A whole family, comprising sponges, corals, and jelly-fish, are called Zoophytes, or plant-animals, from the difficulty of assigning them to one kingdom or the other. On the whole they rather more resemble animals, and are generally classed with them, though they lack many of their most essential qualities, and in form often bear a close resemblance to plants. But when we descend a step lower in the scale of existence we come to a large family—the Protista—of which it is impossible to say that they are either plants or animals. In fact, scientific observers have classed them sometimes as belonging to one and sometimes to the other kingdom; and it was an organism of this class, looking at which through a microscope Huxley pronounced it to be probably a plant, while Tyndall exclaimed that he would as soon call a sheep a vegetable. They are mostly microscopic, and are the first step in organised development from the Monera, which are mere specks of homogeneous protoplasm. Small as they are they have played an important part in the formation of the earth’s crust, for the little slimy mass of aggregated cells has in many instances the power of secreting a solid skeleton, or a minute and delicate envelope or shell, the petrified remains of which form entire mountains. Thus the nummulitic limestone, which forms high ranges on the Alps and Himalayas, and of which the Pyramids are built, consists of the petrified skeletons of a species of Radiolaria, or many-chambered shells, forming the complicated and elegant mansion with many rooms and passages, of the formless, slimy mass which constitutes the living organism. Chalk also, and the chalk-like formation which is accumulating at the bottom of deep oceans, are the results of the long-continued fall of the microscopic snowdrift of shells of the Globigenera and other protistic forms swimming in the sea; and in a higher stage of development the skeletons of corals, one of the family of Zoophytes or plant-animals, form the coral reefs and islands so numerous in the Pacific and Indian Oceans, and are the basis of the vast masses of coralline limestone deposited in the coal era and other past geological periods. As development proceeds the distinction between plants and animals becomes more apparent, though even here the simplest and earliest forms often show signs of a common origin by interchanging some of the fundamental attributes of the two kingdoms. Thus, the essential condition of plant existence is to live on inorganic food, which they manufacture into protoplasm, by working up simple combinations into others more complicated. Their diet consists of water, carbonic dioxide, and ammonia; they take in carbonic dioxide and give out oxygen, while animals do exactly the reverse. But the fungi live, like animals, upon organic food consisting of complicated combinations of carbon, which they assimilate; and, like animals, they inhale oxygen and give out carbonic dioxide.