The fluid confined within the vessel in a closed circulatory system or that contained in a cavity (hemocoel) in an open circulatory system
The circulatory system is the organ system responsible for the circulation and transport of nutrients and other molecules throughout the body of an animal. Multicellular animals have two forms of circulatory system: open circulatory system and closed circulatory system. These two systems differ in terms of the mode of transporting the circulatory fluid within the body of the animal. In open circulatory system, the circulatory fluid is the hemolymph. It is not confined within the blood vessels (thus, the name open circulatory system). It bathes the organs and tissues directly and therefore there is no clear distinction between the interstitial fluid and the blood. The hemolymph is analogous to the blood in closed circulatory system. In closed circulatory system, blood is the circulatory fluid and is contained within the blood vessels. Therefore, there is a distinction between blood and lymph. Examples of animals with closed circulatory system are annelids, few mollusks, and vertebrates. The vertebrates have a cardiovascular system comprised of heart and blood vessels where blood circulates and another system called lymphatic system that circulates lymph. Examples of animals with open circulatory system include the spiders, crustaceans, insects, and certain mollusks.