external gill vs branchia

external gill and branchia both are nouns.

Word	Noun	Adjective	Verb	Adverb
external gill	Yes	No	No	No
branchia	Yes	No	No	No

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

external gill: occurs in some mollusks and in tadpoles and other immature amphibians
branchia: respiratory organ of aquatic animals that breathe oxygen dissolved in water

Other hypernyms of external gill include gill.

external gill (noun)	branchia (noun)
occurs in some mollusks and in tadpoles and other immature amphibians	respiratory organ of aquatic animals that breathe oxygen dissolved in water