<P> Birds, mammals, and crocodilians show complete separation of the heart into two pumps, for a total of four heart chambers; it is thought that the four - chambered heart of birds and crocodilians evolved independently from that of mammals . </P> <P> Circulatory systems are absent in some animals, including flatworms . Their body cavity has no lining or enclosed fluid . Instead a muscular pharynx leads to an extensively branched digestive system that facilitates direct diffusion of nutrients to all cells . The flatworm's dorso - ventrally flattened body shape also restricts the distance of any cell from the digestive system or the exterior of the organism . Oxygen can diffuse from the surrounding water into the cells, and carbon dioxide can diffuse out . Consequently, every cell is able to obtain nutrients, water and oxygen without the need of a transport system . </P> <P> Some animals, such as jellyfish, have more extensive branching from their gastrovascular cavity (which functions as both a place of digestion and a form of circulation), this branching allows for bodily fluids to reach the outer layers, since the digestion begins in the inner layers . </P> <P> The earliest known writings on the circulatory system are found in the Ebers Papyrus (16th century BCE), an ancient Egyptian medical papyrus containing over 700 prescriptions and remedies, both physical and spiritual . In the papyrus, it acknowledges the connection of the heart to the arteries . The Egyptians thought air came in through the mouth and into the lungs and heart . From the heart, the air travelled to every member through the arteries . Although this concept of the circulatory system is only partially correct, it represents one of the earliest accounts of scientific thought . </P>

Parts of the circulatory system and its function