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 zooelogists, 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.

The Mollusks, the next great division of the Animal Kingdom, also include three classes.  With them is introduced that character of bilateral symmetry, or division of parts on either side of a longitudinal axis, that prevails throughout the Animal Kingdom, with the exception of the Radiates.  The lowest class of Mollusks has been named Acephala, to signify the absence of any distinct head; for though their whole organization is based upon the principle of bilateral symmetry, it is nevertheless very difficult to determine which is the right side and which the left in these animals, because there is so little prominence in the two ends of the body that the anterior and posterior extremities are hardly to be distinguished.  Take the Oyster as an example.  It has, like most Acephala, a shell with two valves united by a hinge on the back, one of these valves being thick and swollen, while the other is nearly flat.  If we lift the shell, we find beneath a soft lining-skin covering the whole animal and called by naturalists the mantle, from the inner surface of which arise a double row of gills, forming two pendent folds on the sides of the body; but at one end of the body these folds do not meet,