elements of the newly-formed starch (carbon, hydrogen, and oxygen), and thus the first steps leading to the building up of those wonderful bodies, the proteids, are passed. Nothing of the sort can be done by the protoplasm of an animal cell. Consequently we distinguish among the simplest living things those which are provided with leaf-green, and feed, as do the larger green plants, on dissolved “mineral” solids and gases. There are many thousands of kinds of them—single simple cells. Some are known to microscopists as Diatoms and Desmids—often of curious spindle or crescent-shape, others star-like. The diatoms form on their surface a delicate, wonderfully-sculptured coat of glass-like silica (quartz), which resists destruction and persists long after the protoplasm is dead and washed away. They are favourite objects for examination with the microscope on account of their great beauty and variety. Those simplest living things which have not got leaf-green to enable them to feed on mineral food must—unless they are parasites (as many important kinds are)—get their food, as do bigger animals, by feeding on the solid substance of other living things. All living things are, in fact, ultimately dependent on the green plants—whether microscopic or of larger kinds—not only for food, but for oxygen gas. If you could take away green plants altogether from the world, the animals would eat one another and use up the oxygen gas of the atmosphere, and at the last there would be a few only of the strongest left, like the last survivor of the shipwrecked crew of the _Nancy Bell_, and even they would be suffocating for want of oxygen. The single cells, which are independent animalcules, and feed like animals on whole creatures smaller than themselves, or on bits of the fresh substance of other animals or of plants, are of extraordinary diversity of form and activity. Unlike the unicellular plants, whose food is dissolved in the water in which they live, the single-cell animals of necessity take their food in “lumps” into their inside and digest it, and so their cell-protoplasm has either a soft surface which can take up a food-morsel at any point or it has a firm surface with a definite mouth, or aperture, in it (see Fig. 41) where the mouth is marked by an arrow. Many of them, especially those with soft glutinous protoplasm, which extends from the main-mass in long threads or branching processes searching for food-morsels, form marvellous, perforated shells by chemical deposit, either of silica or limestone (Radiolaria and Foraminifers). The kinds with a firm or tough surface to the cell-protoplasm and a permanent mouth and gullet leading into the cell-substance have very usually a single large lashing-whip (Flagellata), which drives them through the water in search of prey, or they are clothed with hundreds of such lashing threads of smaller size—the “cilia” described above (p. 195)—arranged in rows or circles,