Dorsal and ventral longitudinal vessels are connected by segmental pairs of vessels. Five pairs of hearts circle the esophagus. Numerous tiny vessels in the skin permit gas exchange is across the body surface. A nerve ring around the pharynx connects these ganglia to a subpharyngeal ganglion, from which a pair of fused nerve cords run posteriorly. Along the ventral nerve cords are fused segmental ganglia.
Annelids are hermaphroditic but cross-fertilize during sexual reproduction.
Two earthworms exchange sperm and store it temporarily. A special organ, the clitellum , secretes a mucous cocoon which slides along the worm, picking up its eggs and then the stored sperm. The cocoon slips off the worm into the soil and protects the embryos while they develop. Asexual reproduction occurs in some species by fragmentation followed by regeneration. Movement involves coordinating longitudinal and circular muscles in each segment with the fluid-filled coelom functioning as a hydrostatic skeleton.