earthworm, terrestrial, cylindrical segmented worm of the class Oligochaeta. There are 2,200 earthworm species, found all over the world except in arid and arctic regions and ranging in size from 1 in. (2.5 cm) to the 11-ft (330-cm) giant worms of the tropics. Some earthworms are pallid in color, many are reddish brown to purple, and one Philippino species is bright blue. 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. They come to the surface only on cloudy days and at night (hence the name night crawlers) unless they are flooded out by heavy rainfalls. 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. There is a nerve cord, with ganglia in each segment and an enlarged cerebral ganglion (a primitive brain) at the anterior end. Although they have no prominent sense organs, earthworms are sensitive to light, touch, vibration, and chemicals. The circulatory system is enclosed in vessels; the blood (which contains hemoglobin) is pumped by muscular contractions of five linearly arranged hearts. Earthworms are hermaphroditic, but they cross-fertilize. Two worms exchange sperm cells during copulation; fertilization occurs after the worm's own eggs and the received sperm are encased in a tough sheath secreted by the clitellum, a conspicuous band of tissue near the anterior end. The sheath slips over the worm's head and is deposited underground, where it serves as a cocoon for the developing young. There is no larval stage; the young hatch as miniature adults. The common American and European earthworm, Lumbricus terrestris, up to 10 in. (25 cm) long, with about 150 segments, is used for laboratory dissection and study. Earthworms are also used as live bait and are eaten by some peoples—such as the Maoris, who consider certain species delicacies. 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. Earthworms are classified in the phylum Annelida, class Oligochaeta, order Opisthopora.