cnidarian vs polyp

cnidarian and polyp both are nouns.

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

As nouns, polyp is a hyponym of cnidarian; that is, polyp 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
polyp: one of two forms that coelenterates take (e.g. a hydra or coral): usually sedentary with a hollow cylindrical body usually with a ring of tentacles around the mouth

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

cnidarian (noun)	polyp (noun)
radially symmetrical animals having saclike bodies with only one opening and tentacles with stinging structures; they occur in polyp and medusa forms	one of two forms that coelenterates take (e.g. a hydra or coral): usually sedentary with a hollow cylindrical body usually with a ring of tentacles around the mouth
	a small vascular growth on the surface of a mucous membrane