hydroid vs hydra

hydroid and hydra both are nouns.

Word	Noun	Adjective	Verb	Adverb
hydroid	Yes	No	No	No
hydra	Yes	No	No	No

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

Other hyponyms of hydroid include planula, siphonophore, sertularian.

hydroid (noun)	hydra (noun)
colonial coelenterates having the polyp phase dominant	small tubular solitary freshwater hydrozoan polyp
	trouble that cannot be overcome by a single effort because of its many aspects or its persistent and pervasive quality