free-swimming mostly freshwater flatworms; popular in laboratory studies for the ability to regenerate lost parts
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