Here's why:

* Closed System: In a closed circulatory system, blood is always contained within blood vessels. This allows for efficient transport of oxygen, nutrients, and waste products throughout the body.

* Earthworm's System: Earthworms have a network of blood vessels, including a dorsal (back) and ventral (belly) blood vessel, that run the length of their body. These vessels are connected by smaller capillaries, which allow for exchange of substances with the surrounding tissues.

* Heart-like Structures: Earthworms have five pairs of "hearts" (actually, aortic arches) that pump blood through the system.

In contrast, an open circulatory system:

* Blood is not always contained within vessels.

* Blood bathes the tissues directly, which is less efficient for transporting substances.

* This system is typical of insects and some mollusks.

Therefore, earthworms possess a closed circulatory system for efficient blood flow and nutrient transport.