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Of the invertebrate groups, only annelids and cephalopods have closed circulatory systems. These types of circulatory systems consist of a heart or hearts that pump blood throughout the body through a system of closed vessels. Waste and nutrient exchange happens through the walls of the vessels. On the other hand, in open circulatory systems, fluid called hemolymph is pumped through a large blood vessel into a body cavity called the hemocoel. The hemolymph bathes the tissues and organs, furnishing them with nutrients and picking up waste products. From the tissues, the hemolymph is returned to the heart.