What anatomical characteristic necessitates earthworms needing moist skin conditions?
They breathe through their skin for gas exchange to occur.
Earthworms lack complex respiratory organs, such as lungs or gills, meaning that the entire process of gas exchange—taking in necessary oxygen and expelling carbon dioxide—must happen directly across their external body surface. For this cutaneous respiration to function effectively, the skin must remain consistently moist. Moisture acts as the medium through which gases can dissolve and pass across the epidermal layer into the circulatory system. If the skin dries out, gas exchange halts, leading to suffocation. This biological constraint is the direct reason why heavy rains force earthworms to the surface, as saturated soil can sometimes interfere with necessary gas diffusion, or alternatively, why they emerge when the surface dries slightly following a downpour.
