1) Flatworm 2) Planaria 3) Platyhelminth 1) Planaria 
A planarian is one of many non-parasitic flatworms of the Turbellaria class. It is also the common name for a member of the genus Planaria within the family Planariidae. Sometimes it also refers to the genus Dugesia. Planaria are common to many parts of the world, living in both saltwater and freshwater ponds and rivers. Some species are terrestri...
• (n.) One of the Planarida, or Dendrocoela; any turbellarian worm.
any of a group of widely distributed, mostly free-living flatworms of the invertebrate class Turbellaria (phylum Platyhelminthes). Planaria is the ... [8 related articles]
Pla·na'ri·an noun (Zoology) One of the Planarida, or Dendrocœla; any turbellarian worm. -- 
[n] - free-swimming mostly freshwater flatworms
planaria noun free-swimming mostly freshwater flatworms; popular in laboratory studies for the ability to regenerate lost parts