Members of the Chlorophyta are the green algae or green seaweeds. Although traditionally classified as protists they are now generally considered to be part of the plant kingdom. There are two main divisions; the Ulvophycaea, which are mostly marine and multicellular and form filaments or flat blades as in the sea lettuce, and the Chlorophyceae, which mostly live in fresh water and most of which are single-celled. Like other land plants, they contain chlorophyll a and b, with carotenoids as accessory pigments for photosynthesis. It seems likely that the ancestor of all the land plants was a green alga.