No doubt, also, many of the crustaceans of this period were very ornate in their colors, and in the arrangement of bands, knobs, and ridges on their shells. The numerous corals building in the shallow seas, and carrying their reefs up into the light on the surface, waved purple, orange, and crimson filaments from their limestone towers, until the waters looked like garden beds in full bloom. Among the earliest fossils are worm-casts,[11] from which we learn that even so long ago the worms were helping to build the world. The sponges shared the shallow seas with the corals. Next came the Silurian age, named as we know from a part of Wales where its rocks lie bare. This was a very long age, and is divided into upper and lower Silurian. During all this time the world was becoming richer and richer in life, both of animals and vegetables, and of both land and sea. We find strangely beautiful fossils of this age, as, for instance, the stone lilies. No form of life seemed for a time so hardy and so rich in variety as these stone lilies. These were not plants, but animals, and yet they looked more like beautiful snowy flowers than like animals. In very deep water of the present day we find some few species of these crinoids, or stone-lily animals, and from them we can understand those of the ancient time. They grew rooted in one place, waved on long, pliant stems, and from their snowy cups spread forth filaments that looked like delicate stamens and pistils. Swimming about among the stone lilies, went the nautilus, relative of our “paper nautilus” of which the poet sings:-- “This is the ship of pearl, which poets feign Sails the unshadowed main-- The venturous bark which flings On the sweet summer wind its purpled wings, In gulfs enchanted where the siren sings, And coral reefs lie bare, Where the cold sea-maids rise to sun their streaming hair.” The nautilus of the Silurian time perhaps looked quite as lovely and harmless, yet really was a mighty monster, devouring millions of its fellow-denizens of the deep. To-day we see a relation of this old-time nautilus, in our cuttle-fish, sometimes called the devil-fish. To the Silurian age belong the written stones. The tiny fossils imbedded in these rocks are like little coal-black stains, which appear as minute writing in an unknown tongue. These small cells are indeed writing, and help us to learn the story of the ancient world. The corals increased greatly in the Silurian age, and extended far up to the polar regions, where corals are now unknown. In these early ages the chief home of life was in the sea. The waters brought forth living things before the earth produced them, while the creatures of the air, the birds and insects, came later still. Two very important facts are here to be noted: Just as the Silurian age was closing, several kinds of fish-like creatures appeared, the first vertebrate or backboned animals.