cnidarian vs hydrozoan

cnidarian and hydrozoan both are nouns.

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

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

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

Other hyponyms of cnidarian include polyp, medusa, medusan, medusoid, jellyfish, scyphozoan, hydroid, actinozoan, anthozoan.

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