How do annelids breathe through their body walls?

Annelids primarily perform gas exchange through their skin, a process known as cutaneous respiration. Because their body wall is thin, oxygen can diffuse directly through the epidermis into the underlying circulatory system or coelomic fluid.12

While this method is standard for many species, some annelids utilize specialized structures to supplement or replace skin-based breathing. For instance, certain polychaete worms possess gill filaments, and some aquatic oligochaetes exchange gases through their rectum. To facilitate efficient oxygen uptake, many annelids rely on respiratory pigments—most commonly hemoglobin or chlorocruorin—dissolved in their blood or coelomic fluid to transport oxygen to tissues. Additionally, many tube-dwelling species actively enhance respiration by using muscular or ciliary activity to circulate oxygenated water through their burrows or around their bodies.12