The term 'FISH LUNG' is composed of two words, 'GILL' and 'LUNG'. Gills are organs found in aquatic animals that allow them to breathe underwater. They are composed of thin filaments that absorb oxygen from the water and expel carbon dioxide. Lungs are organs found in terrestrial animals that allow them to breathe air. They are composed of spongy tissue that absorbs oxygen from the air and expels carbon dioxide. Fish have both gills and lungs, allowing them to survive in both aquatic and terrestrial environments.