flatworm vs planaria

flatworm and planaria both are nouns.

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

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

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

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

flatworm (noun)	planaria (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