flatworm vs fluke

flatworm and fluke both are nouns.

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

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

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

flatworm (noun)	fluke (noun)
parasitic or free-living worms having a flattened body	parasitic flatworms having external suckers for attaching to a host
	either of the two lobes of the tail of a cetacean
	flat bladelike projection on the arm of an anchor
	a barb on a harpoon or arrow
	a stroke of luck