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.
So much for the medium in which they live, move, and have their being. Let us pass on to their organization. We have already dwelt on their extreme minuteness; their mean size is a fifth of a line or the sixtieth part of an inch; the largest species scarcely reveal themselves to the naked eye. They are generally colourless; some of them are, nevertheless, green, blue, red, brown, and even blackish. Seen on the object-glass of the microscope, they appear to be gelatinous, transparent, and naked, or invested with an envelope more or less resistant, which we shall designate after Dujardin by the term Sarcoda, a substance which is homogeneous, diaphanous, elastic, contractile, and, above all, destitute of every kind of organization. They are usually ovoid or globular. Those most frequently met with, and which attract the most attention from observers, are furnished with vibratile cilia, which cover the whole body, acting as paddles. These organs are evidently intended to propel the animal from one place to another. At other times they appear to be employed in conveying food to the mouth, if we may use the expression. Some Infusoria are without these cilia, having only one or many very slender filaments, the undulating movement of which suffices to determine their progression through the liquid which surrounds them.
Authors who have written on the Infusoria have sometimes, like Leuwenhoek, Ehrenberg, and Pouchet, attributed to them a very complex structure. Others, like Müller, Cuvier, and Lamarck, have considered them to be gifted with an organization extremely simple. We shall probably find that the truth lies between these two extremes.
In the superior Infusoria, besides the granules, the interior globules, vesicles full of liquid, vibratile cils, and a tegumentary system, more or less complex, we find the substance which is called Sarcoda.
The digestive apparatus of the Infusoria has been the subject of numerous observations, and has been provocative of very animated discussions. In the inferior order of the class, which comprehends the very smallest animalcules, it has not been found possible to observe the organization of the digestive apparatus in a satisfactory manner. Some writers think they have no mouth, what has been taken for that organ being only hollow dimples on the surface of the body; others recognize the existence of a buccal orifice, sometimes furnished with a solid armature. As to the arrangements of the interior cavities in which digestion takes place, we know nothing certain.
The digestive apparatus is better understood in the superior Infusoria, called ciliate, namely, those provided with vibratile cils. These cils seem to determine the currents of the liquid, leading the nutritive corpuscles suspended in the water towards the entrance of the digestive apparatus. They form, in some sort, the prehensile organs which seize the aliment. The cils are, at the same time, the organs intended to facilitate respiration; in short, these little whips playing upon the water unceasingly round the Infusoria, is just the action required for the absorption of the oxygen contained in the water. These cils, then, serve at once for the propulsion of the animal, for its nutrition, and for its respiration, presenting a remarkable example of cumulative functions in physiology.
The corpuscles of nutritive substances directed towards the buccal orifice by the vibratile cils soon disappear in the interior of the animal. Availing himself of this fact and the transparency of the animal, Herr Gleichen, a German physiologist of the last century, conceived the happy idea of colouring the water which contained these animalcules with a finely-powdered carmine; he traced the colouring matter in the bodies of some of them. But it was reserved for Ehrenberg to avail himself of the same artifice in order to study the internal structure and mode of absorbing nutritive matter in these minute creatures. This physiologist fed many groups of Infusoria, some of them with water coloured with carmine, others with indigo and other colouring matters. He saw, besides, some coloured globules, nearly uniform in size, in different individuals of the same species. From this he arrived at the conclusion that the colouring matter was deposited in many of the surrounding dimples. Ehrenberg thought that each of these dimples was a stomach, and that the introduction of the food into the interior of these reservoirs, as well as the evacuations, were produced by means of an intestine around which these stomachs are arranged. In some cases he even thought he could distinguish the outlines of this intestinal canal, and its connection with numbers of ampula or bladders. Generalizing the conclusions drawn from his observations, in short, we find that his class, Infusoria, embraced two very different forms of animal life, which he divided into Infusoria, Polygastrica, and Rotifera, the latter division including those known as Wheel animalcules; the Polygastrica being so called from his idea that the typical forms possessed a number of stomachs. In some, Ehrenberg counted four stomachs, an organization which brings these microscopic beings into a strange kind of comparison with the ox and the goat. In others he counted two stomachs.