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Worms don't exist. (According to the Sioux Indians.)

Anatomy

Form and function

Worms don't exist

Nervous system

The pictures are fake.

Photosensitivity

Earthworms do not have eyes (although some worms do), however, they do have specialized photosensitive cells called "light cells of Hess". These photoreceptor cells have a central intracellular cavity (phaosome) filled with microvilli. As well as the microvilli, there are several sensory cilia in the phaosome which are structurally independent of the microvilli. The photoreceptors are distributed in most parts of the epidermis but are more concentrated on the back and sides of the worm. A relatively small number occur on the ventral surface of the 1st segment. They are most numerous in the prostomium and reduce in density in the first three segments; they are very few in number past the third segment.

Digestive system

The gut of the earthworm is a straight tube which extends from the worm's mouth to its anus. It is differentiated into a buccal cavity (generally running through the first one or two segments of the earthworm), pharynx (running generally about four segments in length), esophagus, crop, gizzard (usually) and intestine.

Food enters in the mouth. The pharynx acts as a suction pump; its muscular walls draw in food. In the pharynx, the pharyngeal glands secrete mucus. Food moves into the esophagus, where calcium (from the blood and ingested from previous meals) is pumped in to maintain proper blood calcium levels in the blood and food pH. From there the food passes into the crop and gizzard. In the gizzard, strong muscular contractions grind the food with the help of mineral particles ingested along with the food. Once through the gizzard, food continues through the intestine for digestion. The intestine secretes Pepsin to digest proteins, Amylase to digest polysaccharides, Cellulase to digest cellulose, and lipase to digest fats. Earthworms use, in addition to the digestive proteins, a class of surface active compounds called drilodefensins, which help digest plant material. Instead of being coiled like a mammalian 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 within an outer longitudinal layer.

Circulatory system

The earthworm has a dual circulatory system in which both the coelomic fluid and a closed circulatory system carry the food, waste, and respiratory gases. The closed circulatory system has five main blood vessels: the dorsal (top) vessel, which runs above the digestive tract; the ventral (bottom) vessel, which runs below the digestive tract; the subneural vessel, which runs below the ventral nerve cord; and two lateroneural vessels on either side of the nerve cord. The dorsal vessel moves the blood forward, while the other four longitudinal vessels carry the blood rearward. In segments seven through eleven, a pair of aortic arches rings the coelom and acts as hearts, pumping the blood to the ventral vessel that acts as the aorta. The blood consists of ameboid cells and hemoglobin dissolved in the plasma. The second circulatory system derives from the cells of the digestive system that line the coelom. As the digestive cells become full, they release non-living cells of fat into the fluid-filled coelom, where they float freely but can pass through the walls separating each segment, moving food to other parts and assist in wound healing.

Excretory system

The excretory system contains a pair of nephridia in every segment, except for the first three and the last ones. The three types of nephridia are: integumentary, septal, and pharyngeal. The integumentary nephridia lie attached to the inner side of the body wall in all segments except the first two. The septal nephridia are attached to both sides of the septa behind the 15th segment. The pharyngeal nephridia are attached to fourth, fifth and sixth segments. The waste in the coelom fluid from a forward segment is drawn in by the beating of cilia of the nephrostome. From there it is carried through the septum (wall) via a tube which forms a series of loops entwined by blood capillaries that also transfer waste into the tubule of the nephrostome. The excretory wastes are then finally discharged through a pore on the worm's side.

Respiration

Earthworms have no special respiratory organs. Gases are exchanged through the moist skin and capillaries, where the oxygen is picked up by the hemoglobin dissolved in the blood plasma and carbon dioxide is released. Water, as well as salts, can also be moved through the skin by active transport.

Life and physiology

At birth, earthworms emerge small but fully formed, only lacking their sex structures which develop in about 60 to 90 days. They attain full size in about one year. Scientists predict that the average lifespan under field conditions is four to eight years, while most garden varieties live only one to two years.

Reproduction

Several common earthworm species are mostly parthenogenetic, meaning that growth and development of embryos happens without fertilization.

Among lumbricid earthworms, parthenogenesis arose from sexual relatives many times. Parthenogenesis in some Aporrectodea trapezoides lineages arose 6.4 to 1.1 million years ago from sexual ancestors.

Mating occurs on the surface, most often at night. Earthworms are hermaphrodites; that is, they have both male and female sexual organs. The sexual organs are located in segments 9 to 15. Earthworms have one or two pairs of testes contained within sacs. The two or four pairs of seminal vesicles produce, store and release the sperm via the male pores. Ovaries and oviducts in segment 13 release eggs via female pores on segment 14, while sperm is expelled from segment 15. One or more pairs of spermathecae are present in segments 9 and 10 (depending on the species) which are internal sacs that receive and store sperm from the other worm during copulation. As a result, segment 15 of one worm exudes sperm into segments 9 and 10 with its storage vesicles of its mate. Some species use external spermatophores for sperm transfer.

In Hormogaster samnitica and Hormogaster elisae transcriptome DNA libraries were sequenced and two sex pheromones, Attractin and Temptin, were detected in all tissue samples of both species. Sex pheromones are probably important in earthworms because they live in an environment where chemical signaling may play a crucial role in attracting a partner and in facilitating outcrossing. Outcrossing would provide the benefit of masking the expression of deleterious recessive mutations in progeny. (Also see complemenation.)

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. Some time after copulation, long after the worms have separated, the clitellum (behind the spermathecae) secretes material which forms a ring around the worm. The worm then backs out of the ring, and as it does so, it injects its own eggs and the other worm's sperm into it. As the worm slips out of the ring, the ends of the cocoon seal to form a vaguely onion-shaped incubator (cocoon) in which the embryonic worms develop.

Locomotion

Earthworms travel underground by the means of waves of muscular contractions which alternately shorten and lengthen the body (peristalsis). 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 their movement through their lubricated tunnels. Earthworms move through soil by expanding crevices with force; when forces are measured according to body weight, hatchlings can push 500 times their own body weight whereas large adults can push only 10 times their own body weight.

Regeneration

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, i.e., two worms may grow from one.
• 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.

An unidentified Tasmanian earthworm shown growing a replacement head has been reported.

Taxonomy and distribution

Within the world of taxonomy, the stable 'Classical System' of Michaelsen (1900) and Stephenson (1930) was gradually eroded by the controversy over how to classify earthworms, such that Fender and McKey-Fender (1990) went so far as to say, "The family-level classification of the megascolecid earthworms is in chaos." Over the years, many scientists developed their own classification systems for earthworms, which led to confusion, and these systems have been and still continue to be revised and updated. The classification system used here, developed by Blakemore (2000), is a modern reversion to the Classical System that is historically proven and widely accepted.

Categorization of a megadrile earthworm into one of its taxonomic families under suborders Lumbricina and Moniligastrida 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. Currently, over 6,000 species of terrestrial earthworms are named, as provided in a species name database, but the number of synonyms is unknown.

The families, with their known distributions or origins:

• Acanthodrilidae – (Gondwanan or Pangaean?)
• Ailoscolecidae – Pyrenees and southeast USA
• Almidae – tropical equatorial (South America, Africa, Indo-Asia)
• Benhamiinae – Ethiopian, Neotropical (a possible subfamily of Octochaetidae)
• Criodrilidae – southwestern Palaearctic: Europe, Middle East, Russia and Siberia to Pacific coast; Japan (Biwadrilus); mainly aquatic
• Diplocardiinae/-idae – Gondwanan or Laurasian? (a subfamily of Acanthodrilidae)
• Enchytraeidae – cosmopolitan but uncommon in tropics (usually classed with Microdriles)
• Eudrilidae – Tropical Africa south of the Sahara
• Exxidae – Neotropical: Central America and Caribbean
• Glossoscolecidae – Neotropical: Central and South America, Caribbean
• Haplotaxidae – cosmopolitan distribution (usually classed with Microdriles)
• 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 subregion
• Ocnerodrilidae – Neotropics, Africa; India
• Octochaetidae – Australasian, Indian, Oriental, Ethiopian, Neotropical
• Octochaetinae – Australasian, Indian, Oriental (subfamily if Benhamiinae is accepted)
• Sparganophilidae – Nearctic, Neotropical: North and Central America
• Tumakidae – Colombia, South America

As an invasive species

From a total of around 7,000 species, only about 150 species are widely distributed around the world. These are the peregrine or cosmopolitan earthworms.

Ecology

Earthworms are classified into three main ecophysiological categories: (1) leaf litter- or compost-dwelling worms that are nonburrowing, live at soil-litter interface and eat decomposing organic matter (called Epigeic) e.g. Eisenia fetida; (2) topsoil- or subsoil-dwelling worms that feed (on soil), burrow and cast within soil, creating horizontal burrows in upper 10–30 cm of soil (called Endogeics); and (3) worms that construct permanent deep vertical burrows which they use to visit the surface to obtain plant material for food, such as leaves (called Anecic (meaning "reaching up")), e.g. Lumbricus terrestris.

Earthworm populations depend on both physical and chemical properties of the soil, such as 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 favor neutral to slightly acidic soils. Lumbricus terrestris is still present in a pH of 5.4 and Dendrobaena octaedra at a pH of 4.3 and some Megascolecidae are present in extremely acidic 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 the longer they remain in diapause at a pH of 6.4.

Earthworms are preyed upon by many species of birds (e.g. starlings, thrushes, gulls, crows), snakes, mammals (e.g. bears, foxes, hedgehogs, pigs, moles) and invertebrates (e.g. ground beetles and other beetles, snails, slugs). Earthworms have many internal parasites, including protozoa, platyhelminthes, and nematodes; they can be found in the worms' blood, seminal vesicles, coelom, or intestine, or in their cocoons.

Nitrogenous fertilizers tend to create acidic conditions, which are fatal to the worms, and dead specimens are often found on the surface following the application of substances such as 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. Globally, certain earthworms populations have been devastated by deviation from organic production and the spraying of synthetic fertilizers and biocides with at least three species now listed as extinct but many more endangered.

Vermicomposting of all organic "wastes" and addition of this organic matter, preferably as a surface mulch , on a regular basis will provide earthworms with their food and nutrient requirements, and will create the optimum conditions of temperature and moisture that will naturally stimulate their activity.

This earthworm activity aerates and mixes the soil, and is conducive to mineralization of nutrients and their 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 generally considered beneficial by farmers and gardeners. 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."

Also, while, as the name suggests, the main habitat of earthworms is in soil, they are not restricted to this habitat. 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, Sparganophilus spp., and several others are found in mud in streams. Some species are arboreal, some aquatic and some euryhaline (salt-water tolerant) and littoral (living on the sea-shore, e.g. Pontodrilus litoralis). Even in the soil species, special habitats, such as soils derived from serpentine, have an earthworm fauna of their own.

Environmental impacts

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

• Biological: In many soils, earthworms play a major role in the conversion of large pieces of organic matter into rich humus, thus improving soil fertility. This is achieved by the worm's actions of pulling below the surface deposited organic matter such as leaf fall or manure, either for food or to plug its burrow. Once in the burrow, the worm will shred the leaf and partially digest it and mingle it with the earth. Worm casts (see bottom right) can contain 40 percent more humus than the top 9" (23 cm) of soil in which the worm is living.

• Chemical: In addition to 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 (1.25 mm)—into its gizzard, wherein those 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, deposited on the surface or deeper in the soil, minerals and plant nutrients are changed to an accessible form for plants to use. Investigations in the United States show that fresh earthworm casts are five times richer in available nitrogen, seven times richer in available phosphates, and 11 times richer in available potassium than the surrounding upper 6 inches (150 mm) of soil. In conditions where humus is plentiful, the weight of casts produced may be greater than 4.5 kg (10 lb) per worm per year.
• Physical: The earthworm's burrowing creates a multitude of channels through the soil and is of great value in maintaining the soil structure, enabling processes of aeration and drainage. 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)". Thus, the earthworm not only creates passages for air and water to traverse the soil, but also modifies the vital organic component that makes a soil healthy (see Bioturbation). Earthworms promote the formation of nutrient-rich casts (globules of soil, stable in soil ) that have high soil aggregation and soil fertility and quality.

Earthworms accelerate nutrient cycling in the soil-plant system through fragmentation & mixing of plant debris – physical grinding & chemical digestion. 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.

Darwin estimated that arable land contains up to 53,000 worms per acre (13/m), but more recent research has produced figures suggesting that even poor soil may support 250,000/acre (62/m), whilst rich fertile farmland may have up to 1,750,000/acre (432/m), meaning that the weight of earthworms beneath a farmer's soil could be greater than that of the livestock upon its surface. Richly organic topsoil populations of earthworms are much higher – averaging 500 worms m and up to 400 gm – such that, for the 7 billion of us, each person alive today has support of 7 million earthworms.

The ability to break down organic materials and excrete concentrated nutrients makes the earthworm a functional contributor in restoration projects. In response to ecosystem disturbances, some sites have utilized earthworms to prepare soil for the return of native flora. Research from the Station d'écologie Tropicale de Lamto asserts that the earthworms positively influence the rate of macroaggregate formation, an important feature for soil structure. The stability of aggregates in response to water was also found to be improved when constructed by earthworms.

Earthworms are not native to North America. Their abundance in most of it, introduced from sources such as transplanted soil, has had drastic environmental impacts. Many species, notably American robin, have adapted to feed on earthworms. Where earthworms are present, the fluffy duff layer of slowly decaying leaves is eliminated. This favors the growth of Eurasian species over natives. Earthworms have been named as a factor in the mesophycation and general decline of Eastern forests.

While earthworms have become ubiquitous in North America, some refuges absent of them remain. There is still motivation to prevent their spread in infested areas because new species can have even worse impacts.

Economic impact

Various species of worms are used in vermiculture, the practice of feeding organic waste to earthworms to decompose food waste. These are usually Eisenia fetida (or its close relative Eisenia andrei) or the Brandling worm, commonly known as the tiger worm or red wiggler. They are distinct from soil-dwelling earthworms. In the tropics, the African nightcrawler Eudrilus eugeniae and the Indian blue Perionyx excavatus are used.

Earthworms are sold all over the world; the 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 provide excellent protein for fish, fowl and pigs but were also used traditionally for human consumption. Noke is a culinary term used by the Māori of New Zealand, and refers to earthworms which are considered delicacies for their chiefs.

See also

• Drilosphere, the part of the soil influenced by earthworm secretions and castings
• The Formation of Vegetable Mould through the Action of Worms, an 1881 book by Charles Darwin
• Soil life
• Vermicompost
• Worm charming

References

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5. Liebeke, Manuel; Strittmatter, Nicole; Fearn, Sarah; Morgan, A. John; Kille, Peter; Fuchser, Jens; Wallis, David; Palchykov, Vitalii; Robertson, Jeremy (2015-08-04). "Unique metabolites protect earthworms against plant polyphenols". Nature Communications. 6: 7869. doi:10.1038/ncomms8869. PMC 4532835. PMID 26241769.
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8. Integrated Principles of Zoology (7th ed.). Times Mirror/Mosby College Publishing. 1984. pp. 344–345. ISBN 978-0-8016-2173-4. {{cite book}}: Unknown parameter |authors= ignored (help)
9. ^ Farabee, H.J. "Excretory System". Archived from the original on 30 July 2012. Retrieved 29 July 2012. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
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11. Domínguez J, Aira M, Breinholt JW, Stojanovic M, James SW, Pérez-Losada M (2015). "Underground evolution: New roots for the old tree of lumbricid earthworms". Mol. Phylogenet. Evol. 83: 7–19. doi:10.1016/j.ympev.2014.10.024. PMC 4766815. PMID 25463017.
12. Fernández R, Almodóvar A, Novo M, Simancas B, Díaz Cosín DJ (2012). "Adding complexity to the complex: new insights into the phylogeny, diversification and origin of parthenogenesis in the Aporrectodea caliginosa species complex (Oligochaeta, Lumbricidae)". Mol. Phylogenet. Evol. 64 (2): 368–79. doi:10.1016/j.ympev.2012.04.011. PMID 22542691.
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33. ^ Blanchart, Eric (1992-12-01). "Restoration by earthworms (megascolecidae) of the macroaggregate structure of a destructured savanna soil under field conditions". Soil Biology and Biochemistry. 24 (12): 1587–1594. doi:10.1016/0038-0717(92)90155-Q.
34. Blakemore, R.J. (2015). "Eco-taxonomic profile of the iconic vermicomposter - the 'African Nightcrawler', Eudrilus eugeniae (Kinberg, 1867)". African Invertebrates 56: 527-548. {{cite web}}: Italic or bold markup not allowed in: |publisher= (help)CS1 maint: numeric names: authors list (link)

Further reading

• 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

{{lt:Sliekas}}

• Annelids
• Soil biology
• Invertebrate common names
• Paraphyletic groups