The earthworm is an astonishing creature but it is nothing new. In 1881, Charles Darwin (1809-82), the English naturalist who is better known for his theory of evolution by natural selection, described the habits of the burrowing type of these organisms. Among his observations and deductions are the following (Darwin 1881):
- They are nocturnal; they wander about at night. They can crawl forward and backward;
- They have no eyes, but they can distinguish day from night; they retreat rapidly into their burrows when brightly illuminated;
- They breathe through their skin, having no special respiratory organs;
- The two sexes occur in the same individual but they mate in pair;
- They can live long under water;
- They are sensitive to dry air, heat, cold, vibrations and to touch;
- They have no jaws or teeth and are completely deaf;
- Their sense of smell is feeble and limited to the perception of certain odors;
- They are omnivorous and prefer one taste over another. They moisten leaves before swallowing with a fluid which is of the same digestive nature as the secretion from the pancreas of higher animals. They are cannibals, eating dead earthworms;
- They can survive for a limited period of time by swallowing soil and extracting its nutrient content. They also swallow sand, small stones, and fragments of concrete;
- They excavate their burrows by pushing the soil sideways and by swallowing it; they excavate burrows on moist soil under large stones, pavements and buildings which, upon collapse of these burrows, cause their sinking;
- They eject castings outside of their burrows. The castings are thoroughly mixed with intestinal secretions and favor plant growth, but large castings neccesitate regular rolling of lawns;
- They are eaten by birds;
- They regularly “plow” the land and mix the top;
- They exhibit some form of intelligence, indicated by their dragging of leaves from the tip and by their covering of their burrows.
However, it appears that interest in the potential uses of these worms have only been recently accelerated, starting with the publication of the “Proceedings of a Workshop on the Role of Earthworms in the Stabilization of Organic Residues” in 1981 (Edwards 2006), 100 years after Darwin’s publication of his last book “The Formation of Vegetable Mould, Through the Action of Worms, With Observations of their Habits”. According to Trautmann and Krasny (1997), only about 5 percent of the numerous species of these worms have been studied.
Earthworms are invertebrate animals, that is, they are without backbones and are thus described also as “soft bodied”. They are found in both temperate and tropical parts of the world. They belong to the phylum Annelida (segmented worms) under class Oligochaeta (few setae) and are further grouped into the terrestrial and the aquatic types. Oligochaeta also includes the leeches, better known because of those blood-sucking species although some are free-living. In contrast, parasitic worms, including flukes, tapeworms, nematodes, Ascaris, Trichinella, and filarial worm, belong to the phyla Platyhelminthes and Nematoda (Mader 1993).
Further, they are described as epigeic if they live and feed in the litter layer on the soil surface, e.g. those that are entirely pigmented or dark colored; anecic if they eat litter collected from the soil surface and transported down into the soil; and endogeic if they live and feed in the subsoil, feeding on organic matter and dead roots, and also ingesting large amounts of mineral materials, e.g. non-pigmented earthworms (Hairiah et al. 2001).
Terrestrial types are usually colored reddish or pink, with an elongated, cylindrical body that is divided crosswise into rings or segments. With the exception of the first and last segment, each segment has 8 or more small, hair-like or spine-like setae. The setae grips the soil or other substrate on which the worm crawls. Near its head, the adult worm has a paler colored, saddle shaped or band-like structure called clitellum which functions in reproduction. The clitellum completely or almost completely encircles several segments and is used to form egg cases and supply the eggs with nutrients (James 2002).
The terrestrial types have male genital openings behind the female (so called opisthoporous Oligochaeta). All other members of Oligochaeta are primarily aquatic. As of April 2001, over 60 new species of terrestrial earthworms have been collected in the Philippines by Samuel W. James of the Maharishi University of Management, Iowa, USA (James 2002). Including the aquatic types, there are about 3,000 earthworm species worldwide. They vary in size from 10 mm to 1.2 m in length and from 10 mg to 600 g in weight (Trautmann and Krasny 1997).
These worms have both male and female reproductive organs in the same individual. However, they usually reproduce by mating in which each partner fertilizes the other even though they are capable of reproduction parthogenetically without mating (Edwards and Arancon 2006).
During mating, two worms align themselves, facing each other in a head-to-tail posture. The clitellum secretes mucus that keeps the sperm hydrated as they pass between the worms. After separating, the clitellum of each worm produces a slime tube into which the eggs and sperm are deposited and fertilization takes place. The slime tube then forms a protective egg case or cocoon within which the worms develop. Worms continue to develop without passing larval stage (Mader 1993). The eggs can withstand adverse conditions, hatching when environmental conditions become favorable. It takes from one to eight months for earthworms to become sexually mature. They can live and regularly reproduce for several years (Edwards and Arancon 2006). The term bed-run is also used to refer to the young individuals.
Earthworms are aerobic organisms but have no breathing organs. They feel, absorb oxygen and excrete waste gases, which must be dissolved in water, through their skin. They feed on bacteria and fungi that thrive on decomposing organic matter, as well as nematodes (Edwards and Arancon 2006; Guerrero 2009).
Those that reside in soil feed on vast quantities of soil with living and decaying organic matter. Much of the food that they eat passes through their body and excreted as small piles of dirt, called worm cast or vermicast, on the soil surface (Mader 1993). According to Hairiah et al. (2001), these worms and other organisms that eat soil have a gut flora of bacteria.
Update: Scientists discovered how these organisms digest plant materials with anti-herbivory defenses. To read news report at our Blog page, click here.