hydrozoan vs cnidarian

hydrozoan and cnidarian both are nouns.

Word	Noun	Adjective	Verb	Adverb
hydrozoan	Yes	No	No	No
cnidarian	Yes	No	No	No

As nouns, cnidarian is a hypernym of hydrozoan; that is, cnidarian is a word with a broader meaning than hydrozoan:

hydrozoan: colonial coelenterates having the polyp phase dominant
cnidarian: radially symmetrical animals having saclike bodies with only one opening and tentacles with stinging structures; they occur in polyp and medusa forms

Other hypernyms of hydrozoan include coelenterate.

hydrozoan (noun)	cnidarian (noun)
colonial coelenterates having the polyp phase dominant	radially symmetrical animals having saclike bodies with only one opening and tentacles with stinging structures; they occur in polyp and medusa forms