Answer: The Respiratory System:
The respiratory system is the set of organs responsible for the gas exchange between the animal's organism and the environment, that is, pulmonary hematose, making possible the cellular respiration. In terrestrial vertebrates, the respiratory system is essentially formed by two lungs. But in aquatic animals, such as fish and mollusks, the system is based on the gills, whereas in terrestrial arthropods, respiration is ensured by a system of trachea. In unicellular organisms and in some animals, such as sponges and coelenterates, as well as in "plants" (in the sense of the Lineu taxonomy), there is no true respiratory system, and cellular respiration is ensured by direct gas exchange between the body's cells and the environment.