a class of multicellular green algae measuring 1-20 cm high. The plants may be bushy or spherical. Bushy species have single-row shoots, sometimes forming anastomoses. The cells are multinucleate. There is one reticulate chloroplast or many small discoid ones. Pyrenoids are present. The cells of the thallus split, forming daughter cells within the maternal cell. Asexual reproduction is by means of zoospores or, less frequently, akinetes. When the plants reproduce by akinetes, the entire cell becomes a spore. The sexual process is isogamy or, sometimes, oogamy. As a rule, the alternation of forms is isomorphic. Meiosis occurs during spore formation or, less commonly, gamete formation.
There are 16 genera, with about 300 species, distributed mainly in tropical seas. Four genera, embracing about 60 species, are found in the USSR. In freshwaters large masses of the algae overgrow hydro-engineering structures. The algae are eaten by aquatic animals. [23–1435–]