KING-CRABS, ETC. 251 



where the spines on the basal segments of these appendages crush and tear it to 



pieces. In May, June, and July, large numbers of king-crabs approach the coast 



in couples to spawn. Choosing spring-tides, they advance along the bottom until 



the water is shallow enough to allow the carapace to project above the surface. 



The female then scrapes a hollow in the mud, lays her eggs, and hurries back with 



her mate into deep water. By the action of the waves the eggs are soon covered 



with a layer of sand, and at ebb-tide are exposed to the warmth of the sun. When 



first it emerges from the egg, the young king-crab is a minute nearly spherical 



creature, with a fringe of stiff bristles running round the body, and differs from the 



parent in having no tail. Subsequently it undergoes a succession of moults, during 



which the form of the adult is gradually acquired, the tail appearing at the second 



change. The casting of the skin is effected by the splitting of the integument of 



the cephalothorax all round, immediately beneath the margin of the carapace. 



Through the aperture thus made the creature struggles forth, leaving its old shell 



behind. Before the growth of the tail the young king-crab is in a helpless state, 



the slightest obstacle turning it upside down. In this emergency it starts a 



vigorous flapping of its gill-plates, which cause it to rise in the water. Then 



ceasing the agitation, it at once descends with a chance of alighting right side up. 



The existing; king-crabs are the typical representatives of the 

 Extinct Types. .. . . & & . . z. f . r • 1 m • 



family Limulidce, and fossil remains of Limulus occur in the Tertiary 



rocks as wells as in the Cretaceous, Jurassic, and Triassic beds of the Secondary 



epoch. In the Palaeozoic strata the class is represented by a number of forms, such 



as Bellinurus from the Carboniferous, Protolimulus from the Devonian, and 



Hemiaspis from the Silurian, which resemble Limulus in most of their characters, 



but differ in having the abdomen composed of at least nine distinct segments. On 



this account they are referred to a distinct family, Hemiaspididce. It is, however, 



interesting to note that in the young king-crab the abdomen is also composed of 



nine segments, so that just as in the life-history of each individual king-crab the 



final and adult stage with a solid abdomen is preceded by a transitory stage in 



which the abdomen is jointed, in the history of the class the existing and final 



stage, represented by the adult king-crab of our own day, was preceded by a 



transitory stage, which, in the segmentation of the abdomen, was on a level with 



the young king-crab. 



The seas in which these fossil forms lived were also inhabited 

 Merostomata. 



by some nearly allied types, differing from the king-crabs, both in 



habits and some important points of structure. The carapace, for instance, was 



much smaller and did not conceal the legs, the last pair of which were generally 



thickened and flattened, and transformed, as in Eurypterus, into powerful short 



paddles. In one form, however, named Slimonia, the legs of the last two pairs 



were enormously elongated, evidently to serve the purpose of oars. The abdomen 



was used as a propeller, and it was long and divided into twelve flexible segments, 



the last of which bore the tail-plate or telson. As in the king-crab, the bases of 



most of the cephalothoracic limbs were armed with teeth and acted as jaws ; 



but those of the anterior pair formed either short tactile organs or long and 



powerful nippers, as in Pterygotus. 



The Merostomata, as these animals are termed, appear to have lived both in 



