Gradually, some cells specialized in the performance of the simplest animal functions, like the catching of food and digestion; and in the course of time, such simple forms of life as sponges and marine worms appeared. From flat worms, life advanced to the annelid or ringed worms—little creatures with a food canal and a body cavity filled with blood. In the worm begins the first development of the brain. A few sensitive cells in the fore end of the body, that beginning of mental life, is a far-flung prophecy of the genius that will one day thrill the world. Primitive depressions, lined with pigment cells, in the worm-like head, represent Nature’s first reaching for eyes; two other sensitive nerve pits are the beginning of the nose; yet a further pair of susceptible concave surfaces are destined to develop into the ears of land animals. Creatures with these primitive structures still exist among Nature’s lowly forms. [Illustration: FIG. 12.—LUNG FISHES. Australian Lung Fish, Ceratodus (top); South African Lung Fish, Protopterus (middle); South American Lung Fish, Lepidosiren (bottom).] From worms and worm-like creatures, were developed snails and a great variety of small animals, covered with shells. The Cambrian Rocks, the earliest rocks that have preserved fossils, have yielded the remains of some of these shell creatures (Fig. 8, upper). The Upper Silurian Rocks, belonging to a period much later than the Cambrian, have entombed the fossils of the molluscs shown in Fig. 8 (lower). We must understand that by this time living things had been evolving for millions of years, yet these shelled animals were the highest forms that had so far appeared. We must understand, too, that only the skeletons of creatures possessing a bony frame could be preserved as fossils. The fleshy part of the body, Nature destroyed. Moreover, it must be realized that of the countless billions of creatures that have lived, the rocks have preserved the fossil remains of only a few. We must not expect too much from Nature’s mutilated record. It is enough to know that the specimens that have been preserved prove the gradual unfoldment of life, and enable us to interpret the wonderful story of evolution. [Illustration: FIG. 13.—THE FLYING FISH.]