A nonparasitic (free-living) freshwater flatworm which is often used in biology experiments studying the regeneration of lost body parts. The term "planarian" can refer to any freshwater flatworm within the taxonomic order Tricladida, in the class Turbellaria, in the phylum Platyhelminthes, or specifically to members of the genus Planaria, in the family Planariidae, in the order Tricladida.