Every animal needs a constant supply of oxygen to produce the energy that keeps it alive. Animals that live in water absorb oxygen from the water. Land animals get oxygen from the air. All but a few kinds of animals use some form of respiration or breathing to draw in oxygen and to give off carbon dioxide.
Most animals with backbones breathe with gills if they live in water, or with lungs if they live on land.
A fish gulps water and absorbs the oxygen it contains. Then the fish expels the water through spaces between its gills.
Different kinds of land animals have lungs that work in various ways. Frogs push air into their lungs under pressure, using the floors of their mouths as pumps. Snakes and lizards use muscles between their ribs to increase body volume. Air, then, flows into their lungs and occupies the empty spaces produced.
Warm blooded animals, including birds and mammals, have special muscles and breathing organs that allow them to obtain large amounts of oxygen. Their bodies must produce more energy than those of cold blooded animals because they usually are more active.
Most animals without backbones are insects. They take in oxygen through air tubes called tracheae. These air tubes extend inward to the body organs from pores on the insect's sides. Movements of body muscles pump air into and out of the tubes.
Some animals without backbones, such as crabs, crayfish and lobsters have gils under thin parts of the body wail above the legs.
Most spiders have a single pair of lungs with thin sheets of tissue that look like the pages of a book. In fact, the spider's lung is called a "book lung."
Some fresh water snails draw air into their lungs through a pore on the side of the body. Other fresh water snails and all salt water snails have gills. They get oxygen from the water and do not have to rise to the surface to breathe.
Some worms and most other animals without backbones absorb ail the oxygen they need through the surfaces of their bodies. Earthworms, for example, breath through their moist skins.
Lungfish can breathe air as well as water. They have gills like other fish and air bladders which are used like lungs.
Lungs give an animal's blood its needed oxygen supply. As the blood circulates, carrying nourishment to the tissue, it gives up oxygen and absorbs carbon dioxide. Blood then returns to the lungs for more oxygen. Oxygen enters the lungs with each breath and carbon dioxide is expelled when the animal breathes out.