Cells mostly radially symmetrical, sometimes bilaterally symmetrical, flattened, dorsoventrally curved or slightly asymmetrical; cell shape ellipsoidal, ovoid, cylindrical, pyriform, or fusiform to almost spherical; cell size and shape may vary within a given species depending on different environmental conditions; cell surface smooth with distinctive mucilaginous cell coat; flagella 2, of equal length; chloroplast single, cup-, dish-, or bell-shaped; pyrenoid basal with continuous starch shell; eyespot anterior; nucleus anterior; Golgi bodies (2-4) parabasal; contractile vacuoles absent (freshwater species of doubtful affiliation); asexual reproduction by longitudinal division of vegetative flagellate cells; asexual cysts subspherical, thick-walled with bumpy surface; sexual reproduction isogamous, gametic fusion involves flagellar agglutination and activation of mating structures; several species are homothallic, the type species is reported to be heterothallic; zygote with a thick, smooth wall; after a resting stage, the zygote forms 32 progeny cells, which are liberated through a rupture in the zygote cell wall; meiosis takes place during germination of the zygote; habitat euryhaline or hypersaline.