VOLVOX

a freshwater, chlorophyll-containing organism that lives in colonies; it is assigned by zoologists to the flagellate protozoan order Volvocida and by botanists to the green algae (Chlorophyta). The oval, hollow colonies, one cell in depth and about the size of a pinhead, contain from 500 to 60,000 cells imbedded 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 possesses differentiation between somatic and reproductive cells, a phenomenon considered highly 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 to form multicellular organisms.