Infusoria, a name given to several classes of active Protozoa, some of which appear in great numbers in stagnant infusions of animal or vegetable matter. The great majority are provided with vibratile locomotor processes of their living matter, usually in the form of cilia or flagella; and, though these may be retracted when the animal occasionally encysts itself, they are practically permanent, and express the predominantly active constitution of these cells. Most are microscopic, but many are readily seen when foul water is held in a glass vessel between the eye and the light. Yet there may be more Infusorians in a cup of stagnant water than there are people on the globe. Infusorians occur both in fresh and salt water, and a few are parasitic; they feed on vegetable or on animal matter, on bacteria or on one another, while some possessed of a green pigment, closely allied to, if not identical with chlorophyll, probably absorb carbonic dioxide after the manner of plants. Most Infusorians possess a 'mouth'—i.e. a special aperture through which the food-particles are wafted in by the cilia or flagella. As single cells, comparable to the units of ciliated epithelium in multicellular animals, to the active spores of plants, and to male cells or spermatozoa, they exhibit the usual protoplasmic structure and the central differentiation or nucleus. There is usually a definite rind, often with cuticular structures; and there are generally contractile vacuoles, probably excretory in function. Many Infusorians occur not as single individuals, but as members of a colony, the results of multiplication remaining clubbed together, and often forming masses easily visible to the unaided eye. They multiply with great rapidity by dividing into two, or by rapid successive division into a larger number (spore-formation); and thus a single Infusorian, with favourable temperature and nutrition, may in four days become the ancestor of a progeny of a million, in six days of a billion, in seven and a half days of a hundred billions—weighing one hundred kilograms! If the life of the species, however, is to be sustained, conjugation or incipiently sexual union of two Infusorians (not of the same family) must occur, for if the descendants of one individual be left by themselves the whole family falls victim to 'senile degeneration,' and the members dwindle away. In many cases among ciliated Infusorians the researches of Maupas and others have shown that the conjugation of two forms means an interchange of nuclear elements; in other cases the two individuals fuse into one. When the two conjugates are of unequal size, as in the common Vorticella or bell-animalecule, it seems justifiable to call the smaller male and the larger female.
The classes included under the title of Infusorians are as follows, beginning with those ciliated forms to which zoologists often restrict the term.
Ciliata.—Infusorians characterised by the predominance of alternately bent and straightened motile processes known as cilia. The usual nucleus is accompanied by a second neighbour nucleus (para- or micro-nucleus), the elements of which are interchanged in conjugation. They are classified according to the relative position and size of their cilia. The slipper-animalecule (Paramaecium), and Opalina parasitic in the intestine of the frog illustrate those which are more or less completely ciliated (Holotricha); the beautifully-coloured species of Stentor, the genus Balantidium, with one species parasitic in man, and the common Bursaria are among those with heterogeneous cilia dissimilar in size and form (Heterotricha); the stalked bell-animalcule Vorticella and its beautiful allies Epistylis and Carchesium, the jumping Halteria, with a girdle of springy, bristle-like processes, and Ophrydium, which multiplies into large hollow colonies, sometimes 5 inches across, have a special wreath of cilia round the mouth (Peritricha); and lastly, those with cilia restricted to the under surface are well illustrated by Euplotes, Oxytricha, and Stylonichia.
Flagellata.—Infusorians with a vibratile or undulatory flagellum, or with more than one, used for locomotor or food-catching purposes, including a vast number of forms, some of which are often called Monads, while others—e.g. Volvox—approach if they do not unite with the Algae. One of the very commonest flagellate genera is Euglena. To the flagellates proper there have to be added the Choanoflagellata, with a single flagellum surrounded by a beautiful wine-glass-like collar—e.g. Salpingeca, and the interesting Proterospongia—a colony with slight division of labour among its members and like a little fragment of sponge flesh; also the Dinoflagellata, with two flagella, one parallel, the other transverse to the long axis of the body—e.g. Peridinium, an extremely common marine form, affording food to some fishes; lastly, the Rhynchoflagellata, with a large locomotor flagellum, including two genera—the phosphorescent marine ‘night-light’ (Noctiluca), and Leptodiscus, a beautiful bell-like form, which seems within the compass of a single cell like a far-off prophecy of medusoid architecture.
Suctoria or Acinetaria.—Infusorians with cilia only in their free-living youth, usually fixed as adults, and always with prehensile or suctorial processes like tentacles, by means of which they prey upon other Protozoa. Acineta and Podophrys are suctorial; the common Acineta is only prehensile.
In beauty of form and movement, in the liveliness of their behaviour, and in the intricate phases of their life-history, Infusorians afford almost inexhaustible material for investigation, which many workers have shown to be at once captivating in itself and full of biological suggestiveness. In the general economy of nature Infusoria are especially important as a food-supply to small animals, and in so far as they unite with Bacteria in working decaying matter once more into the cycle of life, or in reducing it to simpler elements.
See BACTERIA, MONAD, PARAMÆCIUM, PROTOZOA, VORTICELLA; Claparède and Lachmann, Études sur les Infusoires (Geneva, 1858–61); Stein, Organismus der Infusions-Thiere (Leip. 1859–63); Saville Kent, Manual of the Infusoria (Lond. 1880–82); Ray Lankester, article ‘Protozoa,’ Encycl. Brit. (1885); Maupas, Archiv. Zool. Expér. (vi. 1888); Bütschli, ‘Protozoa,’ in Bronn’s Thierreich.