Respiratory Organs in Various Organisms

Simple animals such as sponges, coelenterates (like Hydra and jellyfish), and flatworms have no dedicated respiratory organs at all - every cell lies close enough to the surrounding water that oxygen and carbon dioxide simply diffuse directly across the body surface. Earthworms extend this strategy slightly: their moist, richly vascularised skin serves as the respiratory surface, which is why earthworms must keep their skin damp to survive.

Insects take a very different route. Instead of relying on blood to carry gases, they have a network of branching air tubes called tracheae that open to the outside through small pores and carry air directly to the tissues. Gills are the respiratory organ of choice for most aquatic arthropods (such as prawns) and molluscs, and also for fishes among the vertebrates - gills are thin, highly vascularised, feathery structures that extract dissolved oxygen from water, which explains why aquatic organisms must breathe much faster than land animals, since the amount of dissolved oxygen in water is far lower than the amount of oxygen in air. …