A class of marine algae, most of them red in colour, whose basic shape is filamentous or membranaceous. They tend to occur at greater depths than the green algae (Chlorophyta) and they are among the oldest groups of eukaryotic algae, known from the Cambrian upwards. Epiphyton (Cambrian to Devonian) formed mounds and Solenopora (Lower Cambrian to Cretaceous) formed nodular masses made up of close-packed tubes. The coralline red algae, e.g. Lithothamnion, are important rock builders, constructing rigid structures and contributing lime mud to sediments.