Do Earthworms Have a Closed or Open Circulatory System?

Instructor: Carole Brown

Carole has taught numerous Biology courses at the college level and has a PhD in Neuroscience.

In this lesson, you will learn to differentiate between a closed and an open circulatory system. Read on, and you will be able to characterize the earthworm's circulatory system!

Open versus Closed Circulatory Systems

If you are wondering what type of circulatory system an earthworm has, it is important to first understand that there are two main types of circulatory systems: open and closed. The circulatory system in animals - regardless of type - functions to provide nutrients and get rid of wastes, but what is the difference between an open and a closed system? The answer is actually very simple: vessels.

Closed Circulatory System

In a closed circulatory system, blood is contained within vessels, and will never leave the vessel system. These vessels originate from the heart, which is the pump of the circulatory system.

Example of a closed circulatory system.
Diagram of a closed circulatory system

Try putting your hand to your chest to feel your heart beating; what you are actually feeling is the pumping action of the heart that moves blood through your vessels. Blood circulates unidirectionally from the heart through arteries, arterioles, and capillaries, to reach all tissues and organs in the body. The capillaries are very narrow vessels with a one-cell layer-thick wall, which allows the easy transfer of nutrients and oxygen from the blood to cells.

In addition, cells dump carbon dioxide and wastes back into the blood, which returns to the heart through venules and veins. This closed system is found in all vertebrates and some invertebrates.

Open Circulatory System

In an open circulatory system, the blood is not contained within a vessel system. Instead, tissues and organs are surrounded by the circulatory fluid, which is known as hemolymph. Hemolymph is like blood and includes interstitial fluid, which is the fluid that usually surrounds tissues.

Hemolyph surrounds tissues in an open circulatory system.
Diagram of an open circulatory system

The heart pumping mechanism, in addition to the animal's natural body movement, results in the movement of the hemolymph throughout the body. This movement allows gas and nutrient exchange. Open circulatory systems are found in arthropods such as insects, crustaceans, and most mollusks.

The Earthworm Circulatory System

Now that we know the difference between an open and a closed system, let's explore the earthworm, an invertebrate. Its circulatory system consists of (1) two major vessels (dorsal and ventral blood vessels), (2) smaller lateral vessels that supply blood to all tissues and organs and connect the ventral blood vessel to the dorsal blood vessel, and (3) five pairs of aortic arches that make up the earthworm's hearts.

Image of an earthworm.

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