An earthworm is a long, soft-bodied animal without legs or bones that lives in soil and helps break down dead plant material into nutrients that other plants can use. If you've ever dug in a garden or turned over a rock after rain, you've probably seen these reddish-brown creatures wriggling through the dirt.

Earthworms tunnel through soil by eating their way forward, swallowing dirt and organic matter as they go. As the soil passes through their bodies, they digest the nutrients they need and leave behind castings (worm droppings) that enrich the soil. Their constant tunneling also creates pathways for air and water to reach plant roots, which is why gardeners love having earthworms around.

Charles Darwin, the famous naturalist, spent years studying earthworms and calculated that in some areas, earthworms move ten tons of soil per acre every year. Despite having no eyes, ears, or lungs (they breathe through their skin), earthworms are remarkable at sensing vibrations and light. They can regenerate some body segments if injured, though they can't grow a whole new worm from just any piece.

Without earthworms and similar decomposers, dead leaves and plants would pile up instead of turning into the rich soil that feeds new growth.