Animalcule. a term which, although etymologically applicable to any very small animal, is limited in ordinary language to those which are microscopical. Animalcules exist in prodigious numbers, their size being such that myriads of them find ample space for all the movements of an active life within a single drop of water. Seawater often contains them in enormous numbers, and the luminosity of the sea is often due to this cause (Noctiluca). Although, contrary to a widely diffused belief, they occur only in very small number in drinkable waters, they abound wherever water becomes stagnant, or contains decomposable organic matter. Thus rain-water allowed to stand long in an open cistern, or the water of a vase in which cut flowers are placed, soon becomes more or less turbid and offensive; and if a drop be placed on a slip of glass and examined, even with a pocket lens, a multitude of living beings can be seen moving rapidly in all directions, while minute specks are also to be seen in motion between these. On the application of higher microscopic power, new organisms come again into view, so that the variations of size between the invisible inhabitants of one drop are as great as those between whales and minnows. An immense variety of animalcules can very easily be studied by collecting impure water from a dozen different sources, and keeping it separate in open wide-mouthed bottles in a window, and observing from time to time; for not only do the contents of the different vessels differ from each other, but they also vary greatly with the season, so that an unending source of new surprises is thus open to the most inexperienced microscopist without leaving his room. Besides obtaining numerous varieties of microscopic algæ, diatoms, bacteria, &c. (see ALGÆ, DIATOMS, PROTOPHYTES), examples of all the leading forms of minute animal life are thus to be obtained; and these, at first supposed to belong to the same general type of structure, are now known to be extremely varied. The simplest form which the observer will meet is a naked lump of jelly-like protoplasm, constantly flowing into new shapes, the Ameba (q.v.); while other masses of jelly, the Foraminifera (q.v.), may be found possessed of coverings of sand, or even carbonate of lime, and only protruding their irregular processes (pseudopodia) through its openings. Others again, the sun-animalcules of fresh water, and the Radiolarians, which inhabit the sea, are usually possessed of a beautifully marked flinty skeleton. These groups are usually united under the head of Rhizopoda (q.v.). Another great series, in which the form of the body is usually defuite, the pseudopodia being generally replaced by vibratile threads or cilia, are termed the Infusoria (q.v.). All these are the equivalents only of a single cell of higher animals, and are therefore grouped into the sub-kingdom Protozoa; but many animalcules are of far more complex organisation. Thus the wheel-animalcules (see ROTIFERA) are segmented, worm-like animals; and the larvæ of almost all marine and fresh-water invertebrates are at an early stage free-swimming and microscopic. From its extreme vagueness, therefore, the term animalcule is now disused by scientific writers.
Despite their apparent insignificance, certain animalcules, by virtue of their almost imperishable skeletons, are among the most important agencies which have built up the crust of the earth. The surface of the sea is largely inhabited by Radiolarians and Foraminifera, the former preponderating in cold, the latter in temperate and tropical waters. As they die, their skeletons sink to the bottom, and form mud or ooze, which through time and pressure becomes consolidated into rock. Many polishing stones, &c. are thus mainly composed of Radiolaria; while chalk is principally formed by the skeletons of Foraminifera, and greensand of internal siliceous casts of these. Many limestones, marbles, quartzites, &c. are probably of similar origin, although all trace of organic structure may have been eliminated by metamorphic change. See PROTOZOA, PROTOPHYTES, and other articles named above; also any adequate work on microscopy—e.g. Carpenter On the Microscope, and the Micrographic Dictionary.