The cells contain a single plastid that occupies most of the cell periphery and contains at least one pyrenoid. The cells are organized into branched filaments that, together, produce a plant body less than 2 mm in diameter. In some species, the filaments are laterally compressed, forming a circular thallus with no apparent gaps between cells. Cultured plants that do not become fixed to a substrate may form spherical balls of cells. They are motile by means of two flagella of equal length inserted near but not at the apical end of the cell.
Male gametes tend to be almost colorless, unlike the pigmented zoospores.
Zoosporangia and antheridia look like vegetative cells until the motile cells are released. They remain on the plant body until after fertilization and cyst maturation. Cysts are spherical, are somewhat larger than vegetative cells and have a thickened wall with various types of ornamentation. They are formed from fertilized eggs. A more or less well organized jacket of sterile vegetative cells surrounds the cyst while it is attached to the plant body.