Circulatory Systems in Non-Humans
. 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...