planarian vs flatworm

planarian and flatworm both are nouns.

Word	Noun	Adjective	Verb	Adverb
planarian	Yes	No	No	No
flatworm	Yes	No	No	No

As nouns, flatworm is a hypernym of planarian; that is, flatworm is a word with a broader meaning than planarian:

planarian: free-swimming mostly freshwater flatworms; popular in laboratory studies for the ability to regenerate lost parts
flatworm: parasitic or free-living worms having a flattened body

Other hypernyms of planarian include platyhelminth.

planarian (noun)	flatworm (noun)
free-swimming mostly freshwater flatworms; popular in laboratory studies for the ability to regenerate lost parts	parasitic or free-living worms having a flattened body