Anatomy of the earthworm

Earthworm is the common name for the larger members of the 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 to the outside of body posterior to the female pores, even though the male segments are anterior to the female. Cladistic studies have supported placing them instead in the Haplotaxida, which also includes the family Haplotaxidae. Folk names for earthworm include "dew-worm", "night crawler" and "angleworm".

Earthworms are also called megadriles (or big worms), as opposed to the microdriles, which include the families Tubificidae, Lumbriculidae, and Enchytraeidae, among others. The haplotaxids have been traditionally considered microdriles. The megadriles are characterized by having a multilayered clitellum (which is much more obvious than the single-layered one of the microdriles), a vascular system with true capillaries, and male pores behind the female pores.

## Overview

There are over 2,200 species known worldwide, existing everywhere but Arctic and arid climates. They range in size from two centimeters (less than one inch) to over three meters (almost ten feet) in the Giant Gippsland Earthworm. 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 Aporrectodea caliginosa is deeper burrowing.

In temperate zone areas, the most commonly seen earthworms are lumbricids (Lumbricidae), mostly due to the recent rapid spread of a relatively small number of 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 blood 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. A typical lumbricid will have 5 pairs of hearts. 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.

## Dissection

The classroom dissection of the earthworm and other animals has become controversial in recent years. One response to this has been the development of online "virtual dissections".

## Reproduction

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 external 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.

## Behavior

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, and are vulnerable to predators such as birds.

## Locomotion and importance to 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. The whole process is aided by the secretion of a slimy lubricating mucous. In more compacted soils the earthworm actually eats its way through the soil, cutting a passage with its muscular pharynx and dragging the rest of the body along. 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. In addition, many earthworms will come to the surface and graze on the higher concentrations of organic matter there, mixing it with the mineral soil. 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)

## 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 1/20 of an inch across) into its '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 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)" (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 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).

Professor I. L. Heiberg of State University of New York College of Environmental Science and Forestry has stated that in optimum conditions, the worm population may even reach 250,000,000 per acre (62,000/m²), 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 fewer, scrawny, undernourished specimens.

## Earthworms as invasives

Lumbricid earthworms are invasive to North America and not only have displaced native earthworms in much of the continent, but have invaded areas where earthworms did not formerly exist. There are no native earthworms in much of North America, especially in the north, and the forests there developed relying on a large amount of undecayed leaf matter. The worms decompose that leaf layer, making the habitat unsurvivable for certain species of trees, ferns and wildflowers. Currently there is no economically feasible method for controlling earthworms in forests, besides preventing introductions. Earthworms normally spread slowly, but can be widely introduced by human activities such as construction earthmoving, or by fishermen releasing bait, or by plantings from other areas.

Soils which have been invaded by earthworms can be recognized by an absence of palatable leaf litter. For example, in a sugar maple - white ash - beech - northern red oak association, only the beech and oak leaves will be seen on the forest floor (except during autumn leaf-fall), as earthworms quickly devour maple and ash leaves. Basswood, dogwood, elm, poplar and tuliptree also produce palatable foliage.

## 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 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. Aporrectodea 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.

## Ecology

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 acid soil. However, Lumbricus terrestris are still present in pH of 5.4 and Dendrobaena octaedra at 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 acid the soil, the sooner worms went into diapause, and remain in diapause the longest time at 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, and robins. Mammals such as hedgehogs and moles eat many earthworms as well. Earthworms are also eaten by many invertebrates such as Ground beetles and other beetles, snails, slugs and flatworms. Earthworms have many internal parasites including Protozoa, Platyhelminthes, nematodes. They are found in many part of earthworms' bodies like the blood, seminal vesicles, coelom, intestine, or in the cocoons.

## Threats to earthworms

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 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 fetida or the Brandling worm, also known as the Tiger worm or Red Wriggler, and are distinct from soil-dwelling earthworms.

Earthworms are sold all over the world. The earthworm market is sizeable. According to Doug Collicut (see "Nightcrawler" link below), "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."

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