genus of freshwater, chlorophyll-containing green algae (division Chlorophyta). The spherical or oval hollow colonies, one cell in depth and sometimes exceeding the size of a pinhead, contain from 500 to 60,000 cells embedded in a gelatinous wall.

Asexual colonies have biflagellated somatic cells and reproductive cells (gonidia) that produce small daughter colonies within the parent. Developing ova or spermatozoa replace gonidia in sexual colonies. Fertilization of eggs results in zygotes, which encyst and are released from the parent colony after its death. Thick-walled zygotes formed late in the summer serve as winter resting stages.

Volvox exhibits differentiation between somatic and reproductive cells, a phenomenon considered by some zoologists to be significant in tracing the evolution of higher animals from Protozoa. Certain species, in which somatic cells appear to be joined by cytoplasmic strands, may be considered as multicellular organisms.