What serves as a heart in an earthworm's circulatory system?

The aortic arches serve as a heart in an earthworm's circulatory system. There are five pairs of aortic arches, located near the head of the earthworm, that function much like a human heart.

The function of the aortic arches is to pump blood into the dorsal blood vessels and the ventral blood vessels. These vessels extend all throughout the earthworm's body and are responsible for carrying blood to its organs. The dorsal vessels circulate blood to the front of the earthworm's body, while the ventral vessels circulate blood to the back of its body. Earthworms have a closed circulatory systems, meaning that blood always remains in the blood vessels.