INFUSO'RIA, a class of the sub-kingdom of animals called protozoa (q.v.). The term, originally almost synonymous with animalcules (q.v.), is now very much restricted in its signification. It was first used by Otto Friederich Muller, and was adopted by Cuvier, who made the infusoria the last class of 1•adiata (q v.). But their radiated structure is by no means established. No distinct trace of nervous matter has been found.—After (1773-86), the next to devote himself to the special study of the infusoria was Ehrenberg, the publication of whose work on them (1837) was the com, meucement of a new era in the history of this branch of zoology, which has since been prosecuted with great industry by Dujaroin, Stein, Lachmann and Claparede, Cohn; LieberktIlm, Rymer Jones, and others. Many of the organisms included by Ehrenberg,' as by previous naturalists, among infusoria, are now regarded as vegetable (see DESIIIDE3E and DIATOMACEPE); whilst others, as the CereaTIM (q.v.), have been dis covered to be immature states of entozoa. The rotifera (q.v.) are now also, by very general consent, widely separated from the polygastrica of Ehrenberg, for which alone. the term infusoria, although not unobjectionable (see ANIMALCULE), is retained; the term polygastrica (Gr. many-stomached) being rejected, because it expresses a view of the structure of these creatures which is generally deemed erroneous. Agassiz has gone the length of proclaiming an opinion, not received by other naturalists, that the info soria are all immature or larval worms. But of the forms at present known, it is at all events probable that many are those of immature creatures; it is certain that some species assume very different forms at different stages of their existence; and the whole. life-history of no one species is fully known.
Some of the infusoria are large enough to be, individually visible to the naked eye, but most of them are altogether microscopic. Their bodies are composed of sarcode, a glutinous diaphanous substance, of which the Outer layer sometimes forms a more or less resisting integument. The body has some well-defined form, of which the varieties are very great in different species. Many are furnished with cilia, the motion of which carries them with great rapidity through the fluid in which they live, and by means of which also currents are created in the fluid to bring food to the mouth. Tue 'mouth is very generally surrounded or largely provided with cilia.. Whether these organs are under the control of will, or maintain their motion without will or even consciousness on the part of the creature, like the cilia of the epithelium in higlAr animals, is hot determined. There is an analogy in favor of the latter, opinion, and many appearances —which, however, the phenomena of zoospores, etc., teach us to regard as possibly deceptive—in favor of the latter. Some infusoria, instead of cilia, have a few slender filaments, which they agitate with an undulatory movement; others move by contrac tions and extensions of their bodies. Some have stiff bristle-like organs, which they use as feet for crawling on the surfaces of other bodies; and some have hooks, by which they attach themselves to foreign bodies.
All infusoria have a distinct mouth, and many have also an anal opening, sometimes near the mouth, sometimes at the opposite extremity of the body. Between these, Ehrenberg imagined that he could trace an intestine, straight in some, variously bent in others, with which along its course many small stomachs are connected; whilst in infusoria having only one aperture, he supposed all the stomachs to open immediately from it. But other observers have failed to find the canal and stomachs, although Ehrenberg's experiments, by means of fluids colored with indigo and carmine, have been often repeated. And it seems probable that the food taken into the mouth is simply conveyed into the midst of the soft gelatinous substance of the body, being formed into pellets as it passes from the mouth through a kind of gullet in time firmer integument. The food of infusoria consists of organic particles of various kinds, and different species have been remarked to show it preference, like those of higher animals, for particular kinds of food. Many of them feed on microscopic plants and on other
infusoria. Their great use in the economy of nature is probably to consume organic particles, the decomposition of which would otherwise lie baneful to all life, and the return of which by decomposition to their primitive elements would diminish the fertility and wean of the world. The numbers of the infusoria are prodigious. They are found in all parts of mhe world, and both in fresh and salt water, in slagnant ponds and ditche., in mineral and hot springs, and in moist situations. Any infusion or other liquid containing vegetable or animal matter, expo....ed to the atmosphere, is sure' to be full of them. Their multitudes are so great that leagues of the ocean are some times tinged by them. Some, which, instead of swimming freely, like most of their class, become surrounded with a gelatinous substance, are found adhering together in masses sometimes 4 or 5 in. in diameter, although the individual animals are so small that a cubic inch of the mass may contain 8,000,000 of them. The infusoria contained in a single cup of putrid water may exceed in number the whole human population of the globe I The organization of the infusoria is still very imperfectly known. There appears in many of diem a cavity not far from the mouth, the contractile space—variously regarded as a cavity without proper walls, or as a vesicle—from which branches sometimes radiate through the substance of the body, and which, being capable of contraction and expan sion, is regarded by some as the center of a kind of vascular system. It is with con siderable probability regarded as furnished with proper walls. There is also, probably in all the infusoria, another organ, evidently of great importance, although its use is still uncertain, calred the nucleus, which is usually roundish or a little elongated, some times much elongated and band-like. It is enveloped in a membrane, and is more com pact than the surrounding substance. In the multiplication of these animals by spon taneous division, a fission of the nucleus always takes place. Each of the halves becomes furnished with a complete mouth, set of cilia, and other organs. The division, in the same species, is sometimes longitudinal, sometimes transverse, perhaps alternately longitudinal and transverse. The multiplication of the infusoria in this Nvay is extremely rapid. A paramecium, well supplied with food, has been observed to undergo division every 24 hours, from which would result 16,384 individuals in a fortnight, or 268,435,456 in four weeks. Reproduction also takes place by gemination; buds or gemmules forming on the outer surface of the body, and gradually assuming the shape) of the parent animal, although they do not attain to their full size till after separation. More is another mode of reproduction by encysting or encapsulation. The animal contracts, closes its mouth, becomes surrounded by a viscid secretion, and finally by a membrane, becomes attenuated, and dissolves, all but the nucleus, into a mere liquid containing granules, which afterwards form within the cyst a new infusorium, different in form and appearance from that by which the cyst was produced. The metamorphoses of the infusoria have been traced to a certain extent in some kinds, but not fully in any. Whether any truly sexual propagation takes place has not been per fectly ascertained, although the observations of Balbiani have made it extremely prob able as to some of them. A reproduction, different from all that has yet been mentioned, has been observed to take place in some, by the formation of internal germs, to which this character has been ascribed, but the subject is still involved in doubt; nor is it improbable that there may be amongst these minute creatures a production of real eggs, which has hitherto eluded observation.
In the integument of some infusoria, very minute fusiform bodies are thickly imbedded. called trichocysts, which are capable of throwing out long filaments. Their use is unknown, although they arc supposed to be urticating organs. The filaments are thrown out when the animal is subjected to annoyance by the drying up of the liquid in which it lives, or by the application of some irritating liquid.