In his recent classification of the Arachnida, Lankester[[207]] has grouped the Xiphosura or King-crabs with the extinct Eurypterids or Gigantostraca under the name of Delobranchiata, better known under the name Merostomata[[208]] of Dana. The chief character of this group, and one which differentiates it from all the animals placed together by Lankester in the group Embolobranchiata, is that they have gills patent and exposed. The Xiphosura are, in fact, with the exception of a few marine Mites, the only Arachnids which now live in the sea as did their allies the Eurypterids in Palaeozoic times. With a few fresh-water exceptions, all other Arachnids have taken to life on land, and with a change from water-breathing to air-breathing came a change in the respiratory system, the gills becoming “lung-books,” or possibly tracheae, or disappearing altogether.
A few years ago Pocock re-classified the Xiphosura, and his classification will be found on pp. 276, 277. It will be noticed that in his classification the generic name Limulus has disappeared. I have, however, retained it in this article, partly because I regard the name as so well established that every one knows what it denotes, and partly because in a group which contains confessedly very few species, differing inter se comparatively slightly, it seems unnecessary to complicate matters with sub-families and new names.
Looked at from above a Limulus presents a horse-shoe-like outline, from the posterior end of which projects a long spine. It is often called in America the Horsefoot-crab, but its common or vulgar name is the King-crab. Across the middle of the body is a joint, and this joint separates the prosoma from the meso- and metasoma which are in King-crabs fused together. The prosoma comprises all the segments up to and including the segment which carries the chilaria;[[209]] the mesosoma begins with the segment bearing the genital pores, and ends with the last segment which bears appendages; the metasoma comprises all the segments posterior to the last segment which carries appendages. The prosoma corresponds with the “cephalothorax” of some authors, and the meso-plus the metasoma are equivalent to their “abdomen.”
Dorsally, then, the prosoma is a vaulted structure with a smooth, horse-shoe-shaped anterior and lateral margin. Its posterior edge, the line where the meso-plus the metasoma are hinged, is a re-entrant bay with three sides. The meso- and metasoma are in the King-crabs fused together and form a hexagon. Three sides of this hexagonal double region form the hinge, two form the lateral margins and slope inwards; these bear six fused and six-jointed spines which have a segmental value. The sixth or posterior side is indented, and its concavity forms the area to which the large post-anal, unsegmented tail or spine is hinged.
The whole body is covered by a smooth chitinous sheath varying from sage-green to black in colour, and it is kept very clean, probably by some excretion which hinders various sessile animals attaching themselves to it as they do, for instance, on many Copepods. Burrowing animals like Limulus are usually free from these messmates. King-crabs have a self-respecting, well-groomed appearance. On the rounded dorsal surface the chitinous covering is produced into a certain number of spines arranged in a median and two lateral rows. The anterior median spine overhangs the median eyes, and the anterior lateral spine on each side overshadows the large lateral eyes.
Fig. [152].—Dorsal view of the King-crab, Limulus polyphemus, × ½. From Shipley and MacBride. 1, Carapace covering prosoma; 2, meso- and metasoma; 3, telson; 4, median eye; 5, lateral eye.
The vaulted carapace is turned in on the under side, where there is a flat rim which widens anteriorly, and on the inner edge this rim borders a sunken area, into the concavity of which the numerous appendages project. Thus, although when viewed from above a Limulus looks as though it had a solid body shaped something like half a pear, when viewed from below, especially if the appendages be removed, it will be found that the body is thin and hollowed, and almost leaf-like, as if most of the edible part of the half-pear had been scooped out. Within the hollow thus formed the appendages lie, and here they move about, seldom or never protruding beyond the edge of the carapace,—in fact, all except the pedipalps and ambulatory legs are too short to project beyond this limit.
Fig. [153].—Ventral view of the King-crab, Limulus polyphemus, × ½. From Shipley and MacBride. 1, Carapace covering prosoma; 2, meso- and metasoma; 3, telson; 4, chelicera; 5, pedipalp; 6, 7, 8, 9, 3rd to 6th appendages, ambulatory limbs; 10, genital operculum turned forward to show the genital apertures; 11, 12, 13, 14, 15, appendages bearing gill-books; 16, anus; 17, mouth; 18, chilaria.