Volvox is a coenobial and colonial alga. Volvox colonies multiply so rapidly during the rainy season that the entire pond is covered by them and appears greenish. The colonies are oval to spherical in shape , having 500 to 60, 000 cells. The cells have a typical chlamydomonad type of structure. There is a single nucleus , cup shaped chloroplast , two contractile vacuoles , an eye spot, two flagella inserted at the anterior end. Reproduction in Volvox is of two types — asexual reproduction and sexual reproduction . Asexual reproduction occurs by means of special types of cells located at the posterior end of the colony . These special cells are called as gonidia. Sexual reproduction occurs at the end of the growing season. Male sex organ is known as antheridium and the female sex organ is known as oogonium.