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Earthworm
Lumbricus terrestris, the Common Earthworm
Scientific classification
Kingdom:	Animalia
Phylum:	Annelida
Class:	Clitellata
Subclass:	Oligochaeta (but disputed, see taxonomy)
Order:	Haplotaxida
Suborder:	Lumbricina NODC v. 8.0, 1996[1]
Families
Acanthodrilidae Ailoscolecidae Almidae Benhamiinae Criodrilidae Diplocardiinae Enchytraeidae Eudrilidae Exxidae Glossoscolecidae Haplotaxidae Hormogastridae Kynotidae Lumbricidae Lutodrilidae Megascolecidae Microchaetidae Moniligastridae Ocnerodrilidae Octochaetidae Octochaetinae Sparganophilidae Tumakidae

An earthworm is a tube-shaped, segmented animal that is commonly found living in soil. An earthworm has a digestive system that runs straight through its body, conducts respiration through the cuticle covering its skin, and has a simple, closed circulatory system. Earthworms are hermaphrodites--each individual carries both the male and female sex organs. Having no skeleton, an earthworm maintains its structure with fluid-filled chambers.

"Earthworm" is the common name for the largest members of Oligochaeta (which is either a class or subclass depending on the author) in the phylum Annelida. In classical systems they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, even though the internal male segments are anterior to the female. Theoretical cladistic studies have placed them instead in the suborder Lumbricina of the order Haplotaxida, but this may again soon change. Folk names for the earthworm include "dew-worm", "Rainworm", "night crawler" and "angleworm" (due to its use as fishing bait).

Earthworms are also called megadriles (or big worms), as opposed to the microdriles (or small worms) in the families Tubificidae, Lumbriculidae, and Enchytraeidae, among others. The megadriles are characterized by having a distinct clitellum (which is much more obvious than the single-layered one of the microdriles) and a vascular system with true capillaries.

Anatomy[link]

Body[link]

Depending on the species, an adult earthworm can be anywhere from 10mm long and 1mm wide to over 2m long and over 25mm wide, but the typical Lumbricus terrestris grows to about 360mm long.^[2]

From front to back, the basic shape of the earthworm is a cylindrical tube, divided into a series of segments that compartmentalize the body. Grooves called "furrows" are (generally^[3]) externally visible on the body demarcating the segments; some furrows contains pores that exude a fluid that moistens and protects the body surface. Except for the mouth and anal segments, each segment carries bristle-like hairs called "setae"^[4] used for movement^[5]; most species have four pairs of setae on each segment.^[4]

Generally, within a species, the number of segments found is consistent across specimens, and individuals are born with the number of segments they will have throughout their lives. The first body segment (segment number 1) features the both the earthworm's mouth and, underneath the mouth, a fleshy lobe called the prostomium, which seals the mouth entrance when the worm is at rest, but is also used to feel and chemically sense the worm's surroundings. Some species of earthworm can even use the prostomium to grab and drag items like grasses and leaves.

Adult earthworms develop a belt-like glandular swelling, called the clitellum, which covers several segments toward the back part of the front half of the animal. This is part of the reproductive system, and it harbors the egg capsule. The last, or posterior segment is called the periproct, and it is most commonly cylindrical like the body, but depending on the species may also be quadrangular, octagonal, trapezoidal, or flattened. A short vertical slit, the earthworm's anus, is found on this segment.^[4]

Cross section of the body of an earthworm (Oligochaeta) showing the disposition of the more important organs : the body wall (w) consists of dermis, circular and longitudinal muscles; the body cavity is divided by membranes (c) into a series of chambers, in each of which opens the mouth of a coiled nephridium (n); the axis of the cavity is occupied by the intestine (i); above and below it is a longer blood vessel (v), and below it is also the central nerve cord (nc)

The exterior of an individual segment is a thin cuticle over skin, commonly pigmented red to brown, which has specialized cells that secrete mucous over the cuticle to keep the body moist and ease movement through soil. Under the skin is a layer of nerve tissue, and two layers of muscles--a thin outer layer of circular muscle, and a much thicker inner layer of longitudinal muscle.^[6] Inside of the muscle layers is a fluid-filled chamber called a coelom^[7] that provides structure for the earthworm's body; in segments along the intestine, this chamber houses a structure called a nephridium, which removes metabolic waste and expels it through pores on the sides^[8]. At the center of a segment is the digestive tract, which runs straight through the body from mouth to anus without coiling, flanked above and below by blood vessels and the nerve cord. The segments are separated from each other by septa^[9] perforated with pores, which allow the coelomic fluid to pass between segments.^[10]

Many earthworms can eject coelomic fluid through pores in the back in response to stress; the Didymogaster sylvaticus (known as the "squirter earthworm") can squirt fluid as high as 30 cm.^[10]

Circulatory system[link]

Earthworms have a closed circulatory system, with three main blood vessels: the dorsal vessel, which runs above the digestive tract, and the ventral vessel and the subneural vessel, which run below it.^[11] There is not a distinction between 'arteries' and 'veins.' In each segment, a vessel rings the coelom and connects the three longitudinal vessels.^[12] Five segments in the esophageal region contain muscular commissural vessels that connect the top and bottom vessels and function like hearts to pump the blood.^[11]

Digestive system[link]

The gut of the earthworm is a straight tube which extends from mouth to anus. It is differentiated into a buccal cavity (generally running through the first 1 or 2 segments of the earthworm), pharynx (running generally about 4 segments in length), esophagus, crop, gizzard (typically) and intestine.^[13]

Food enters the mouth. The pharynx acts as a suction pump; its muscular walls draw food back. In the pharynx, the pharyngeal glands secrete mucus. Food moves into the esophagus, and then the crop and gizzard. In the gizzard, strong muscular contractions grind the food up with the help of mineral particles ingested along with the food. Once through the gizzard, food continues through the intestine for digestion. Instead of being coiled like a mammal intestine, an earthworm's intestine increases surface area to increase nutrient absorption by having many folds running along its length. The intestine has its own pair of muscle layers like the body, but in reverse order--an inner circular layer inside an outer longitudinal layer.^[14]

Reproduction[link]

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Earthworm reproduction

Earthworm cocoons from L. terrestris

An earthworm cocoon from L. rubellus

Earthworms typically have two pairs of testes, surrounded by 2 pairs of testes sacs. There are 2 or 4 pairs of seminal vesicles which produce, store and release the sperm via the male pores, and ovaries and ovipores in segment 13 that release eggs via female pores on segment 14. However, most also have one or more pairs of spermathecae (depending on the species) that are internal sacs which receive and store sperm from the other worm in copulation. Some species use external spermatophores for transfer instead.

Copulation and reproduction are separate processes in earthworms. The mating pair overlap front ends ventrally and each exchanges sperm with the other. The clitellum becomes very reddish to pinkish in color. The cocoon, or egg case, is secreted by the clitellum band which is near the front of the worm, but behind the spermathecae. Some time after copulation, long after the worms have separated, the clitellum secretes the cocoon which forms a ring around the worm. The worm then backs out of the ring, and as it does so, injects its own eggs and the other worm's sperm into it. As the worm slips out, the ends of the cocoon seal to form a vaguely lemon-shaped incubator (cocoon) in which the embryonic worms develop. They emerge as small, but fully formed earthworms, except for a lack of the sex structures, which develop later in about 60 to 90 days. They attain full size in about one year, sometimes sooner. Scientists predict that the average lifespan under field conditions is 4–8 years, still most garden varieties live only one to two years. Several common earthworm species are mostly parthenogenetic, that is, with asexual reproduction resulting in clones.

Regeneration[link]

Earthworms have the ability to regenerate lost segments, but this ability varies between species and depends on the extent of the damage. Stephenson (1930) devoted a chapter of his monograph to this topic, while G.E. Gates spent 20 years studying regeneration in a variety of species, but “because little interest was shown”, Gates (1972) only published a few of his findings that, nevertheless, show it is theoretically possible to grow two whole worms from a bisected specimen in certain species. Gates’s reports included:

• Eisenia fetida (Savigny, 1826) with head regeneration, in an anterior direction, possible at each intersegmental level back to and including 23/24, while tails were regenerated at any levels behind 20/21.^[15]
• Lumbricus terrestris Linnaeus, 1758 replacing anterior segments from as far back as 13/14 and 16/17 but tail regeneration was never found.
• Perionyx excavatus Perrier, 1872 readily regenerated lost parts of the body, in an anterior direction from as far back as 17/18, and in a posterior direction as far forward as 20/21.
• Lampito mauritii Kinberg, 1867 with regeneration in anterior direction at all levels back to 25/26 and tail regeneration from 30/31; head regeneration was sometimes believed to be caused by internal amputation resulting from Sarcophaga sp. larval infestation.
• Criodrilus lacuum Hoffmeister, 1845 also has prodigious regenerative capacity with ‘head’ regeneration from as far back as 40/41.^[16]
• Lumbriculus veriega Able to split into two segments and survive.

An unidentified Tasmanian earthworm shown growing a second head is reported here:^[17]

Locomotion and importance to soil[link]

Close up of an earthworm in garden soil

Earthworms travel underground by the means of waves of muscular contractions which alternately shorten and lengthen the body. The shortened part is anchored to the surrounding soil by tiny claw-like bristles (setae) set along its segmented length. In all the body segments except the first, last and clitellum, there is a ring of S-shaped setae embedded in the epidermal pit of each segment (perichaetine). The whole burrowing process is aided by the secretion of lubricating mucus. Worms can make gurgling noises underground when disturbed as a result of the worm moving through its lubricated tunnels. They also work as biological "pistons" forcing air through the tunnels as they move. Thus earthworm activity aerates and mixes the soil, and is constructive to mineralization and nutrient uptake by vegetation. Certain species of earthworm come to the surface and graze on the higher concentrations of organic matter present there, mixing it with the mineral soil. Because a high level of organic matter mixing is associated with soil fertility, an abundance of earthworms is beneficial to the organic gardener. In fact as long ago as 1881 Charles Darwin wrote: It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures. ^[18]

Benefits[link]

The major benefits of earthworm activities to soil fertility can be summarized as:

• Biological. In many soils, earthworms play a major role in converting large pieces of organic matter (e.g. dead leaves) into rich humus, and thus improving soil fertility. This is achieved by the worm's actions of pulling down below any organic matter deposited on the dried dirt, such as leaf fall or manure, either for food or when it needs to plug its burrow. Once in the burrow, the worm will shred the leaf and partially digest it, then mingle it with the earth by saturating it with intestinal secretions. Worm casts (see below) can contain 40% more humus than the top 9" (23 cm) of soil in which the worm is living.

Faeces in form of casts

• Chemical. As well as dead organic matter, the earthworm also ingests any other soil particles that are small enough—including sand grains up to 1/20 of an inch (1.25mm) across—into its gizzard wherein minute fragments of grit grind everything into a fine paste which is then digested in the intestine. When the worm excretes this in the form of casts which are deposited on the surface or deeper in the soil, minerals and plant nutrients are made available in an accessible form. Investigations in the US show that fresh earthworm casts are 5 times richer in available nitrogen, 7 times richer in available phosphates and 11 times richer in available potash than the surrounding upper 6 inches (150 mm) of soil. In conditions where there is plenty of available humus, the weight of casts produced may be greater than 4.5 kg (10 lb) per worm per year, in itself an indicator of why it pays the gardener or farmer to keep worm populations high.
• Physical. By its burrowing actions, the earthworm is of great value in keeping the soil structure open, creating a multitude of channels which allow the processes of both aeration and drainage to occur. Permaculture co-founder Bill Mollison points out that by sliding in their tunnels, earthworms "act as an innumerable army of pistons pumping air in and out of the soils on a 24 hour cycle (more rapidly at night)".^[19] Thus the earthworm not only creates passages for air and water to traverse, but is itself a vital component in the living biosystem that is healthy soil. Earthworms continue to move through the soil due to the excretion of mucus into the soil that acts as a lubricant for easier movement of the worm. (See Bioturbation.)

The earthworm's existence cannot be taken for granted. Dr. W. E. Shewell Cooper observed "tremendous numerical differences between adjacent gardens", and worm populations are affected by a host of environmental factors, many of which can be influenced by good management practices on the part of the gardener or farmer^[20].

Darwin estimated that arable land contains up to 53,000 worms per acre (13/m²), but more recent research from Rothamsted Experimental Station has produced figures suggesting that even poor soil may support 250,000/acre (62/m²), whilst rich fertile farmland may have up to 1,750,000/acre (432/m²), meaning that the weight of earthworms beneath a farmer's soil could be greater than that of the livestock upon its surface.

Earthworms as an invasive species[link]

From a total of around 6,000 species, only about 120 species are widely distributed around the world. These are the peregrine or cosmopolitan earthworms.^[21][22]

Special habitats[link]

While, as the name earthworm suggests, the main habitat of earthworms is in soil, the situation is more complicated than that. The brandling worm Eisenia fetida lives in decaying plant matter and manure. Arctiostrotus vancouverensis from Vancouver Island and the Olympic Peninsula is generally found in decaying conifer logs. Aporrectodea limicola and Sparganophilus and several others are found in mud in streams. Some species are arboreal. Even in the soil species, there are special habitats, such as soils derived from serpentine which have an earthworm fauna of their own.

Ecology[link]

Earthworms are classified into three main ecophysiological categories: (1) leaf litter/compost dwelling worms (epigeic) e.g. Eisenia fetida; (2) topsoil or subsoil dwelling worms (endogeics); and (3) worms that construct permanent deep burrows through which they visit the surface to obtain plant material for food, such as leaves (anecic), e.g. Lumbricus terrestris.^[23]

Permanent vertical burrow

Earthworm populations depend on both physical and chemical properties of the soil, such as soil temperature, moisture, pH, salts, aeration and texture, as well as available food, and the ability of the species to reproduce and disperse. One of the most important environmental factors is pH, but earthworms vary in their preferences. Most earthworms favor neutral to slightly acidic soil. However, Lumbricus terrestris are still present in a pH of 5.4 and Dendrobaena octaedra at a pH of 4.3 and some Megascolecidae are present in extremely acid humic soils. Soil pH may also influence the numbers of worms that go into diapause. The more acidic the soil, the sooner worms go into diapause, and remain in diapause the longest time at a pH of 6.4.

Ocypus olens trying to prey on Lumbricus sp.

Earthworms form the base of many food chains. They are preyed upon by many species of birds (e.g. starlings, thrushes, gulls, crows, European Robins and American Robins), snakes, mammals (e.g. bears, foxes, hedgehogs, moles) and invertebrates (e.g. ground beetles and other beetles, snails, slugs). Earthworms have many internal parasites including Protozoa, Platyhelminthes, Nematodes; they can be found in the worms' blood, seminal vesicles, coelom, intestine, or in the cocoons.

The application of chemical fertilizers, sprays and dusts can have a disastrous effect on earthworm populations^{[citation needed]}. Nitrogenous fertilizers tend to create acidic conditions, which are fatal to the worms, and often dead specimens are to be found on the surface following the application of substances like DDT, lime sulphur and lead arsenate. In Australia, changes in farming practices such as the application of superphosphates on pastures and a switch from pastoral farming to arable farming had a devastating effect on populations of the Giant Gippsland earthworm leading to their classification as a protected species.

Therefore, the most reliable way to maintain or increase the levels of worm population in the soil is to avoid the application of artificial chemicals. Adding organic matter, preferably as a surface mulch, on a regular basis will provide them with their food and nutrient requirements, and also creates the optimum conditions of heat (cooler in summer and warmer in winter) and moisture to stimulate their activity.

Economic impact[link]

Earthworms being raised at the La Chonita Hacienda in Mexico

Various species of worms are used in vermiculture, the practice of feeding organic waste to earthworms to decompose and compost food waste. These are usually Eisenia fetida (or its close relative Eisenia andrei) or the Brandling worm, also known as the Tiger worm or Red Wiggler, and are distinct from soil-dwelling earthworms.

Earthworms are sold all over the world. The earthworm market is sizable. According to Doug Collicut, "In 1980, 370 million worms were exported from Canada, with a Canadian export value of $13 million and an American retail value of $54 million."

Earthworms are also sold as food for human consumption. Noke is a culinary term used by the Māori of New Zealand, to refer to earthworms which are considered delicacies.

Taxonomy and distribution[link]

Within the world of taxonomy, there is considerable controversy over how to classify earthworms, with Fender and McKey-Fender (1990) going so far as to say "The family-level classification of the megascolecid earthworms is in chaos."^[24] Over the years, many scientists have developed their own classification systems for earthworms, and these systems have been and still continue to be revised and updated.^[25][26] The classification system used here, developed by Jamieson (1988), is commonly used^[27] and is the system used by the United States Integrated Taxonomic Information System.^[1]

Categorization of an earthworm into one of its taxonomic families under suborder Lumbricina is based on such features as the makeup of the clitellum, the location and disposition of the sex features (pores, prostatic glands, etc.), number of gizzards and body shape.^[26] There are over 3,500 described species of earthworm.^[27]

The families, with distribution of the main ones:^[26]

• Acanthodrilidae - (Gondwanan or Pangaean?)
• Ailoscolecidae - Pyrenees and southeast USA
• Almidae - Tropical equatorial (South America, Africa, Indo-Asia)
• Benhamiinae - ?Ethiopian, Neotropical
• Criodrilidae - Southwestern Palaearctic: Europe, Middle East, Russia and Siberia to Pacific coast; Japan (Biwadrilus); mainly aquatic
• Diplocardiinae/-idea - (Gondwanan or Laurasian?)
• Enchytraeidae - cosmopolitan distribution but uncommon in tropics
• Eudrilidae - Tropical Africa south of the Sahara
• Exxidae - Neotropical: Central America and Caribbean
• Glossoscolecidae - Neotropical: Central, S. America, Caribbean
• Haplotaxidae - cosmopolitan distribution
• Hormogastridae - Mediterranean
• Kynotidae - Malagasian: Madagascar
• Lumbricidae - Holarctic: North America, Europe, Middle East, Central Asia to Japan
• Lutodrilidae - Louisiana southeast USA
• Megascolecidae - (Pangaean?)
• Microchaetidae - Terrestrial in Africa especially South African grasslands
• Moniligastridae - Oriental and Indian sub-region
• Ocnerodrilidae - Neo-Tropics, Africa; India
• Octochaetidae - Australasian, Indian, Oriental, Ethiopian, Neotropical
• Octochaetinae - ?Australasian, Indian, Oriental
• Sparganophilidae - Nearctic, Neotropical: North and Central America
• Tumakidae - Columbia, South America

See also[link]

• Caecilians of the Western Ghats
• Drilosphere, the part of the soil influenced by earthworm secretions and castings
• The Formation of Vegetable Mould through the Action of Worms
• Invasive earthworms of North America
• Soil life
• Vermicompost
• Worm charming

References[link]

Referenced works[link]

• Blakemore, Robert J. (2012). Cosmopolitan Earthworms -- an Eco-Taxonomic Guide to the Peregrine Species of the World. (5th Ed). Yokohama, Japan: VermEcology So(i)lutions. 
• Sims, Reginald William; Gerard, B (1985). Earthworms: Keys and Notes for the Identification and Study of the Species. London: Published for The Linnean Society of London and the Estuarine and Brackish-Water Sciences Association by E. J. Brill/Dr. W. Backhuys. 
• Edwards, Clive Arthur; Bohlen, Patrick J. (1996). Biology and Ecology of Earthworms, 3rd Ed. Springer. 

Further reading[link]

• Edwards, Clive A., Bohlen, P.J. (Eds.) Biology and Ecology of Earthworms. Springer, 2005. 3rd edition.
• Edwards, Clive A. (Ed.) Earthworm Ecology. Boca Raton: CRC Press, 2004. Second revised edition. ISBN 0-8493-1819-X
• Lee, Keneth E. Earthworms: Their Ecology and Relationships with Soils and Land Use. Academic Press. Sydney, 1985. ISBN 0-12-440860-5
• Stewart, Amy. The Earth Moved: On the Remarkable Achievements of Earthworms. Chapel Hill, N.C.: Algonquin Books, 2004. ISBN 1-56512-337-9

External links[link]

Wikimedia Commons has media related to: Earthworms

General[link]

• Earthworm Digest - information website and magazine
• WormWatch - Field guide to earthworms
• Earthworms as pests and otherwise hosted by the UNT Government Documents Department

Academic[link]

• Infography about Earthworms
• Earthworm resources
• Opuscula Zoologica Budapest, online papers on earthworm taxonomy
• A Series of Searchable Texts on Earthworm Biodiversity, Ecology and Systematics from Various Regions of the World -- maintained by Rob Blakemore, Ph.D., an earthworm taxonomy specialist

Agriculture and ecology[link]

• Earthworm Information at University of California, Davis
• Minnesota Invasive Earthworms Minnesota DNR information on the negative impacts of earthworms

Worm farming[link]

• The Three Tiered Worm Farm A technical guide for constructing a tiered worm farming system
• How to Make a Worm Farm Good for Composting and Fishing

For kids[link]

• Kids Discovery
• BioKIDS -- Segmented worms