Respiratory System

The earthworm has no lungs, gills or any other dedicated breathing organ. Instead, respiration is entirely cutaneous - gas exchange happens across the thin, permanently moist body wall. The outer skin is richly supplied with blood capillaries lying close to the surface, so dissolved oxygen simply diffuses inward into the blood while carbon dioxide diffuses outward into the surrounding film of moisture. Mucous secretions and coelomic fluid escaping through the dorsal pores keep the skin surface wet, which is essential because gases can only cross a moist membrane; an earthworm that dries out cannot breathe and quickly …