earthworm

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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 AnnelidaAnnelida
[Lat., anellus=a ring], phylum of soft-bodied, bilaterally symmetrical (see symmetry, biological), segmented animals, known as the segmented, or annelid, worms.
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, class Oligochaeta, order Opisthopora.

earthworm

[′ərth‚wərm]
(invertebrate zoology)
The common name for certain terrestrial members of the class Oligochaeta, especially forms belonging to the family Lumbricidae.

earthworm

any of numerous oligochaete worms of the genera Lumbricus, Allolobophora, Eisenia, etc., which burrow in the soil and help aerate and break up the ground
References in periodicals archive ?
alatus individuals are extracted every year in the region, the supply of giant earthworms to be used as bait has always met the demand (Drumond, 2008; Drumond et al.
The abundance of the giant earthworms was estimated using the Capture Per Unit Effort (CPUE = number of individuals/person/hr or number of individuals/person/metres) calculated from the data obtained in the field by following the harvesting activities of the local extractors during the dry seasons of 2006, 2007, 2010 and 2011.
At the beginning of the rainy season, the giant earthworms leave their aestivation chambers to feed and reproduce.
But these searches lack the gravitas of the hunt for the giant earthworm.
Xerces hopes to sponsor a search for the giant earthworm at some point.
SCIENTISTS KNOW THAT THE OREGON GIANT EARTHWORM burrows deep in the ground--as much as five yards down--and that it subsists on the underside of decaying pine needles, bits of wood, and the odd insect.