which are carried the reproductive organs and branchiae, the former protected by a thoracic plate or "operculum," the latter borne by five pairs of lamellar appendages. Lastly, articulated to the posterior margin of the abdominal shield, is a long sword-like spine or "telson" (fig. 150, t). The circulatory system of Limulus is of a very high type, though the heart is tubular. The venous blood, instead of being contained in the mere interspaces and lacunae between the tissues, is to a large extent confined within proper vessels. A remarkable peculiarity, also, is that the ventral nerve-cord is enclosed within the abdominal artery, and most of the nerves are similarly ensheathed within the arteries.
The eggs of Limulus are laid in the sand, and are fertilised by the male. Just prior to the time of hatching, six segments can be recognised in the cephalothorax; the abdomen consists of nine well-marked somites; the bases of the legs are hardly spinose; and the abdominal spine is quite rudimentary. In this stage (fig. 152), the larva closely resembles some of the Trilobites, such as Trinucleus and Asaphus. After hatching, the previously existing segmentation is soon obliterated, and, three or four weeks later, the telson assumes the ensiform shape characteristic of the adult. According to the views of Van Beneden, the development of Limulus so closely resembles that of the Scorpions, that the former should properly be removed from the Crustacea, and placed in the Arachnida.
Fig. 150. - Xiphosura. Limulus polyphe-mus, viewed from below, c The cephalic shield carrying the sessile eyes upon its upper surface; o " Operculum," covering the reproductive organs ; b Branchial plates ; a First pair of antennae (antennules) ending in chelae. Below these is the aperture of the mouth, surrounded by the spiny bases of the remaining five pairs of appendages, which are regarded by Woodward as being respectively, from before backwards, the great antennae, the mandibles, the first maxillae, the second maxillae, and a pair of maxillipedes. All have their extremities chelate.
Fig. 151. - Eurypterida. Pterygotus An-glicus, restored (after H. Woodward). c c Chelate antennae ; o o Eyes situated at the anterior margin of the carapace ; m m The mandibles, and the first and second maxillae ; n n The maxillipedes - the basal margins of these are serrated, and are drawn as if seen through the me-tastoma or post-oral plate, which serves as a lower lip. Immediately behind this is seen the operculum or thoracic plate which covers the two anterior thoracic somites. Behind this are five thoracic and five abdominal somites, and lastly there is the telson (t).
The King-crabs are found in the Indian and Japanese seas, on the coasts of North America, and in the Antilles. They sometimes attain a large size, and both the eggs and the flesh are eaten by the Malays.