The earthworm does not have a heart. The organ in an earthworm that acts as a heart is called the aortic arch. An earthworm has five aortic arches.
No matter how big an earthworm gets, the total mass of the aortic arches does not become greater than 5 percent of its body. The aortic arches along with the dorsal blood vessel move blood around the earthworm's body. The earthworm needs all five aortic arches to achieve sufficient blood flow. Unlike mammals, earthworms do not have lungs to pass oxygen into the bloodstream. Instead, earthworms absorb oxygen through their skin into the their blood vessels.