In adding Infusoria to the Radiates, Cuvier was false to his own principle of founding all classification on plan. He was influenced by their seeming simplicity of structure, and placed them in the lowest division of the Animal Kingdom on that account. But even this simplicity was only apparent in many of them. At certain seasons of the year myriads of these little Animalcules may be seen in every brook and road-side pool. They are like transparent little globules, without any special organization, apparently; and were it not that they are in constant rotation, exhibiting thus a motion of their own, one would hardly suspect that they were endowed with life. To the superficial observer they all look alike, and it is not strange, that, before they had been more carefully investigated, they should have been associated together as the lowest division of the Animal Kingdom, representing, as it were, a border-land between animal and vegetable life. But since the modern improvements in the microscope, Ehrenberg, the great master in microscopic investigation, has shown that many of these little globules have an extraordinary complication of structure. Subsequent investigations have proved that they include a great variety of beings: some of them belonging to the type of Mollusks; others to the type of Articulates, being in fact little Shrimps; while many others are the locomotive germs of plants, and so far from forming a class by themselves, as a distinct group in the Animal Kingdom, they seem to comprise representatives of all types except Vertebrates, and to belong in part to the Vegetable Kingdom, Siebold, Leuckart, and other modern zoölogists, have considered them as a primary type, and called them Protozoa; but this is as great a mistake as the other. The rotatory motion in them all is produced by an apparatus that exists not only in all animals, but in plants also, and is a most important agent in sustaining the freshness and vitality of their circulating fluids and of the surrounding medium in which they live. It consists of soft fringes, called Vibratile Cilia. Such fringes cover the whole surface of these little living beings, and by their unceasing play they maintain the rotating motion that carries them along in the water.