any of a group of green algae (division Chlorophyta) that are common in fresh water. Colonies vary from loosely associated flat disks of similar organisms (Gonium) to the complex spherical arrangement of Volvox. Each cell has a central nucleus and two or four flagella protruding from an opening in the anterior end of the closely fitting cellulose wall. Chromatophores contain green pigments. Sexual reproduction occurs by isogamy, anisogamy, or oogamy.