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a class of the most highly developed Protozoa.
The principal characteristics of Infusoria include the presence of cilia (for locomotion and feeding) and two types of nuclei (a polyploid macronucleus and a diploid micronucleus, differing in structure and function); the sexual process takes the form of conjugation (contact between or fusion of individuals rather than of gametes). Infusorians vary in size from 12 μ to 3 mm. Their external appearance is varied. Infusorians are mobile and sessile, solitary and colonial, stalked and stalkless, retractile and testaceous. The body often grows out in odd patterns, especially in tentacled Infusoria. Many retractile forms have a shell. The body of an infusorian consists of a solid covering, or cortex (a pellicle and a layer of ectoplasm, sometimes with nematocyst filaments, or trichocysts) and a jellylike endoplasm with nuclei, vacuoles, and various kinds of granules (mitochondria, secretory bodies). The bases (basal granules) of the cilia lie under the pellicle; the cilia are arranged in rows, joined together here and there into membranes, membranelles, and cirri. The fibrillar systems of the cortex—support, skeletal, and muscular—are well developed. Most infusorians feed by swallowing small algae, fungi, and bacteria; some are predators. Some infusorians (astomatous en-doparasites) feed osmotically, by pinocytosis. The food is digested in digestive vacuoles that periodically become detached from the gullet and sink into the endoplasm. The undigested particles are eliminated through the cytopyge. Most infusorians have one or more contractile vacuoles, which regulate osmotic pressure in the cell.
Reproduction is only asexual; by division into two, by repeated division (strobilation), by simultaneous multiple division (palintomy), or, in many sessile forms, by various kinds of budding (external or internal, single or multiple). The sexual process has been modified and is no longer associated with reproduction; rather, temporary contact between (or, less commonly, the fusion of) two individuals occurs in order to exchange the products of the division of micronuclei and to replace the old nuclear apparatus with a new one. In exceptional cases the nuclei can be reconstructed without contact between individuals, by en-domixis and autogamy.
Class Infusoria embraces about 6,000 species (belonging to 400 genera) and is divided into five subclasses: evenly ciliated infusorians (Holotricha), spirally ciliated infusorians (Spirotricha), circularly ciliated infusorians (Peritricha), conically ciliated infusorians (Chonotricha), and sucking infusorians (Suctoria). Infusorians live equally in freshwater and saltwater, where they form part of the plankton and benthos. They also inhabit the soil, wet mosses, and sandy littorals. They play a very important role in sewage treatment plants. Most sessile infusorians and some mobile groups (totaling 2,000–2,500 species) have adapated to ectocommensalism, endocommensalism, and parasitism on practically all aquatic and many groups of terrestrial animals (except birds). Some infusorians are found only on (or in) organisms of a particular taxonomic group. For example, Chonotricha live only on crustaceans; Entodiniomorpha, in ungulates; and Thigmotricha, mostly in mollusks. Sessile infusorians are often found on algae. Many parasitic infusorians, after massive reproduction, become pathogenic for the hosts: Ichthyophthirius for fish and balantidium for domestic animals and man, in whom they cause balantidiasis. The spread of the symbionts and parasites is limited by the area of distribution of their hosts; that of free-living infusorians, by the ambient temperature and other factors. The spread of infusorians is promoted by their ability to form protective wind-borne cysts when bodies of water dry up. Infusorians probably originated from primitive colorless heterotrophic flagellates.
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A. V. IANKOVSKII