32 1 8 THE JOINTED ANIMALS 



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 molts, 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 cephalo- 

 thorax all round, immediately beneath the margin of the carapace. Through the 

 aperture thus made the creature struggles forth, leaving its old shell behind. Be- 

 fore 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 family 

 ' s Limulidce, and fossil remains of Limulus occur in the Tertiary rocks 

 as well as in the Cretaceous, Jurassic, and Triassic beds of the Secondary epoch. 

 In the Paleozoic 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 characteristics, 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 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 pro- 

 peller, 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 cephalo- 

 thoracic 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 Merostornata, as these animals are termed, appear to have lived both in 

 fresh and salt water, and their organization seems to show that they were powerful 



