cnidarian vs hydroid

cnidarian and hydroid both are nouns.

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

As nouns, hydroid is a hyponym of cnidarian; that is, hydroid 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
hydroid: colonial coelenterates having the polyp phase dominant

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

cnidarian (noun)	hydroid (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