It is clear that evidence from the last-named source, when it is available, is the most important of all, since the order of succession of the various types is given by that of the rock strata in which they occur, and we can be quite certain that we are dealing, if not with the actual ancestors, at least with the forerunners of existing species. The "imperfection of the geological record," however, is so great that the organisms preserved in the fossil state represent only an insignificant part of the whole number of organisms that have lived on the globe since life began; and it is not surprising, therefore, that in many groups the study of fossils has hitherto afforded little help towards the working out of their genealogical history. Thus, among Crustacea there are many important groups such as the Copepoda, which are entirely unknown as fossils, their small and delicate bodies being ill adapted for preservation, although there is every reason to suppose that they are a very primitive and very ancient group. In many fossil Crustacea only the hard shell or carapace has been preserved, the appendages being lost or represented only by indecipherable fragments, and in some cases it is hardly possible to guess at the affinities of the animals. Further, several important groups are already represented in some of the oldest of the fossil-bearing rocks at present known, and the differentiation of these groups must have taken place in the dark ages before the record of the fossils begins. In spite of these disadvantages, however, the study of fossil Crustacea does throw considerable light on the evolution of the group, and it is likely that interesting results in this direction await future investigations.
Fig. 81—Restoration of a Trilobite (Triarthrus becki), showing the Appendages. Upper Side on Right, Under-side on Left. Slightly enlarged. (After Beecher.)
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In the earliest fossiliferous rocks the most abundant and important Arthropods are the Trilobites ([Fig. 81]), an extinct group which appears to have been related to the primitive Crustacea. The name Trilobite refers to the three-lobed form of the body when seen from the dorsal side, most species having a pair of grooves running lengthwise which divide off a middle lobe containing the principal organs of the body from two lateral "pleural" expansions covering the limbs. The head-shield shows indications of being composed of five segments, and bears a pair of sessile compound eyes. It is followed by a number (up to twenty-six) of free somites, and the body ends in a tail-shield, or "pygidium," which is often plainly composed of several somites fused together. Although Trilobites are among the commonest and most familiar of fossils in the older rocks, the nature of their appendages remained quite unknown until within recent years, when specimens of several species showing the structure of the limbs and under-side of the body were discovered in America. From these it appears that the head bore in front a pair of long thread-like antennæ and four pairs of two-branched appendages, each with a jaw process, or "gnathobase," turned towards the mouth, which is covered below by a large anterior lip, or "hypostome." It seems probable that the five pairs of head-appendages correspond respectively to the antennules, antennæ, mandibles, maxillulæ, and maxillæ, of Crustacea; but the second pair appear to have acted as jaws, retaining the gnathobase which, among Crustacea, is only hinted at by the hooked spine on the antenna of the nauplius larva.
Each of the free somites and of those forming the tail-shield bears a pair of two-branched appendages, not differing greatly from the posterior appendages of the head, but becoming smaller and more flattened towards the hinder end of the body. The numerous genera and species of Trilobites present great differences in the form and ornamentation of the dorsal surface of the body, and it is probable that considerable differences may also have existed in the structure of the limbs, which are only known in two or three species. Some Trilobites are among the most ancient of known fossils, being found in rocks of the Lower Cambrian epoch. The group reaches its maximum development in the Ordovician, and the number of the species and size of the individuals gradually diminish through the Silurian and Devonian till they become extinct at the close of the Carboniferous epoch, except for a single species found in rocks of Permian age in America.
Although zoologists are not all agreed as to the precise systematic place to be assigned to the Trilobites, there can be little doubt that they were related more or less closely to the most primitive Crustacea, and they are of special interest as preserving for us the stage in which the second pair of appendages were still used as biting jaws, and had not moved forwards in front of the mouth to become antennæ, as in all living Crustacea.
Contemporary with some of the earliest Trilobites, however, are undoubted Crustacea, which, so far as we know their structure, are not very different from types now living. In the Cambrian epoch the Branchiopoda appear to be represented by Protocaris, which in its general form resembles Apus; and there are a variety of genera and species of Ostracoda, although, since their shells alone are preserved, it is not possible to determine their exact relations to existing forms. In the succeeding Ordovician and Silurian epochs we first meet with the remains of Barnacles, and it is interesting to note that some of them are referred to the genera Pollicipes and Scalpellum, which are represented by numerous species in the seas of the present day. Along with these, however, are some strange-looking forms (Turrilepas, etc.), having the body covered with rows of overlapping plates. If these are really Cirripedes, they must have differed considerably in structure from the more modern types.