An open circulatory system is a type of circulatory system in which nutrients and waste are moved through the body with the assistance of a fluid which flows freely through the body cavity, rather than being contained in veins. Many invertebrates like insects and shellfish have an open circulatory system, with the exact composition of the circulating fluid varying, depending on the animal species involved. In contrast, vertebrates have a closed circulatory system which circulates blood through a series of vessels in the body, with the interstitial fluid known as lymph moving slowly between the cells and through a series of lymph nodes.
In the case of an animal with an open circulatory system, all of the organs and internal structures are constantly bathed in a mixture of the components of blood and lymph. This fluid brings nutrition and often oxygen to these structures, while carrying away waste for processing. Because the system is not closed, it is not possible to create blood pressure; instead, the animal circulates this fluid with muscle contractions. In some animals, the fluid is not oxygenated; instead, tissues receive oxygen directly through the tracheal system.
The fluid in an open circulatory system is sometimes referred to as hemolymph, referencing the fact that it is a cross between blood and lymph. It spends a brief amount of time in the heart, but instead of being pumped out of the heart and into a network of arteries and veins, it is dumped directly into the body cavity of the animal. If you have ever handled fresh shellfish, you have seen hemolymph; it is the pale pinkish to green fluid which squirts out when you cut the shellfish open.
One advantage to an open circulatory system is that it renders the animal much less vulnerable to pressure. This can be an advantage for mollusks which live at great depths, because it prevents compression of their bodies. Because many insects use their tracheal systems to transport oxygen, they tend to have open circulatory systems because they place less demands on their bodies; efficiency is not as urgent when oxygen and carbon dioxide are handled through a separate system.
The open circulatory system also gives animals greater control over their body temperature, which can be a distinct advantage at times. For example, it can help to quickly dissipate heat, allowing insects to survive in extremely hot environments. However, animals with a closed circulatory system have greater control over oxygen delivery to specific tissues, and they are also able to filter their blood and lymph more accurately because these systems are separated.