A structure associated with an open circulatory system is

The haemocoel is a structure specifically associated with an open circulatory system. In an open circulatory system, found in arthropods (insects, crustaceans, spiders) and most molluscs, blood (called haemolymph) is not confined to blood vessels throughout its entire circuit. Instead, the heart pumps haemolymph through short arteries into open body cavities called haemocoels, where the fluid directly bathes the tissues and organs, allowing exchange of nutrients, gases, and waste products. The haemolymph then drains back into the heart through openings called ostia.

Veins, arteries, and arterioles are all components of a closed circulatory system, in which blood is always contained within a continuous network of vessels (as in vertebrates, annelids, and cephalopods). In a closed system, exchange between blood and tissues occurs across capillary walls, not by direct bathing of organs.

The presence of a haemocoel is one of the defining features that distinguishes an open circulatory system from a closed one.