Earthworms breathe through their skin, which is also known as their cutaneous respiration. Oxygen from the air diffuses through the thin, moist skin and into the bloodstream, where it is distributed throughout the body. Similarly, carbon dioxide from the bloodstream diffuses out through the skin and into the air.

To facilitate this exchange of gases, earthworms require a moist environment. They are typically found in soil or other damp areas, where their skin can stay moist enough to allow for respiration. In fact, earthworms can suffocate if their skin dries out too much.

Earthworms also have a simple circulatory system that helps to transport oxygen and nutrients throughout their bodies. This system consists of a series of vessels that connect the worm's dorsal blood vessel to its ventral blood vessel. The dorsal blood vessel carries oxygenated blood from the worm's skin to its head, while the ventral blood vessel carries deoxygenated blood from the head back to the skin, where it can release carbon dioxide and pick up more oxygen.