Thalli of uniseriate branched filaments with apical and/or intercalary growth. Branches sparse to profuse with branches inserted laterally below apex of cell or apically on cell (a pseudodichotomy). If attached, branching rhizoids arise from basal cell and other cells in basal region, or simple discoid holdfast produced. Chloroplasts parietal, either densely packed discoid and/or united in a reticulum. Pyrenoids in many chloroplasts, bilenticular, flanked by two bowl-shaped starch bodies. Cells multinucleate, nuclei dividing more or less synchronously with nuclear membrane remaining intact. Cell wall polysaccharide mainly crystalline cellulose I, forming numerous lamellae of microfibrils in crossed fibrillar pattern. Asexual reproduction by biflagellate or quadriflagellate zoospores only method of reproduction in some species, some species only reproducing by thallus fragmentation. Akinetes produces by most species under unfavourable conditions, these swollen, thick-walled, filled with starch. Thick walled rhizoids and basal portions of main axes also perennating devices. Sexual reproduction by regular alternation of generations producing biflagellate isogametes and quadriflagellate zoospores. Meiosis precedes spore production. Zoidangia with 1-3 pores, cruciate zooids released under pressure. Zoids of C. rupestris with well developed layer of granules outside the plasmalemma and inside the 'fibrous-layer' which may be involved in cell wall synthesis.