A class of the phylum Platyhelminthes commonly known as the flatworms. These animals are chiefly free-living and have simple life histories. The bodies are elongate and flat to oval or circular in cross section. Their length ranges from less than 0.04 in. (1 mm) to several centimeters, but may exceed 20 in. (50 cm) in land planaria. Large forms are often brightly colored. Smaller forms may have black, gray, or brown parenchymal pigment or may be white or transparent except for the color of ingested food and symbiotic algae. This class, which numbers some 3400 described species, is ordinarily subdivided into the 12 orders: Acoela, Catenulida, Haplopharyngida, Lecithoepitheliata, Macrostomida, Nemertodermatida, Neorhabdocoela, Polycladida, Prolecithophora, Proseriata, Temnocephalida, and Tricladida.