branchia vs respiratory organ

branchia and respiratory organ both are nouns.

Word	Noun	Adjective	Verb	Adverb
branchia	Yes	No	No	No
respiratory organ	Yes	No	No	No

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

branchia: respiratory organ of aquatic animals that breathe oxygen dissolved in water
respiratory organ: any organ involved in the process of respiration

branchia (noun)	respiratory organ (noun)
respiratory organ of aquatic animals that breathe oxygen dissolved in water	any organ involved in the process of respiration