A planarian is one of many flatworms of the traditional class Turbellaria.[2][3] It usually describes free-living flatworms of the order Tricladida (triclads),[4] although this common name is also used for a wide number of free-living platyhelminthes.[2] Planaria are common to many parts of the world, living in both saltwater and freshwater ponds and rivers. Some species are terrestrial and are found under logs, in or on the soil, and on plants in humid areas.