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any living biological entity, such as an animal, plant, fungus, or bacterium



any living being.

Unicellular and multicellular organisms are distinguished from nonliving matter by several basic vital properties: cellular organization (seeCELL); metabolism, by which proteins and nucleic acids regenerate the organism and maintain a constant internal environment (see, HOMEOSTASIS); movement in all its specific forms—muscular, cytoplasmic, ciliary, and flagellar (seeMOVEMENT); excitability; growth and development; reproduction; variability and heredity (seeVARIATION); and adaptability (seeADAPTATION). Prokaryotes are organisms that lack chromosomes and a typical cell nucleus; examples are bacteria, blue-green algae, Rickettsia, and members of the order Mycoplasmatales. Prokaryotes are simpler in structure and smaller in size than the smallest cellular organisms; for example, the diameter of an animal cell is more than 3 microns (μ), while that of a bacterial cell is usually less than 3 μ. One of the smallest bacteria consists of a total of 5 × 107 atoms once its water is removed.

In its interaction with the environment, an organism is coordinated on the cytoplasmic, cellular, tissular, organic, and organismic levels so as to act as a unified system. The formation of a complete organism in phylogeny consists in the differentiation and integration of cells, tissues, organs, and functions (seeDIFFERENTIATION, INTEGRATION). In unicellular organisms vital functions are effected by special organelles. Over the course of evolution, the development of multicellularity permitted the progressive morphophysiological complexification and differentiation of organisms. This complexification and differentiation is made possible by the structural and functional coordination of cells, tissues, and organs, which is achieved through nervous and humoral means.

The interdependence of organs over the course of animal evolution was comprehensively studied by A. N. Severtsov and students of his school. The phylogenetic aspects of the evolution and differentiation of tissues that arose from cells with common structures, functions, and developmental courses were studied by A. A. Zavarzin, N. G. Khlopin, and A. V. Rumiantsev and their students. The differentiation and integration of organs and functions were also studied by many other Russian and Soviet scientists, including I. I. Mechnikov, I. P. Pavlov, I. I. Shmal’gauzen, and V. A. Dogel’, as well as by foreign scientists, including E. Haeckel, A. Dohrn, and G. de Beer.

Modern biology, especially genetics, has elucidated the genetic connection between generations of organisms and the connections between phylogeny and ontogeny on every level of organization in an individual organism. (SeeHISTOGENESIS, , , MORPHOGENESIS, PHYLOGENY.)


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Khlopin, N. G. Obshchebiologicheskie i eksperimental’nye osnovy gistologii. Moscow, 1946.
Severtsov, A. N. Morfologicheskie zakonomemosti evoliutsii: Sobr. soch., vol. 5. Moscow-Leningrad, 1949.
Zavarzin, A. A. Izbr. trudy, vols. 1–4. Moscow-Leningrad, 1950–53.
Shmal’gauzen, I. I. “Integratsiia biologicheskikh sistem i ikh samoreguliatsiia.” Biul. Moskovskogo obshchestva ispytatelei prirody; Otdel biologicheskii, 1961, vol. 66, fasc. 2.
Shmal’gauzen, I. I. Reguliatsiia formoobrazovaniia v individual’nom razvitii. Moscow, 1964.
Amlinskii, I. E. “Nekotorye problemy stanovleniia mnogokletochnosti.” In the collection Struktura i formy materii. Moscow, 1967.
Ryzhkov, V. L. “Mesto individa sredi biologicheskikh sistem.” In the collection Razvitie kontseptsii strukturnykh urovnei v biologii. Moscow, 1972.
De Beer, G. R. Embryos and Ancestors. Oxford, 1958.
Regulation and Control in Living Systems. Edited by H. Kalmus. New York, 1967.



An individual constituted to carry out all life functions.
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