Exchange of respiratory gases occurs by simple diffusion over their entire body surface in

Lower invertebrates like sponges, coelenterates, flatworms, etc., exchange O₂ with CO₂ by simple diffusion over their entire body surface. Earthworms also use their body surface (moist cuticle). Insects have a network tracheal tubes to transport atmospheric air within the body. Special vascularised structures called gills are used by most of the aquatic arthropods and molluscs (branchial respiration). Vascularised bags called lungs are used by the terrestrial forms for the exchange of gases (pulmonary respiration). Among vertebrates, fishes use gills whereas amphibians, reptiles, birds and mammals respire through lungs. Amphibians like frogs can respire through their moist skin (cutaneous respiration) also.