Earthworms play an important role in many ecosystems, breaking down rotting organic matter into its simple components for use by plants. While earthworms may seem simple because they lack many visible external organs, they have complex inner organs including five pairs of heart-like structures called aortic arches, which they use to pump oxygenated blood to the rest of their bodies. In fact, depending on the definition of "heart," earthworms can be said to have either 10, or zero hearts.