Both types meet us in the Early Cambrian, but while the Mollusk is there represented only by low forms, the Articulate is then not only in the humble guise of the worm, but in the complex and highly organised form of the Trilobite (Figs. 28 and 75). What older phases they may have passed through we know not; but in the Lower Cambrian we have various forms of these animals, including some of the largest known as well as some of the smallest; some of the most complex in number of parts as well as some of the simplest. These animals, in short, seem to have appeared at once all over the world fully formed, and in a variety of generic and specific forms; and nothing short of a very large faith in the imperfection of the geological record can suffice to account for their evolution. [Illustration: FIG. 76_a_.—Burrows of Trilobite and of modern King-crab. The Trilobite burrow is known as Ruschinites, and has been supposed to be a sea-weed of the kind called _Bilobites_.] 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. [Illustration: 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.