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If a little hay is steeped in water fora few days, and the infusion is then examined, it will be found to teem with microscopic life. Similarly the natural infusions offered by accumulations of water containing decaying animal and vegetable matter, or ponds where the simpler forms of vegetation flourish, are tenanted by countless millions of minute animals of various kinds, which, from their mode of occurrence, were named by the earliest observers Infusoria. The term at first included many organisms which, further investigation showed, could not be retained in the same class; plants mistaken for animals, because they moved about; animals of higher organisation, such as the Rotifera; others of a lower type, like the Amœba. This sifting process is even yet far from complete. It is not improbable that many forms now placed in this class, when their life history comes to be written, will have to be separated from the Infusoria properly so called. The number of species may also be reduced, For in the imperfection of our knowledge it is well to remember that forms which to-day are counted as distinct species may hereafter prove to be only different stages of the same animal.

Like other members of the sub-kingdom Protozoa, the creatures we are considering possess a simple body net divided into segments, and one which cannot be cut into two exactly corresponding halves. There is no definite alimentary canal, hut digestion is effected indifferently in any portion of the fluid contents of the body. Pellets of food may he lodged in vacuoles extemporised in various parts of the interior, but they are not enclosed in stomachs separated by any sort of wall from the surrounding mass. Compared with other Protozoa, the Infusoria exhibit a more advanced differentiation of structure. The fluid protoplasm or sarcode, of which the bulk of the body consists, passes externally into a denser portion, the so-called cortical layer; which again is often protected by a still firmer covering termed the cuticle. Food is admitted by a distinct ciliated month opening into a short ciliated gullet, whence it passes, together with a small quantity of water, into the general body-cavity. When a proper month is not present, there is at least an oral region where a mouth may be extemporised. Refuse is excreted at a particular spot situated near to or remote from the mouth; but the discharging orifice is not, as a rule, permanently visible. One, two, or more contractile vesicles, having a fixed position in the cortical layer and connected with channels leading inwards, serve by their slow expansion and quick contraction to keep up a sort of circulation in the fluid interior, and to purify the sarcodic contents. In the same layer are found a nucleus, the female element, and, attached thereto, a nucleolus, the male element of the reproductive process, Reproduction takes place either by self-division lengthwise or crosswise, or by the conjugation of two individuals; the former method being characteristic of the sedentary, the latter of the free-swimming Infusoria, The same species may multiply in both ways.