Earthworms rely on their moist bodies for oxygen to diffuse into the blood
circulating near the surface of their skin. The reason that earthworms
come to the surface when the ground is water saturated is because it needs
the air circulating in the soil to diffuse enough oxygen into their blood.
Earthworms do not have a respiratory system such as we have. They have very
thin skin and can exchange oxygen and CO2 through their skin.