Colonies large (up to 1.5 mm), more or less spherical to ellipsoidal, usually containing many hundreds to thousands (up to 60,000) cells in a single layer on periphery of a common gelatinous matrix; colonial boundary tripartite, each cell also surrounded by an individual extracellular matrix; 2 cell types, somatic and reproductive; somatic cells identical, spherical, ellipsoidal, pyriform or stellate, with 2 outwardly directed flagella; neighbouring cells attached by cytoplasmic connections; chloroplast single, cup-shaped or irregularly disciform, with a single (rarely several) pyrenoid(s); eyespot single, usually largest in anterior cells and progressively smaller or absent in posterior cells; nucleus more or less central; contractile vacuoles 2-6; reproduction, both asexual and sexual, restricted to relatively few reproductive cells (gonidia) which lose flagella and are somewhat larger than somatic cells; asexual reproduction by formation of 2-80 daughter colonies; initial sequence of cell divisions leading to small hollow spheres of cells or plakeas, in which flagellar bases and nuclei are oriented inwards; new gonidial cells being formed by unequal division of certain cells early in cleavage process; developing daughter colonies undergo inversion; sexual reproduction strictly oogamous; most species heterothallic, some homothallic, either monoecious or dioecious, some species heteromorphic with dwarf male colonies; female reproductive cells (2 to 700 per colony) enlarging to form oogonia with single non-flagellated eggs (oospheres); male reproductive cells dividing and developing into plate-like or globose sperm packets or plakeas of 16-512 biflagellate spindle-shaped antherozoids; zygotes thick-walled, smooth or variously ornamented, often with reddish contents; on germination, a single (rarely 4) biflagellate (sometimes aflagellate) gone cell escaping from zygote wall and dividing into a gone colony; nutrition phototrophic or photo-organotrophic; common, in bogs, pools, ponds, ditches, but most species with restricted geographical distribution.