Four general types of respiratory surfaces are found in animals. The first is simply the body surface. This will work if it is thin and moist enough. Examples of animals that take up oxygen over their whole body surface are sponges, cnidarians, and newts. The second type is the gill, a respiratory surface folded out from the body.
Gills are chracteristic of many aquatic animals, include fish, aquatic insects, and mollusks.
The third type is a system of tracheal tubes that delivers air directly to the tissues. This specialized system works well for small animals, and is found in insects, millipedes, and centipedes. The fourth type is called the lung. Diffusion moves oxygen across respiratory surfaces. Diffusion also moves oxygen from the blood to the cells that need it.