The closed circulatory system of some mollusks and all higher invertebrates and the vertebrates is a much more efficient system. Here blood is pumped through a closed system of arteries, veins, and capillaries. Capillaries surround the organs, making sure that all cells have an equal opportunity for nourishment and removal of their waste products. However, even closed circulatory systems differ as we move further up the evolutionary tree. One of the simplest types of closed circulatory systems is found in annelids such as the earthworm.
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.