Means of distribution of materials through the tissues of animals that does not keep the circulating fluid within vessels.

Open circulatory systems are seen for example in various mollusks and insects. The fluid that flows in association with open circulatory systems is known as hemolymph. Contrast open circulatory systems with closed circulatory systems.

Note that the return of fluid to the heart in animals possessing open circulatory systems is somewhat equivalent to the return of lymph in animals possessing closed circulatory systems.