A group of branched, filamentous green algae, commonly known as the stoneworts, brittleworts, or muskgrasses, that occur mostly in fresh- or brackish-water habitats. They are important as significant components of the aquatic flora in some locales, providing food for waterfowl and protection for fish and other aquatic fauna; as excellent model systems for cell biological research; and as a unique group of green algae thought to be more closely related to the land plants.