Circulatory System

Animal circulation depends on the contraction of a pump—usually a heart that pumps blood in one direction through vessels along a circulatory path. In a closed path, the network of vessels is continuous. Alternately, an open path has vessels that empty into open spaces in the body. The closed system in the earthworm uses five pairs of muscular hearts (the aortic arches), to pump blood. Located near the anterior or head end of the animal, the aortic arches contract and force blood into the ventral blood vessel that runs from head to tail. Blood then returns back to the hearts in the dorsal blood vessel. Small ring vessels in each segment connect dorsal and ventral blood vessels. As blood circulates throughout the body, it delivers nutrients and oxygen to cells and picks up carbon dioxide and other wastes.

Most arthropods and some advanced molluscs such as squid and octopuses have an open circulatory system. In the grasshopper, a large blood vessel runs along the top of the body, and enlarges at the posterior or tail end to form a tubelike heart. Openings in the heart (ostia) have valves that permit only the entry of blood into the heart. The heart contracts, forcing blood forward in the blood vessel and out into the head region. Outside the heart, the blood goes into spaces that surround the insect's internal organs. The blood delivers food and other materials to cells and picks up wastes. Animals with open circulatory systems depend on the respiratory system to transport oxygen and carbon dioxide. The blood moves slowly from the head to the tail end of the animal. At the posterior, the blood re-enters the heart through the openings. Contraction of muscles helps speed up the blood flow.