flatworm vs cestode

flatworm and cestode both are nouns.

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

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

flatworm: parasitic or free-living worms having a flattened body
cestode: ribbonlike flatworms that are parasitic in the intestines of humans and other vertebrates

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

flatworm (noun)	cestode (noun)
parasitic or free-living worms having a flattened body	ribbonlike flatworms that are parasitic in the intestines of humans and other vertebrates