From that time the Infusoria have been considered as forming a special group among the radiate animals; afterwards, in the pages of Baer and of De Blainville, we see in these creatures, so imperfect in appearance, only the indeterminate prototype of other classes. But ideas changed altogether respecting them when microscopes of great power, and armed with achromatic lens, were employed in their study. Thanks to the labours of Ehrenberg and Dujardin, we have arrived at a better comprehension of the organization of these infinitely small beings. Naturalists have established, with more exactness, the limits of the zoological group to which they belong. Some stagnant waters are so filled with Infusoria that it is only necessary to dip at random into the liquid medium to procure them in abundance. In other waters they form a bed, occupying the whole basin. In general, it is necessary to search for them where the water is calm, and occupied by vegetation of some kind, such as the _confervæ_, or _lemna_, &c., in the marshes, and _ceramium_ if in the sea. Certain Infusoria live not only in water, but also in places habitually moist, as among tufts of mosses; in beds of _oscillaria_, on moist soil, or on air-damp walls. Others live as parasites on the exterior or in the interior of animals, such as _hydra_, _lombrics_, and _naïads_. Quantities of them are found in the liquid excrements and other products of certain organisms, and they have been noted even in women's milk. But, as their name indicates, it is in aqueous infusions, vegetable or animal, that these animalcules abound. Armed with a microscope, the reader may, with very little trouble, afford himself the pleasure of studying these animals. It is only necessary to place some organic débris--the white of an egg, or some grass, for example--in a vase with a large mouth, filled with water, and expose it to the light and air. Certain reagents, as phosphate of soda, the phosphates, nitrates, or oxalates of ammonia, or carbonate of soda added to these infusions, will singularly favour the development of Infusoria. There are also some accidental infusions which seem to furnish these microscopic beings in great abundance. Water which stagnates in garden soil or in vegetable mould, in the watering-cart or in flower vases, is filled with myriads of these beings. * * * * *