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a genus of mobile colonial Chlorophyta (green algae) living in stagnant fresh waters. In the USSR there are four species. Volvox has the shape of a sphere measuring up to 3 mm in diameter; on the periphery of the sphere there is a single layer of from 200 to 50,000 cells, which are connected by protoplasmic threads: the cavity is filled with a liquid mucus. Two flagella extend outward from each cell; their movement propels the organism. A few larger cells serve for asexual and sexual reproduction; the sexual process is oogamy.