polychaete vs sea mouse

polychaete and sea mouse both are nouns.

Word	Noun	Adjective	Verb	Adverb
polychaete	Yes	No	No	No
sea mouse	Yes	No	No	No

As nouns, sea mouse is a hyponym of polychaete; that is, sea mouse is a word with a more specific, narrower meaning than polychaete:

polychaete: chiefly marine annelids possessing both sexes and having paired appendages (parapodia) bearing bristles
sea mouse: any of several large worms having a broad flattened body with a mat of coarse hairs covering the back

Other hyponyms of polychaete include lobworm, lug, lugworm, bloodworm.

polychaete (noun)	sea mouse (noun)
chiefly marine annelids possessing both sexes and having paired appendages (parapodia) bearing bristles	any of several large worms having a broad flattened body with a mat of coarse hairs covering the back