These are the organs regarded with the gaseous exchange, i.e., intake .of oxygen and output of carbon dioxide. They have generally greater rate of gas exchange per unit area than the general body surface. Thus respiratory organ may be a part or special region of the body or may be an organ specifically meant for this purpose such as lung. In non-chordates various types of respiratory organs are found. These organs may different in their structure but all are provided with a large surface of contact with the surrounding environment and are richly supplied with blood vessels and capillaries which make sure rapid gaseous exchange between the external environment and the blood. The external environment is generally water. Even when it is air, there is a thin film of water covering the respiratory surface and exchange of gases takes place through this aqueous film. The respiratory organs that work in water are gills; those which work in air, are lungs. They may project outwards (gills) of they may be invaginations (lungs).