In open circulatory systems the transport medium is pumped from the heart and into an open body cavity called the haemocoel.The transport medium is under low pressure in the haemocoel where it bathes the tissues and organs.Substances are exchanged between the cells and transport medium by diffusion.The transport medium returns to the heart through an open-ended vessel.Open circulatory systems are found in invertebrates such as insects, molluscs such as snails and clams, arthropods and arachnids (like spiders), and crustaceans like crabs and shrimp.Insect blood is called haemolymph. It is a mixture of blood and interstitial fluid.It transports nitrogenous waste and cells involved in defence against disease but transports no oxygen or carbon dioxide.The heart extends the length of the thorax and abdomen.The hemolymph returns to the heart through open-ended vessels.