Blood is moved along the dorsal vessel by waves of contraction in the wall of the vessel. These connecting vessels function as rudimentary hearts and force the blood into the ventral vessel. There are also special organs in the earthworm for the removal of nitrogenous wastes. Still, blood can flow backward and the system is only slightly more efficient than the open system of insects. As we come to the vertebrates, we begin to find real efficiencies with the closed system.
Fish possess one of the simplest types of true heart.
A fish's heart is a two-chambered organ composed of one atrium and one ventricle. The heart has muscular walls and a valve between its chambers. Blood is pumped from the heart to the gills, where it receives oxygen and gets rid of carbon dioxide. Blood then moves on to the organs of the body, where nutrients, gases, and wastes are exchanged. However, there is no division of the circulation between the respiratory organs and the rest of the body.