Members of the kingdom Rhodophyta are the red algae or seaweeds. Almost all are marine and they vary greatly, some being filaments, some cushions, some feathery and others flattened discs. Many have a coating of calcium carbonate and form crusts or calcified trees that resemble corals. They contain phycobilins and chlorophyll a, but not chlorophyll b or c, and there are about 4,000 species known.