ctenidium vs branchia

ctenidium and branchia both are nouns.

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

As nouns, branchia is a hypernym of ctenidium; that is, branchia is a word with a broader meaning than ctenidium:

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

Other hypernyms of ctenidium include gill.

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