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contractile vacuole

noun, Cell Biology
1.
a membrane-enveloped cellular organelle, found in many microorganisms, that periodically expands, filling with water, and then contracts, expelling its contents to the cell exterior: thought to be important in maintaining hydrostatic equilibrium.
Origin
1875-1880
1875-80
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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Encyclopedia Article for con tractile vacuole

contractile vacuole

regulatory organelle, usually spherical, found in freshwater protozoa and lower metazoans, such as sponges and hydras, that collects excess fluid from the protoplasm and periodically empties it into the surrounding medium. It may also excrete nitrogenous wastes. In amoebas it changes position with the animal's movement; in most ciliates it follows a definite path through the cell; in the Euglena and other flagellates it remains stationary. The filling and emptying cycle may last from seconds to a minute, depending on the species

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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