A Trilobite is a creature in whose structure the number three is dominant. Seen from above, it presents three divisions from front to rear:—first, a cephalic shield or head-piece; secondly, a thorax, divided into several segments movable upon each other; and thirdly, a tail-piece or pygidium, which, when brought against the head by the rolling up of the body segments, effectually covers the lower parts. This lower portion was until lately little known; but the discoveries of Billings and of Wolcott have enabled us to restore the jaws under the head, the jointed legs and spiral gills under the thorax, and thus to complete the structure of the animal, and understand better its relations to modern crabs and shrimps ([Fig. 76]). Of these it certainly comes nearest to the King-crabs and Horseshoe-crabs, a somewhat limited group at present, and one which reaches back in geological time only to the Upper Silurian, when the Trilobites had perhaps already passed their culmination.
Constructed as above described, the Trilobite could swim, as is supposed, usually on its back or side. It could crawl on the bottom. Using its snout as a shovel, it could burrow like a modern King-crab ([Fig. 76]a); and when pressed by danger some species could roll themselves into balls and defy their enemies.
Fig. 77.—Silurian Trilobites.
a, Isotelus. b, Homalonotus. c, Calymene.
This type of animal, entering on the stage in full force in the Older Cambrian, continues under many forms through the whole Palæozoic age, dying out finally in the Carboniferous. [Figs. 77 and 78] show a few of the forms of the Silurian, Devonian, and Carboniferous.
Contemporaneously with the dawn of the Trilobite group, appear some small shrimp-like forms ([Fig. 28]),[19] and others with bivalve shells ([Fig. 79]), which are closely allied to modern forms,[20] and, like the Lingulæ, persist through the succeeding formations with little more than specific change—presenting in this a strange contrast to the Trilobites. While the latter were still flourishing, about the close of the Lower Silurian, a remarkable group of large and highly-developed creatures, allied to the Trilobites, but suited for rapid swimming rather than creeping, was introduced; and in the Upper Silurian and Devonian these creatures[21] attained to gigantic sizes, exceeding, probably, any modern Crustaceans, and were tyrants of the seas. Pterygotus anglicus ([Fig. 80]) is supposed to have attained the length of six feet. Yet these noble representatives of the Crustaceans became extinct in the Carboniferous. On the other hand, a few small king-crabs appear in the Upper Silurian, and this type still continues, and seems to culminate as to size in modern times; so diverse have been the fortunes of these various groups.
