Protozoans, which include all the tiny single-celled animals, are known in fossil form even in late Proterozoic rocks and, as proved by the fossil records, they have been more or less abundant ever since, even now swarming in large portions of the surface sea waters. One of the most remarkable facts in the history of animal life is, that such exceedingly simple creatures persisted almost without change through the tens of millions of years when such profound and even revolutionary changes took place in the animal kingdom in general. The only fossil Protozoans are those which developed delicate shells either of carbonate of lime (the foraminifers) or silica. Special mention should be made of the Cretaceous period when foraminifers must have been exceedingly profuse in clear sea waters which spread over the Gulf Coastal Plain of the United States, parts of southern England, much of France, and other areas, as proved by their accumulated shells which make up formations of chalk hundreds of feet in thickness and many miles in extent. [Illustration: Fig. 49.--A compound colony of fossil graptolites characteristic of late Ordovician time, fully 20,000,000 years ago. Each little prong once held a tiny individual living graptolite which was a very simple type of animal belonging to the subkingdom called "Coelenterates." (Modified after Ruedemann.)] The Coelenterates, which comprise the simplest of the many-celled animals, are saclike forms with mouth openings, but with few other differentiations of parts. All are marine animals. Of these the sponges are porous, and the other types (including corals) have tentacles around their mouths. Sponges have been more or less common from early Paleozoic time to the present, and they have undergone relatively little change. "Jellyfishes," which are in truth not fishes at all, are wholly soft or gelatinous Coelenterates which have left some very remarkable impressions and casts in strata of very early Paleozoic age, those very ancient forms evidently having been almost exactly like those of to-day. Graptolites were slender, plumelike, delicate forms consisting of colonies of tiny individuals, in many cases in branching or radiating combinations. They existed only during the first half of the Paleozoic era. Both because they floated in the open sea, thus permitting widespread distribution, and because they underwent many distinct species changes during short geologic intervals, they are among the most useful fossils for separating the various subdivisions of strata of the earlier Paleozoic. [Illustration: Fig. 50.--Corals, representing the very simple subkingdom of animals called "Coelenterates": a, fossil shell of an individual "cup coral" found only in Paleozoic strata; b, a compound or "chain coral" skeleton found only in relatively old Paleozoic strata; and c, part of a modern coral colony showing living corals.]