Any of about 3,000 species of widely distributed, mostly free-living flatworms of the family Planariidae and related families (class Turbellaria), usually found in freshwater but also in marine and terrestrial environments.
The soft, ciliated body is leaf-shaped when elongated. The spade-shaped head has two eyes and sometimes tentacles. The tail is pointed. The mouth is on the lower side, often more than halfway toward the tail. Most species are 0.1–0.6 in. (3–15 mm) long; some grow to about 1 ft (30 cm). Planarians swim with an undulating motion or creep like slugs. Most feed at night on protozoans, snails, and other worms.