M. Ehrenberg has estimated that the Monas corpusculum is not more than the 1/24,000th part of an inch in diameter; whence Dr. M. C. White, assuming that the ova of the Infusoria and the spores of minute fungi are only the 1/10th part in linear dimensions of their parent organisms, concludes that there must be an incalculable amount of germs no larger than the 1/240,000th or 1/100,000th part of an inch in diameter; and since according to MM. Sullivant and Wormley, vision with the most powerful microscope is limited to objects of about the 1/80,000th part of an inch in diameter, we need not be surprised if Infusoria and other organisms appear in putrescible liquids in far greater numbers than the germs in atmospheric dust visible by the aid of microscopes would lead us to expect. The ferments are the least in size and lowest in organization of all the Infusoria. The higher group which abounds in stagnant pools and ditches are exceedingly numerous, and their forms are varied beyond description. They are globular, ovoid, long and slender, short and thick, many have tails, one species is exactly like a swan with a long bending neck, but whatever the form may be, all have a mouth and gullet. Although the skin of the Infusoria is generally a mere pellicle, that of the red Paramœcium and some others resembles the cellulose covering of a vegetable cell, engraved with a pattern; but in all cases respiration is performed through the skin. Whatever form the cell which constitutes the body of the Infusoria may have, the highly contractile diaphanous pellicle on its exterior is drawn out into minute slender cilia which are the locomotive organs of these creatures. Vibrating cilia form a circlet round the mouth of some of these animalcules, a group of very long ones are placed like whiskers on each side of it, as in the Paramœcium caudatum, and in some cases there is a bunch of bristles in front. Certain Infusoria have cilia in longitudinal rows, and in many the whole body is either partially or entirely covered with short ones. In some Infusoria their vibrations are constant, in others interrupted, and so rapid that the cilia are invisible. These delicate fibres which vary from the 1/500th to the 1/13,000th part of an inch in length, move simultaneously or consecutively in the same direction and back again, as when a fitful breeze passes over a field of corn. These animalcules seize their prey with their cilia, and swim in the infusions or stagnant pools, in which they abound, in the most varied and fantastic manner; darting like an arrow in a straight line, making curious leaps and gyrations, or fixing themselves to an object by one of their cilia and spinning round it with great velocity, while some only creep. These motions, which bring the animalcules into fresh portions of the liquid, are probably excited by the desire for food and respiration. [Illustration: Fig. 104. Paramœcium caudatum. _a_ _a_, contractile vesicles; _b_, mouth.]