Flatworm, major group of simple animals that includes the parasitic flukes and tapeworms and the free-living flatworms. Flatworms have long, unsegmented, extremely thin bodies. The largest free-living species grow 1 ft (30 cm) long, though parasitic species may attain 40 ft (12 m). They are found under bark, in fresh water, and in the sea. Freshwater planarian flatworms are often used in biological research.