Siphonophyceae

a class of green algae. The noncellular thallus is large, reaching 0.5 m and greater in length, and has several nuclei and many chloroplast disks. The algae contain si-phonein and siphonoxanthine, as well as the pigments characteristic of all green algae. The families Caulerpaceae and Udo-teaceae have leucoplasts, and they have xylan instead of membrane cellulose. In other families the membrane cellulose is replaced by mannan, and there are no leucoplasts. The thallus is filamentous, globose, or segmented into cylindrical and leaflike shoots; sometimes it is calcified. In Codiaceae the thallus resembles tissue: there are fine colorless threads inside and a layer of large bubbles with chloroplasts outside. The plants reproduce vegetatively, sexually, and asexually. The sexual process is anisogamy; there is heteromorphic alternation of sexual and asexual forms of development.

There are approximately 30 genera, with 300 species, growing mainly in warm seas along the littoral zone and at depths to 70 m. More than ten species, belonging to six genera, are found in the seas of the USSR. In tropical countries some species, for example, caulerpa, are used as food. Fossil Siphonophyceae dating back to the Ordovician period have been found.