Earthworms burrow in the ground, swallowing soil from which the organic material is extracted and ground up in the gizzard and depositing the residue as castings outside the burrow. In cold and dry weather they retreat into their burrows and remain dormant. The segments of the earthworm, visible externally as rings, are separated by internal partitions. On each segment are four pairs of bristles, or setae, with which the worm anchors itself to the walls of the burrow, drawing itself forward by rhythmic muscular contractions. Although they have no prominent sense organs, earthworms are sensitive to light, touch, vibration, and chemicals.
Earthworms are hermaphroditic, but they cross-fertilize.
The sheath slips over the worm's head and is deposited underground, where it serves as a cocoon for the developing young. The common American and European earthworm, Lumbricus terrestris, up to 10 in. The earthworm's greatest service, however, of immense importance to agriculture, is aerating and mixing the soil. Earthworm castings bring to the surface from 7 to 18 tons of soil per acre annually. This invaluable function of the earthworm was first pointed out in a detailed study by Charles Darwin.