That fluid is either hemolymph or blood, depending on the type of circulatory system. Instead of a heart, there are In this type of system, there is neither a true heart nor capillaries as are found in humans. Inside these animals is an open cavity called a hemocoel into which a blood-like fluid called hemolymph is pumped ( … Animals such as arthropods and mollusks possess an open circulatory system. Open circulatory systems are systems where blood, rather than being sealed tight in arteries and veins, suffuses the body and may be directly open to the environment at places such as the digestive tract. Arthropods are a group of animals that include insects and crustaceans. Open circulatory system: This type of system is found in animals such as insects and some mollusks (snails, clams). An open circulatory system is beneficial because animals don't have a blood pressure, so they can live at great ocean depths. In this type of system, there is neither a true heart nor capillaries as are found in humans. Arthropods and mollusks have an open circulatory system. Animal hearts come in different sizes and shapes, but they have the same function: to pump fluid throughout the circulatory system. Open Circulatory System Definition. Animals can have one of two kinds of circulatory systems: open or closed.