dorsal blood vessel
The dorsal blood vessel is an important part of an earthworm's circulatory system as well as its respiratory system. Since an earthworm has neither lungs nor gills, oxygen must be absorbed through its skin and passed onto the bloodstream through the dorsal blood vessel. After oxygen (from surrounding moist soil) enters the dorsal blood vessel, the worm's five pairs of muscles (hearts) pump it through the ventral blood vessel to other parts of the body. The dorsal blood vessel also recirculates blood after it has flown through the ventral blood vessel (Musurca).