branchia, gill - respiratory organ of aquatic animals that breathe oxygen dissolved in water

internal organ, viscus - a main organ that is situated inside the body

lung - either of two saclike respiratory organs in the chest of vertebrates; serves to remove carbon dioxide and provide oxygen to the blood

respiratory system, systema respiratorium - the system for taking in oxygen and giving off carbon dioxide; in terrestrial animals this is accomplished by breathing

book lung - organ in many arachnids containing many thin folds of membrane resembling the leaves of a book