Which respiratory organ is most often found only in fully aquatic animals?

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Gill is the most common respiratory organ found only in fully aquatic animals. Gills are specialized organs that extract dissolved oxygen from water and excrete carbon dioxide. Aquatic animals such as fishes, crabs, shrimp, and mollusks rely on gills for respiration. The trachea and lungs are respiratory organs found in terrestrial animals, while the skin surface serves as a respiratory organ only in some amphibians and reptiles.

Which Respiratory Organ Is Most Often Found Only in Fully