ANIMALCULE, a term etymologically applicable to any very small animal, and limited in actual use to those which are microscopical. Animalcules exist in prodigious numbers, and of many different kinds, their size being such that multitudes of them find ample space for all the movements of an active life within a single drop of water; and they abound almost wherever there is moisture, at least wherever organic matter is present. The monas crepusculus of Ehrenberg is only 1-2000th part of a line, or 1-24,000th part of an inch in diameter. " Take any drop of water from the stagnant pools around us," says prof. Rymer Jones, "from our rivers, from our lakes, or from the vast ocean itself, and place it under the microscope; you will find therein countless living beings moving in all directions with considerable swiftness, apparently gifted with sagacity, for they readily elude each other in the active dance they keep up Increase the power of your glasses, and you will soon perceive, inhabiting the same drop, other animals, com pared to which the former were elephantine in their dimensions, equally vivacious and equally gifted. Exhaust the art of the optician, strain your eyes to the utmost, until the aching sense refuses to perceive the little quivering movement that indicates the presence of life, and you will find that you have not exhausted nature in the descending scale." Animals belonging to different classes are, however, microscopical, and the term A. is either applied to them all with reference to their mere size, or it is restricted to those which received from Muller, with whom the scientific study and classification of them may be said to have begun, the name of animalcula infusaria, and which are by Cuvier made the fifth and last class, under the name infusoria, of his fourth great division of the ani mal kingdom, radiata. See INFUSORIA. The name infusoria, indeed, etymologically considered, is not more appropriate than animakula, perhaps not quite so much so, as only a small proportion of the animals of this class are actually found in infusions, but it continues to be generally employed by zoologists. Attempts have been made to classify
them according to their structure, and to assign them their proper places accordingly in the general arrangement of the animal kingdom ; and one part of them have been formed into a class under the name rotifera (q.v.), regarded as probably belonging to the articulated division; another part, formed into a class called poloastrica (q.v.), consisting of the simpler kinds, have been in like manner somewhat doubtfully referred to the radi ated division. Agassiz unhesitatingly describes the class infusoria as "an unnatural combination of the most heterogeneous beings." He regards many as locomotive alga; and of those which are true animals, he expresses the opinion that many are merely the chrysalis states of other animals. There still remain, however, many kinds which are perfect animals.
Among the most remarkable discoveries of modern science must be reckoned that of fossil animalcules, in such abundance as to form the principal part of extensive strata. This discovery was made by Ehrenberg, who found the polierschiefer (polishing-slate or tripoli) of Bilin to be almost entirely composed of the silicious shields of a minute fossil A., the length of one of which is about thtli of a line, so that about 23 millions of ani malcules must have gone to form a cubic line, and 41,000 millions to form a cubic inch of the rock. Ehrenberg succeeded in detecting the formation of similar strata in deposits of mud at the bottom of lakes and marshes, the mud swarming with living animalcules, probably in their turn to be fossilized. The bergmehl or mountain meal of Sweden and other parts of Europe, which is sometimes used as an article of food, is entirely composed of the remains of animalcules; not merely, however, of their silicious shields, for it contains a considerable percentage of dry animal matter. Some animalcules prefer waters impregnated with iron, and their death gives rise to an ocherous substance, in which iron is the principal ingredient.