From Wikipedia, the free encyclopedia - View original article
|Suborder:||Lumbricina + Moniligastrida|
NODC v. 8.0, 1996
|Suborder:||Lumbricina + Moniligastrida|
NODC v. 8.0, 1996
An earthworm is a tube-shaped, segmented animal that is commonly found living in soil. Its digestive system runs straight through its body, it conducts respiration through the cuticle covering its skin, and it has a simple, closed blood circulatory system. Earthworms are hermaphrodites--each individual carries both male and female sex organs. As an invertebrate it lacks a skeleton, but an earthworm maintains its structure with fluid-filled chambers functioning like a hydro-skeleton.
"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).
Larger terrestrial earthworms are also called megadriles (or big worms), as opposed to the microdriles (or small worms) in the semi-aquatic families Tubificidae, Lumbriculidae, and Enchytraeidae, among others. The megadriles are characterized by having a distinct clitellum (which is more extensive than that of microdriles) and a vascular system with true capillaries.
Depending on the species, an adult earthworm can be anywhere from 10 mm long and 1 mm wide up to 3 m long and over 25 mm wide, but the typical Lumbricus terrestris grows to about 360 mm long.
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) externally visible on the body demarcating the segments; dorsal pores and nephropores 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" used for movement; species may have four pairs of setae on each segment or more than eight sometimes forming a complete circle of setae per segment.
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 both the earthworm's mouth and, overhanging the mouth, a fleshy lobe called the prostomium, which seals 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 prehensile 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 front part of the animal. This is part of the reproductive system, and it harbors the egg capsule. The posterior is most commonly cylindrical like the rest of the body, but depending on the species may also be quadrangular, octagonal, trapezoidal, or flattened and the last segment is called the periproct. A short vertical slit, the earthworm's anus, is found on this segment.
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. Inside of the muscle layers is a fluid-filled chamber called a coelom that provides structure for the earthworm's body. A structure called a nephridium removes metabolic waste and expels it through pores on the sides and two or more nephridia are found in most segments. At the center of a body is the digestive tract, which runs straight through from mouth to anus without coiling, flanked above and below by blood vessels and the ventral nerve cord. The segments are separated from each other by septa perforated with pores, which allow the coelomic fluid to pass between segments.
Many earthworms can eject coelomic fluid through pores in the back in response to stress; Australian Didymogaster sylvaticus (known as the "blue squirter earthworm") can squirt fluid as high as 30 cm.
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. There is not a distinction between 'arteries' and 'veins.' In each segment, a vessel rings the coelom and connects the three longitudinal vessels. Segments in the esophageal region contain muscular commissural vessels connecting the top and bottom vessels that function like hearts to pump the blood. There are five pairs of hearts, more or less.
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), (o)esophagus, crop, gizzard (usually) and intestine.
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 (o)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.
The excretory system contains a pair of nephridia in every segment, except for the first three and last one.There are three types of nephridia : integumentary, septal and pharyngeal nephridia. 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 4th, 5th and 6th segment. The nitrogenous wastes are removed by the rhythmic beating of the cilia of the nephridiostomes. The excretory wastes are then finally discharged into the gut.
|This article may need to be rewritten entirely to comply with Wikipedia's quality standards. (August 2010)|
Earthworms have one or two pairs of testes contained within sacs. There are two or four pairs of seminal vesicles which produce, store and release the sperm via the male pores. Ovaries and oviducts in segment 13 release eggs via female pores on segment 14. One or more pairs of spermathecae are present (depending on the species) which 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 four to eight 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.
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. Despite denial of this phenomenon by some current experts,[who?] Gates’s reports included:
An unidentified Tasmanian earthworm shown growing a second head is reported here:
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. 
The major benefits of earthworm activities to soil fertility can be summarized as:
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/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.
From a total of around 6,000 species, only about 150 species are widely distributed around the world. These are the peregrine or cosmopolitan earthworms.
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, some aquatic and some euryhaline (salt-water tolerant) and littoral (living on the sea-shore, e.g. Pontodrilus litoralis. Even in the soil species, there are special habitats, such as soils derived from serpentine which have an earthworm fauna of their own.
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) anecic worms that construct permanent deep burrows through which they visit the surface to obtain plant material for food, such as leaves (anecic meaning "reaching up"), e.g. Lumbricus terrestris.
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.
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, pigs, 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 are believed to have a disastrous effect on earthworm populations. 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.
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. In the tropics, the 'African Nightcrawler' Eudrilus eugeniae and the 'Indian blue' Perionyx excavatus are used.
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."
Within the world of taxonomy the stable 'Classical System' of Michaelsen (1900) and Stephenson (1930) was gradually eroded with considerable 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 back 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. There are currently over 6,000 named species of terrestrial earthworms, as provided in an species name database , but the number of synonyms is unknown.
The families, with their known distributions or origins:
|Wikimedia Commons has media related to: Earthworms|