[Illustration: FIG. 36.—Group of modern Hydroids allied to Graptolites. Magnified, and natural size. _a_, _Sertularia_. _b_, _Tubularia_. _c_, _Campanularia._] [Illustration: FIG. 37.—Silurian Graptolitidæ. _a_, _Graptolithus_. _b_, _Diplograpsus_. _c_, _Phyllograpsus_. _d_, _Tetragrapsus_. _e_, _Didymograpsus_.] [Illustration: FIG. 38.—Central portion of Graptolite, with membrane, or float (_Dichograpsus octobrachiatus_, Hall).] [Illustration: FIG. 39—_Ptilodictya acuta_ (Hall). Bryozoan. Siluro-Cambrian.] A stage higher than the sponges are those little polyp-like animals with sac-like bodies and radiating arms or tentacles, which form minute horny or calcareous cells, and bud out into branching communities, looking to untrained eyes like delicate sea-weeds—the sea-firs and sea-mosses of our coasts (Fig. 36). These belong to a very old group, for in the oldest Cambrian we have a form referred to this type (Fig. 33), and in the Upper Cambrian another still more decided example (Fig. 34).[13] This style of life, once introduced, must have increased in variety and extended itself with amazing rapidity, for in the Siluro-Cambrian age we find it already as characteristic as in our modern seas, and so abundant that vast thicknesses of shale are filled and blackened with the _débris_ of forms allied to the sea-firs, and masses of limestone largely made up of the more calcareous forms of the sea-mosses. As examples of the former we may take the _Graptolites_, so named from their resemblance to lines of writing, and of which several forms are represented in Fig. 37. The little teeth on the sides of these were cells, inhabited probably by polyps, like those represented in the modern _Sertularia_ in Fig. 36. Some of them were probably attached to the bottom. In others the branches radiated from a central film which may have been a hollow vesicle or float, enabling them to live at the surface of the water (Fig. 38). These Graptolites are specially characteristic of the Upper Cambrian and Lower Silurian. The netted ones (_Dictyonema_), as may be seen from Figs. 34 and 35, came in before the close of the Cambrian, and continue unchanged to the Silurian, where they disappear. The branching forms, seen in Fig. 37, have scarcely so great a range. They thus form most certain marks of the period to which they belong, and being oceanic and probably floaters, they diffused themselves so rapidly that they appear to indicate the same geological time in countries so widely separated as Europe, North America, and Australia. It is curious, too, that while the Graptolites thus mark a definite geological time, and seem to disappear abruptly and without apparent cause, they are the first link in the long chain of the Hydroids, which, though under different family forms, continue to this day, apparently neither better nor worse than their perished Palæozoic relatives. There is a group of little Stony Corals (_Monticuliporidæ_),