branchia vs ctenidium

branchia and ctenidium both are nouns.

Word	Noun	Adjective	Verb	Adverb
branchia	Yes	No	No	No
ctenidium	Yes	No	No	No

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

branchia: respiratory organ of aquatic animals that breathe oxygen dissolved in water
ctenidium: comb-like respiratory structure serving as the gill of certain mollusks

Other hyponyms of branchia include ceras, external gill, tracheal gill.

branchia (noun)	ctenidium (noun)
respiratory organ of aquatic animals that breathe oxygen dissolved in water	comb-like respiratory structure serving as the gill of certain mollusks