System of vessels in an animal's body that transports essential substances ( blood or other circulatory fluid) to and from the different parts of the body. It was first discovered and described by English physician William Harvey . All animals except for the most simple – such as sponges, jellyfish, sea anemones, and corals – have some type of circulatory system. Some invertebrates (animals without a backbone), such as insects, spiders, and most shellfish, have an ‘open’ circulatory system which consists of a simple network of tubes and hollow spaces. Other invertebrates have pump-like structures that send blood through a system of blood vessels. All vertebrates (animals with a backbone), including humans, have a ‘closed’ circulatory system which principally consists of a pumping organ – the heart – and a network of blood vessels. Fish have a single circulatory system in which blood passes once around the body before returning to a two-chambered heart. In birds and mammals, there is a…