flatworm vs planarian

flatworm and planarian both are nouns.

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

As nouns, planarian is a hyponym of flatworm; that is, planarian is a word with a more specific, narrower meaning than flatworm:

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

Other hyponyms of flatworm include planaria, fluke, trematode, trematode worm, cestode, tapeworm.

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