Earthworms have a long straight tube as their digestive tract. As they burrow into the ground, they create tunnels that aerate the soil (much to the happiness of gardeners); their mouths are also ingesting the decaying organic matter along with the soil. The food moves to their esophagus, then to the crop, where it is stored. The gizzard—a sand-and soil-filled structure with thick, muscular walls—then grinds up the food. From there, the food passes through the intestines (that also holds a large fold in its upper surface, called the typhlosole, that increases the intestine’s surface area), and chemical digestion and absorption occurs.