branchia vs ceras

branchia and ceras both are nouns.

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

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

branchia: respiratory organ of aquatic animals that breathe oxygen dissolved in water
ceras: one of the often brightly colored and branching hornlike structures on the back of the nudibranch (and other related mollusks) that serve as gills

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

branchia (noun)	ceras (noun)
respiratory organ of aquatic animals that breathe oxygen dissolved in water	one of the often brightly colored and branching hornlike structures on the back of the nudibranch (and other related mollusks) that serve as gills