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Earthworm is a common name referering to the segmented worms, phylum Annelida, class Clitellata, subclass Oligochaeta, order Opisthopora.

Overview

There are over 2,200 species known worldwide, existing everywhere but Arctic and arid climates. They range in size from two centimeters (about one inch) to over three meters (eleven feet). Amongst the main earthworm species commonly found in the soil are the red coloured Lumbricus terrestris, which dwells close to and leaves its deposits on the surface, whilst the greyish blue Allolobophora caliginosa is deeper burrowing.

In temperate zone areas, most commonly seen earthworms are lumbricids (Lumbricidae), mostly due to the recent rapid spread of a relatively few European species, but there are several other families, e.g. Megascolecidae, Sparganophilidae, Glossoscolecidae, Haplotaxidae, and others. These other families are often very different from the lumbricids in behavior, physiology and habitat.

Anatomy

Earthworms have a closed circulatory system. They have two main blood vessels that extend through the length of their body- a ventral bood vessel which leads the blood to the posterior end, and a dorsal blood vessel which leads to the anterior end. The dorsal vessel is contractile and pumps blood forward, where it is pumped into the ventral vessel by a series of "hearts" which vary in number in the different taxa. The blood is distributed from the ventral vessel into capillaries on the body wall and other organs and into a vascular sinus in the gut wall where gases and nutrients are exchanged. This arrangement may be complicated in the various groups by suboesophageal, supraoesophageal, parietal and neural vessels, but the basic arrangement holds in all earthworms.

Earthworms are hermaphrodites (both female and male organs within the same individual) but cannot fertilize their own eggs. They have testes, seminal vesicles and male pores which produce, store and release the sperm, and ovaries and ovipores. However, they 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. Copulation and Reproduction are separate processes in earthworms. The mating pair overlap front ends ventrally and each exchanges sperm with the other. The cocoon, or egg case, is secreted by the clitellum, the glandular band which is near the front of the worm, but behind the spermathecae. Some indefinite 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 lacking the sexual structures, which develop later. Some earthworm species are mostly parthenogenetic, in which case the male structures and spermathecae may become abnormal, or missing.

One often sees earthworms come to the surface in large numbers after a rainstorm. There are three theories for this behavior. The first is that the waterlogged soil has insufficient oxygen for the worms, therefore, earthworms come to the surface to get the oxygen they need and breathe more easily. Secondly, some species (notably Lumbricus terrestris) come to the surface to mate. This behavior is, however, limited to a few species. Thirdly, the worms may be using the moist conditions on the surface to travel more quickly than they can underground, thus colonizing new areas more quickly. This is in any event a dangerous activity in the daytime, since earthworms die quickly when exposed to direct sunlight with its strong UV content.

Earthworm anatomy

Above; anatomy of the earthworm

Functions

The Earthworm travels underground by employing a combination of a series of tiny bristles (setae) set along its segmented length and the secretion of a slimy lubricating mucous. The worm is thus able to propel itself forward by means of rippling muscular contractions, ingesting organic materials from even the heaviest soil as it burrows, which it helps to decompose.

The ingested soil is ground up, digested, and the waste deposited behind the worm. This process aerates and mixes the soil, and is often considered greatly helpful by gardeners and farmers. Because a high level of organic matter is associated with soil fertility, an abundance of earthworms is a happy sight for the organic gardener. In fact as long ago as 1881 Charles Darwin wrote;

"It may be doubted whether there are any other animals which have played so important a part in the history of the world, as have these lowly creatures"

(The Formation Of Vegetable Mould Through The Action Of Worms, Charles Darwin)

Indeed, it is probably not much of an exaggeration to state that the humble earthworm is one of the most vital living creatures on the planet, for its actions are essential for the creation and vitality of soil, upon which every living thing is dependent.

Benefits

The major benefits of earthworm activities to soil fertility can be summarised as;

• Biological; The earthworm is essential to composting; the process of converting dead organic matter into rich humus, a medium vital to the growth of healthy plants, and thus ensuring the continuance of the cycle of fertility. This is achieved by the worm's actions of pulling down below any organic matter deposited on the soil surface (eg, leaf fall, manure, etc) 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 6" of soil in which the worm is living.

• Chemical; As well as dead organic matter, the earthworm also ingests any other soil particles that are small enough (including stones up to one-twentieth of an inch across) into it's 'crop' wherein minute fragments of grit grind everything into a fine paste which is then digested in the stomach. When the worm excretes this in the form of casts which are deposited on the surface or deeper in the soil, a perfectly balanced selection of minerals and plant nutrients is made available in an accessible form. Investigations in the USA 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 of soil. In conditions where there is plenty of available humus, the weight of casts produced may be greater than 4.5 kg (lOlb) 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-founderBill 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)" (Permaculture- A Designer's Manual, Tagari Press, 1988)- 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.

It is important that we do not take the humble earthworm for granted. Dr W E Shewell Cooper observed "tremendous numerical differences between adjacent gardens" (Soil, Humus And Health), 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 carries up to 53,000 worms to the acre (131,000 per hectare), but more recent research from Rothamstead Experimental Station has produced figures suggesting that even poor soil may support 250,000 worms to the acre, whilst rich fertile farmland may have up to 1,750,000.

Professor I L Heiberg of New York College of Forestry has stated that in optimum conditions the worm population may even reach 250,000,000 per acre (6,200,000 per hectare), meaning that the weight of earthworms beneath the farmer's soil could be greater than that of his livestock upon its surface. One thing is certain however. Rich, fertile soil that is cared for organically and well fed and husbanded by its steward will reap its reward in a healthy worm population, whilst denuded, overworked and eroded land will almost certainly contain little more than a few scrawny, undernourished specimens.

Special habitats

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 foetida (or fetida) lives in decaying plant matter and manure. Arctiostrotus vancouverensis from Vancouver Island and the Olympic Peninsula is generally found in decaying conifer logs or in extremely acid humus. Alollobophora limicola and Sparganophilus and several others are found in mud in streams. Even in the soil species, there are special habitats, such as soils derived from serpentine which have an earthworm fauna of their own.

Threats

The application of chemical fertilisers, sprays and dusts can have a disastrous effect on earthworm populations. Nitrogenous fertilisers tend to create acid 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, the use of superphosphate on pastures almost totally wiped out the giant 9' Gippsland earthworm.

In addition, as earthworms are processors of large amounts of plant and mineral materials, even if not killed themselves they can accumulate pollutants such as DDT, lead, cadmium, and dioxins at levels up to 20 times higher than in the soil, which in turn are passed on at lethal dosages to the wildlife which feed upon them such as foxes, moles or birds.

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, as well as adding organic matter, preferably as a surface mulch, on a regular basis. This will not only provide them with their food and nutrient requirements, but also creates the optimum conditions of heat (cooler in summer and warmer in winter) and moisture to stimulate their activity.

A recent threat to earthworm populations in the UK is the New Zealand Flatworm (Artiposthia triangulata), which feeds upon the earthworm, but in this country has no natural predator itself. At present sightings of the NZFW have been mainly localised, but this is no reason for complacency as it has spread extensively since its introduction in 1960 through contaminated soil and plant pots. Any sightings of the flatworm should be reported to the Scottish Crop Research Institute, who are monitoring its spread.

Economic Impact

Various species of worms are used in vermiculture, the practice of feeding organic waste to earthworms to decompose (digest) it, a form of composting by the use of worms. These are usually Eisenia foetida or the Brandling worm, also known as the Tiger worm or Red Wriggler, and are distinct from soil dwelling earthworms.

Earthworms make big contributions to our economy.They are sold all over the world for the profit of some lucky ones. The earthworm market is enormous. As Collicut mentioned, 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.

See also

• soil life

External References

• Earthworm Information (UC Davis)
• earthworm on Encyclopedia.com
• Biology of the Night Crawler (Lumbricus terrestris)
• WormWatch - Field guide to earthworms