Earthworm, name of a large number of common worms of the Lumbricidae family with simple tubular bodies made up of a series of rings. The body tapers toward the head end, where there is a mouth. Earthworms move by extending and contracting these rings, anchoring them by means of short bristles. They feed on plant material in the soil. Some species eject the indigestible remains on the surface as worm casts, thereby improving the soil. The population of earthworms in the soil varies widely, but in some grazing lands it may reach 5 million per acre. A few species climb trees and are found under bark. Each earthworm is both male and female but mates with another earthworm before laying eggs. These are deposited in a cocoon which is secreted by the clitellum, the broad band near the front of the worm. Size varies, with some species as small as 1/25 in (1 mm) long, and one Australian species growing to 11 ft (3 m).