Earthworms primarily rely on diffusion as their main method of gaseous exchange, taking in oxygen and releasing carbon dioxide through their moist skin. Unlike many animals, earthworms lack lungs or gills, the specialized organs used for respiration in other creatures. Instead, they breathe through their skin, which serves as the primary surface for gas exchange. 

The earthworm's skin must be kept moist for efficient gas exchange. This is because gases, like oxygen and carbon dioxide, dissolve in water and can then diffuse across the skin's surface.