A small class of mostly motile, photosynthetic, unicellular algae in the chlorophyll a-b phyletic line (Chlorophycota). It is segregated from the Chlorophyceae primarily on the basis of ultrastructural characters, especially the possession of one or more layers of polysaccharide scales outside the plasmalemma. Prasinophytes are mainly members of the marine plankton, but they are also found in brackish- and fresh-water habitats. A few are benthic, with both coccoid and colonial forms known, while others live symbiotically within dinoflagellates, radiolarians, and turbellarian worms. Approximately 180 species are known in 13 genera.