The earthworm is a useful animal. Out of the ground, it is food for other animals. In this ground, it makes rich soil for fields and gardens. Earthworms dig tunnels (隧道) that loosen the soil and make it easy for air and water to reach the roots of plants. These tunnels help keep the soil well drained( 排水 ). Earthworms drag dead leaves, grass and flowers into their burrows ( 洞穴) this plant material decays , it makes the soil more fertile (肥沃的) . No other animal is so useful in building up good topsoil. It is estimated that in one year fifty thousand earthworm carry about eigh tons of a proud of earth to the topsoil.