The most highly organized type of which we have any certain evidence is that of the Crustacea, the group to which our modern lobsters and crabs belong, and its most prominent representatives are the trilobites (Figs. 1, 2), so called from the three lobes into which the body is divided. These creatures are indeed remarkable for the twofold property of bilateral symmetry, and fore and aft jointed structure, both based on the number three. From front to rear we have a large head, usually with well-developed eyes and oral organs, a middle or thoracic part composed of a series of movable segments, and a tail-piece sometimes small, sometimes nearly as large as the head. Transversely, the body is divided into a central and two lateral lobes, which can be seen in the head, the thorax, and usually in the tail as well. The organization of these animals must have been as complex as that of most existing Crustaceans. Their nerve system must have been well developed; a vast number of muscles were required to move the different parts of the trunk, and the numerous and complex limbs which have been observed in some of the species, and no doubt were possessed by all. Their digestive and circulatory organs must have been in proportion to the complexity of their locomotive organs. [Illustration: Fig. 2.--_Triarthrus Becki_, Green. A Trilobite of primitive type, showing its limbs and antennæ. (After Beecher.)] Figure 2, borrowed from Beecher,[4] shows the limbs of a species, not of the Lower Cambrian, but of a somewhat later formation. There can be no doubt, however, that those of earlier species were equally perfect, more especially as Triarthrus is an animal of an old type approaching to extinction in the age succeeding the Cambrian, and its representatives in the earlier and palmy days of the family could not have been inferior in organization. These creatures swarmed in every sea in the Cambrian period, and were represented by a great number of species, some of them of large size, others very small; some many-jointed, others few-jointed, and with a great variety of tubercles, spines, and other ornamental and protective parts. If we ask for their affinities and place in the great group of Crustacea, the answer must be that, while in some points allied to the higher forms, they approach most nearly to those which occupy a medium position in the class, and are, in fact, a composite type, presenting points of structure now distributed among different groups. If we ask for affinities with lower groups, we have to reply that their nearest allies in this direction are the bristle-footed marine worms; but there is a vast gap, both in the Cambrian and Modern seas, between any of these worms and the Crustacea, which, either as embryos or as adults, have any resemblance to them. [Footnote 4: _American Journal of Science_, 1896.]