THE oldest forms of life that we have so far been able to trace through fossils in this way are those of the Cambrian period. These fossils appear suddenly and in great variety, but all are the remains of marine forms. All the chief kinds of invertebrate animals (I.e. those without backbones) now recognised in zoology are already represented by well-developed groups—a fact which not only indicates how old their history is but what a lamentable blank there is in the earlier history of the development of life.
The simplest forms of this assemblage are those belonging
developments of the different invertebrate groups have taken place, forms have come into being, have flourished, and finally died away, some without descendants and others leaving their characteristics to be transmitted more or less faithfully by some related form. None, at any rate, has carried on from these early days until the present.
Of the groups we have named, the Foraminifera and Radiolaria have had great development since Cambrian times. Foraminifera were especially abundant in the long Carboniferous period and, because they were widespread in certain localities during the formation of the oil-bearing strata, they are of great interest to oil-field prospectors. Indeed, some hundreds of persons in America devote their time to the study of these little fossils which help them to unravel the history of the oil-bearing deposits, and thus serve to indicate the presence and extent of oil.
THE ONE-ROOMED TENEMENTS OF THE GRAPTOLITES A SERIES of animals important in the earlier days were the graptolites, which are now long extinct. Their soft parts are quite unknown and we can only guess by analogy at the meaning of the structure. We know them from the abundant remnants of their skeleton of chitin, though that substance itself has generally been carbonised or replaced by iron compounds in the long course of time.
They appear to have consisted essentially of a long hollow chitinous rod from which many little cups projected on one or more sides, so that the whole animal was a sort of tenement of one-roomed cuplets each of which was occupied by a single living polyp, while the whole communicated by means of the central cavity. There are many different varieties of dissimilar appearance, some with several polyp-bearing branches. Others, with only one set of tiny cups, look in the fossil state like tiny saws. Their remains are to be found particularly in the slates of Wales and Shropshire, and they range in antiquity from the Cambrian to the close of the Silurian periods.
v The animals known as the corals, which were in some ways related to the graptolites, continued to be important and to grow great reefs in the warmer seas just as they do to-day, and the remains of many of these reefs have been examined. The Sea-Lilies, or crinoids, were common enough too at some periods to be what are known as rock-formers, for we
know of several places where beds of limestone have been built from the remains of these graceful animals. Sea-Lilies, which were so common in the Silurian age that this period has been called ‘The Age of Sea-Lilies,’ are like slender-fingered starfish, but with numerous fingers. They are attached to the ground by a flexible stalk of varying length.