An illustration of an earthworm. The basic body plan of an earthworm is a tube, the digestive system, within a tube, the muscular outer body. The body is annular, formed of segments that are most specialized in the anterior. Earthworms have a simple circulatory system. They have two main blood vessels that extend through the length of their body: a ventral blood vessel which leads the blood to the posterior end, and a dorsal blood vessel which leads to the anterior end. Most earthworms are decomposers feeding on undecayed leaf and other plant matter, others are more geophagous.