CIRCULATORY SYSTEMS IN NONHUMANS
One-celled organisms and many simple multicelled animals, such as sponges, jellyfishes, sea anemones, flatworms, and roundworms, do not have a circulatory system. All of their cells are able to absorb nutrients, exchange gases, and expel wastes through direct contact with either the outside or with a central cavity that serves as a digestive tract.

More complex invertebrates have a wide range of circulatory system designs. These invertebrate circulatory systems are classified as either open or closed. Open systems—found in starfishes, clams, oysters, snails, crabs, insects, spiders, and centipedes—lack capillaries, and the blood bathes the tissues directly. In closed systems, the blood is confined to a system of blood vessels. Invertebrates with closed systems include segmented worms, squids, and octopuses.

All vertebrate animals have closed circulatory systems. These systems are classified by the number of chambers in the heart, which determines the basic configuration of blood flow. Fish have two-chambered hearts with one atrium and one ventricle. Blood pumped from the ventricle travels through arteries to the gills, where it diverges into capillaries and exchanges gases. Leaving the gills, the capillaries reconvene into blood vessels that carry the oxygenated blood to the rest of the body, where the vessels again diverge into capillaries before reconvening into veins that return to the heart. In this way, the blood passes through first the respiratory organs (the gills) and then the systemic circulation between each pass through the heart.

Frogs and amphibians have three-chambered hearts, with two atriums and one ventricle. Blood pumped from the ventricle enters a forked artery. One fork, the pulmonary circulation, leads to the lung. The other fork, the systemic circulation, leads to the rest of the body. Blood returning from the pulmonary circulation enters the left atrium, while blood from the systemic circulation enters the right atrium. Although there is some mixing of oxygenated and deoxygenated blood in the ventricle, a ridge within the ventricle assures that most of the oxygenated blood is diverted to the systemic circulation and most of the deoxygenated blood goes to the pulmonary circulation. In reptiles, this ridge is more developed, forming a partial wall. In crocodiles, the wall is complete, forming a four-chambered heart like that found in mammals and birds.