Animalcules, a name familiarly applied to the more minute forms of animal life, for the knowledge of which we are mainly indebted to the microscope. Leeuwenhoeck led the way in this as in most other branches of microscopic study; but it is to Gleichen that we are indebted for the first attempt at the systematic study of the subject. He was followed by the Danish microseopist O. F. Muller, who made the first regular classification of animalcules. Subsequent observation has detected many errors in the classification of Muller, and it has now little other than a historic interest. It is to Ehrenberg that we arc indebted, directly or indirectly, for almost all our knowledge of these forms. Since the appearance of his work, Die Infusionsthierchen. the study of minute animal forms has been ably pursued by Dujardin in France, Siebold, Kolliker. and others in Germany, Owen in England, and Bailey in the United States. - The earlier observers grouped together, under the term animalcules, a vast variety of living beings having nothing in common except their minuteness of size. Plants and animals, mollusks, crustaceans, insects and worms, larvro and perfect forms, all were aggregated together under the vague term animalcules.

The labors of modern scientific men have been in great part, exhausted in the distribution of this mass of animal and vegetable life among the various classes, families, and orders to which its heterogeneous materials properly belong, and the formation of a class to which the name infusoria, first proposed by Muiller, is now generally applied. To this class we shall confine ourselves, and shall generally use the term infusoria, not that it is absolutely accurate, for though the greater number of these animals are developed in infusions, yet this rule is not without some striking exceptions. - If a drop of water in which animal or vegetable matter is decaying be placed upon the object-holder of a microscope of adequate magnifying power, say 200 diameters, it will be found to swarm with living beings in active and incessant motion. They vary in size from 1/100 of an inch, when they are just within the limit of unassisted vision, to a minuteness which it tasks the power of the glass to detect. These are infusoria; they abound in every ditch, pond, lake, or river, are equally numerous in salt as in fresh water, have been found in thermal springs of high temperature, and in the melted snow of the Alps and the Andes; in short, wherever water and decaying vegetable or animal matter exist, these infusorial animals will be found in vast numbers.

There is no doubt that they are often drawn up into the atmosphere in watery vapor, and borne to and fro by the winds. Many forms are not deprived of life by complete desiccation, and may therefore be mingled with the dust, and in this condition carried about by the winds, to resume their active vitality so soon as they chance to fall into water. The suddenness with which they appear in water, even distilled water, when exposed to the air, furnished the advocates of spontaneous generation with one of their strongest arguments. - Infusorial animalcules have neither vessels nor nerves, and are made up of a uniform tissue, called by Dujardin sarcode, and by Huxley protoplasm. This is in some classes of nearly uniform consistence; in others the external layer possesses considerably more density than the internal, while in yet others a distinct pellicle or skin can be made out. They have no true feet; a few of the very lowest type have the power of protruding portions of their homogeneous structure in the form of limbs, which they use both for the prehension of their food and for locomotion. In the higher forms the locomotion is by cilia, or very minute hairs. This motion is probably automatic, as it is constant day and night, the animal never sleeping, nor appearing to take rest.

Yet it certainly has in some cases many of the characteristics of spontaneity, the animal in his rapid course seeming to avoid obstacles; but the subject of the character of the locomotion of these animals is very obscure. Some of these higher forms have a shell or outer coat, called carapace or lorica; these are spoken of as loricated. - We have already intimated that the systematic classification of the infusoria lias been a matter of great difficulty. That of Ehrenberg, to which we shall in the main conform, though possessing great merit, has also very great defects. He includes among his infusorial animals very many large and important families which are now known to belong to the vegetable kingdom. His desmidieae are now very generally, we might almost say universally, admitted to be alga; and his diatomaceae are now also placed in the vegetable kingdom. The classification of Dujardin, though it has some great advantages over that of Ehrenberg. is deformed by a multitude of new terms, or. what is worse, old terms to which he affixes new significations.

The two great obstacles which at present forbid even the hope of success in any attempt at systematic classification of infusoria are: 1, the great difficulty of distinguishing the lower forms of animal from the corresponding forms of vegetable life; 2, that of deciding whether a given form is permanent, or whether we have to do with the larva} of an insect, or some one of those forms which crustaceans, polyps, and other of the lower animals assume in the progress of their alternations of generation. A motion apparently spontaneous was formerly supposed to decide the question in favor of an animal nature: but Vaucher of Geneva (1700) proved that a motion not to be distinguished from the spontaneous movements of animals is common in the spores of the simpler aquatic plants, and is indeed nature's provision for their dispersion. That animals absorb oxygen and give out carbon, while plants give out oxygen and absorb carbon, affords in the opinion of many naturalists the desired test. But although this is a very general, it is not found to be a uniform law. A third distinctive mark, and probably the most, useful, is found in the character of their nutritive material - plants being nourished by inorganic, animals by organic food.