Open circulatory systems are found in mollusks, arthropods and several other invertebrates. Open circulatory systems work by pumping blood into a hemocoel, which in turn diffuses blood and pumps it back into the circulatory system. Blood is then pumped by the heart into cavities, organs and tissues of the organism.
Open circulatory systems contain three primary organs: a heart, gill and pericardial sinus. These systems are more basic in structure and complexity of organs than closed circulatory systems, which are found in a few invertebrates and most vertebrates, including humans. The hearts of organisms with open circulatory systems, such as crayfish, contain a single atrium, which serves as a central terminal for recycling and circulating blood.