An order of the green algae (Chlorophyceae), also called Caulerpales, Codiales, or Siphonales, in which the plant body (thallus) is a coenocytic filament (tube or siphon). The filaments may be discrete with free or laterally coherent branches, or organized into a dense plexus exhibiting distinctive morphological features. Septa, which are generally infrequent and incomplete, are formed by centripetal deposition of wall material. A large, continuous central vacuole restricts the cytoplasm to a thin layer just beneath the wall. The cytoplasm contains innumerable nuclei, discoid plastids, and other organelles. Pigments are similar to those in higher plants but include two distinctive xanthophylls, siphonein and siphonaxanthin. Some genera have distinct plastids for the storage of starch. The chief constituent of the wall is frequently xylan or mannan, polysaccharides in which the monomer is xylose and mannose, respectively. Vegetative reproduction is common, usually by rhizomes or fragments. The life history, known for only a few genera, comprises either one somatic phase, with meiosis occurring in the production of gametes, or an alternation of strikingly dissimilar somatic phases, with meiosis occurring in the production of zoospores. Motile gametes are biflagellate, while zoospores have a subapical crown of flagella. Reproductive cells may be formed in unmodified or slightly modified portions of the filament or in special organs.