INFUSORIA, protozoa or organisms of the primary division of the animal kingdom, consisting of single cell or groups of cells, of the classes Flagellata and Ciliata, so-called from propagating and abounding in infusions of o• ganic matter. While the term, first used in 1763, is now restricted to the ciliate protozoans, it often includes the flagellate protozoans as well. The latter are represented by the monads. These are exceedingly minute, round or pear shaped animals, which move by one or two lash-like processes called flagella. They contain a nucleus and contractile vesicles. Some of them are fixed by a stalk, and are provided with a collar, as in Codosiga, out of which the flagellum projects. One of the simplest monads (Heteronnta) is obtained by placing a cod's head in water at a temperature of about 70° F. In a few days the water will swarm with these monads. The young germs will live in boiling water, but perish at a temperature of from 212° to F., while the adults are destroyed at 142° F.
In the ciliata infusoria the body is more or less flattened and covered with cilia (Para mecium, etc.). They have on the under side of the body a slightly defined mouth (or cyto stome), which is permanently open, and the food is swept into it by the action of the cilia around it. The mouth leads into a funnel shaped throat or cytopharynx, which ends in the protoplasm of the body. The food-particles swept into this throat and pressed into the protoplasm form a small enlargement which finally sinks farther in forming the '
nucleus (micronucleus) is much smaller and is concerned with reproduction. Reproduction oc curs usually by self-division, and more rarely the infusorians contract into a ball and divide " ' • into sres, which grow to become adults. The periods of fission are at times interrupted by the process of conjugation, which only differs from sexual reproduction in the 'fact that two individual infusorians meet and fuse together and then separate, the remit being a process of fertilization which leads to a complete new formation of the nucleus, and thus
a new organisation of the animal.
The more specialized infusoria are Senator and Vorticeila. The former is large enough to be seen without a lens. It is purplish, and under the shows itself top be,alieautiful creature. It is trumpet-shap6d: with a''spiral tract of thicker cilia around the mouth-end' The most highly organized infusoria are the bell-animalcules (earelsesiutn, etc.); which are compound bell-shaped forms, forming colonies with forked branched stalks. The nucleus is sausage-shaped, and near it is the micronucleus. They form a white mass like mold 'on the stems and leaves of aquatic plants. Some of the infu soria are parasitic in the digestive and circula tory organs of the higher animals. For a more detailed and illustrated account, consult, W. Saville-Kent's 'Manual of the infusoria, in cluding a description of all known ciliate and tentaculiferous protozoa, British and foreign, and an account of the organization and affinities of the sponges' (London 1880-82).
Bibliography.— Bennett, J. H., 'On the at mospheric germ theory and origin of infusoria> (Edinburgh 1868) ; Ehrenberg, C. G., 'Verbrei' tung und Einfluss des mikroskopischen Lebent in Siid und Nord-Amerika" (Berlin 1843)•; Jennings, H. S. 'Behavior of the lower organ isms' York 1906); Lund, E. J., 'T* relations of bursaria to food' (Baltimore 1914Y; Pritchard, A., 'History of Infusoria) (London 1861) ; Stein, F. von, 'Der Organismus der Infusionstiere' (Leipzig 1859-83) ; Stevens, N. M., 'Studies on the ciliate infusoria licno phora and boveria' (Bryn Mawr 1903) ; Smith, I. P., 'The Infusoria of Kansas' (University Of Kansas 1914); Woodruff,. L. L, 'Study on the life history of hypotnchous (Baltimore 1905).