external image earthworm.jpg

"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 organised creatures." —Charles Darwin (1881)

external image earthworm01.JPG([1])(JLev)

external image worm.jpg?w=300&h=278([2])(BS)

Table of Contents

  1. Classification and Diagnostic Characteristics
  2. Relationship to Humans
  3. Habitat and Niche
  4. Predator Avoidance
  5. Nutrient Acquisition
  6. Reproduction and Life Cycle
  7. Growth and Development
  8. Integument
  9. Movement
  10. Sensing the Environment
  11. Gas Exchange
  12. Waste Removal
  13. Environmental Physiology
  14. Internal Circulation
  15. Chemical Control
  16. References

Classification and Diagnostic Characteristics

Earthworms are small, tube-like invertebrates that burrow and ingest soil, from which they extract food particles. They are the most common members of the Oligochaetes. Earthworms have segmented bodies, each segment having nearly identical structures except for those with key organs. These segments are linked together and coordinate to help the earthworm function.

Earthworms are a reddish-gray color and are usually only a few inches long. Many parts of the United States call the common earthworm night crawlers because they are commonly seen above the ground at night, whereas they are usually buried during the day.[3] (LC)

Earthworms must reside in warm, damp enviroments. While other annelids may have tentacles called parapodia, earthworms do not. They do however have small bristles, setae, between their rides of their segments. Further, Earthworms are blind, as they do not have eyes. The wall of a earthworm is permeable, causing a smooth but gooey surface. Earthworms are extremely flexible, long and tubular.[4] (AWC)

Eukarya (domain) ==> Animalia (kingdom) ==> Annelida (phylum) ==> Clitellada (class) ==> Oligochaeta (subclass)[5]

external image digestive-system-of-earthworm-diagram-i19.jpg


Relationship to Humans

Earthworms are very common around the world and are important to humans. Earthworms aerate the soil and replenish it with nutrients, allowing farmers to grow more and better crops. Chemical fertilizers used on many farms are deadly for earthworms. Earthworms are used to decompose organic waste. They are also studied in labs, used as fishing bait, or even prepared as food. In some places, earthworms are a dangerous invasive species.

Earthworms have been found to be an excellent source of animal feed protein, essential amino acids, fats, vitamins, and minerals for livestock and fish. Chemical analysis of the body tissues of earthworms showed the presence of protein to be 60-70%, fat 7-10%, carbohydrate 8-20%, and minerals 2-3%.[7] (TM)

Habitat and Niche

Earthworms mostly live under the ground, where they spend most of their time digesting soil, taking in nutrients, and excreting waste products.

Earthworms prefer to live in damp, and cool conditions. Any glimpse of sunlight can be harmful to earthworms, therefore they will burrow to safety. Unless an earthworm is in a suitably dark and damp habitat, it will shut down bodily functions until it is in a suitable setting.[8] (SJ)

As herbivores and decomposers, earthworms are usually indicators of healthy soil. By tunneling through the soil, earthworms create favorable conditions for other organisms. Because they eat soil and excrete their waste in the form of casts, earthworms move large amounts of soil both from the deeper layers to the surface and from the surface to the deeper layers. This keeps the soil well-mixed and suitable for other organisms. Furthermore, the tunneling of earthworms also loosens the soil, allowing the soil to not only hold more water but also allowing water to flow through the soil more easily.[9] (FZ)

Although there name may suggest otherwise, earthworms do not all directly live in the earth. One species called Eisenia fetida is found living in decaying plant material and manure, another species, Arctiostrotus vancouverensis is mostly found in decaying conifer logs. Several other species are found in the mud on the bottom of freshwater rivers. Still more, some species are arboreal (living in trees) and some are even aquatic with some species who are euryhaline (salt-water tolerant) and some that live on the sea shore.[10] (BH)

Predator Avoidance

Earthworms have many natural predators including birds, snakes, rodents, and insects. They mostly avoid predators by burrowing deep under the ground.

More predators include ants, mites, centipedes, earwigs, fly larvae, termites, toads, turtles, and slugs. (6) (KG)

They have small bristles, known as setae, which are both sensing devices that can identify any soil vibrations and digging aids. The setae stick to dirt and the earthworm then contracts its body to force itself through soil. The worm also secretes a mucus that helps it slide through dirt more quickly. (TM)

Nutrient Acquisition

Earthworms have a linear tubular gut starting with the mouth and ending with the anus. They ingest food through their mouths, from where it travels to the crop (storage sac that holds food) and then the gizzard (organ that grinds food). The ground food then travels through the intestine where enzymes secreted by intestinal cells break down macromolecules into monomers that can be absorbed into the blood. The intestine of earthworms have a typhosole, an infolding of the intestinal wall which increases the total surface area for maximum absorption. Finally, any undigested food is expelled as feces through the anus in the back of the earthworm.[11]

Earthworms eat mostly anything that is natural, including dead leaves, dead plant material, dead animals, and some living plants. They especially enjoy to eat leaves that have high sugar concentrations from trees such as: maples, birches, ashes, American Sycamore, and American Elm. It is not uncommon for earthworms to eat small live animals accidentally, which live in the soil that they burrow in. (?)

Reproduction and Life Cycle

Earthworms are hermaphroditic, meaning that they are both male and female; they produce both sperm and ova simultaneously. When two earthworms mate, they exchange their sperm so that each one's ova are fertilized by the other's sperm, thus ensuring genetic diversity. The eggs and sperm are deposited in a cocoon outside of their bodies. Fertilization takes place inside the cocoon and new earthworms eventually emerge from the cocoon, beginning life independently and growing until they reach maturity and can also reproduce.[12]

Earthworms can live for about 3 years. They can regenerate small parts of their bodies if they are cut off (6) (KG).

After fertilizing with another worm, a worm can produce new egg capsules 7-10 days later. Those eggs hatch and the new worms reach breeding stage at about 60-90 days old. Then the earthworm grows to its final size. (?) (ES)

Two earthworms mating. [Source: Wikipedia public domain]
Two earthworms mating. [Source: Wikipedia public domain]

Growth and Development

Earthworm embryos develop in small cocoons. As soon as they hatch, they begin life independently, burrowing through dirt and ingesting food to grow into mature adults.[13]

The embryos emerge as small worms without sex structures such as the clitellum or the spermathecae (which develop later in about 60–90 days).[14] (Shwetha)

Earthworms grow through 4 stages in their lives. They begin in the egg stage as eggs in an egg sac. When the eggs hatch they enter the juvenile stage. At this time they are about half and inch and have no reproductive organs. During the mature stage they enter adulthood and start growing their reproductive organs. Once the earthworms are sexually mature they begin mating quite vigorously especially in warmer weather. (MDS). (3)


Earthworms do not have a rigid shell or external covering. Instead, they have a thin, flexible body wall that is permeable.[15]

The integument of the earthworm is very important. As it is responsible for letting oxygen into the blood, the earthworm absorbs oxygen from its surroundings, which then diffuses into the blood by passing through the integument. The outer layer of a earthworm is moist as a special mucous secretes from its glands in the skin. The importance of the mucous is great as it allows microbes to permeate the skin and lines the tunnels earthworms creates (keeping them solid), which allows for better aeration of the skin. The integument of the earthworm is also sensitive to light, touch, and chemicals which allow the earthworm to better sense its environment.[16] (MC)

Anatomy of an earthworm (SM)
([17]) (?)


Earthworms have hydrostatic skeletons, which consist of a body cavity filled with fluid surrounded by muscle. The body cavity is split into many segments, each with a circular muscle layer and longitudinal muscle layer. The contraction of muscles in some segments causes the fluid to bulge out in other segments. By alternating contractions in the two muscle layers, earthworms are able to lengthen their body in some segments while shortening it in others. The short, bulging segments help anchor the earthworm as the long segments stretch out and pull the rest of the earthworm forward. Earthworms are also equipped with setae, tiny bristles on their stomachs which help hold them in place against the dirt.[18]

Setae are bristles or hair-like structures that help attach the earthworm to the ground and create friction so if an earthworm is going down a hill, it would slide down but actually continue to move through hydrostatic pressure. (?) (HSC)

Sensing the Environment

Earthworms sense the environment through segmental nerves. Each segment of an earthworm has a pair of ganglia, clusters of neurons that are attached to adjacent ganglia through a ventral nerve chord. Each segment is thus sensitive to a change in the environment and they coordinate movement together. The ganglia at the anterior end of the earthworm form the brain and control more complex movement.[19]

Even though they cannot see, earthworms have tissue at their head that is sensitive to light. Thus, they have a developed ability to detect light and avoid surfacing during the daytime when they could be affected by the sun by losing water quickly. By surfacing at night, earthworms are able to retain their water and remain in a moist environment, essential for their gas exchange by diffusion.[20] (JF)

Gas Exchange

Earthworms do not have lungs. Gas exchange takes place directly through their skin, which is a thin, permeable membrane. They must remain in moist environments (such as soil) because they lose water quickly in hot, dry air.[21]

Earthworms must also remain in moist environments so their skin can be kept moist and in order for diffusion to occur in which oxygen and carbon dioxide pass through earthworms' skin.[22] (JF)

Additionally, the oxygen that passes through the permeable skin must first mix with the moist slime on the skin. This mixture is what passes through the skin and through the thin tissue of capillaries under the skin. (2) (JL)

Waste Removal

Earthworms have a coelom, a body cavity filled with coelomic fluid. Blood pressure inside the earthworm's circulatory system causes the blood to filter through the capillaries and into the walls of the coelom. In order to get rid of waste products, each segment has a pair of metanephridia, kidney-like structures that have openings inside the coelom, long tube structures, and pores that open to the outside. The nephrostome is the funnel-like opening of the metanephridium with cilia (short hairs) that sweep in coelomic fluid. The tubules that it travels through have cells that actively absorb some molecules while secreting others, producing a dilute urine with waste products (nitrogen and other solutes). The urine finally exits through the nephridiopores, small pores on the stomach that allow the waste to be excreted completely.

Environmental Physiology

Earthworms are ectotherms and must regulate their internal body temperature by staying in a carefully controlled environment. This is why earthworms usually remain underground, and only come to the surface when the air is cool and moist.

Earthworms regulate salt and water concentrations through pores on their skin. See Waste Removal above.

When conditions are not optimum for an earthworm, the first line of defense is to go further underground. This is what most earthworms do when it is too hot or dry. Deep underground, they roll themselves into a ball and estivate, going into a state of suspended bodily function in order to conserve water and other resources.[24] (AA)

Internal Circulation

Earthworms have a closed circulatory system, in which blood remains within a continuous system of vessels. An earthworm has a large ventral blood vessel that carries blood from the anterior end to the posterior end. The ventral vessel branches off into smaller vessels, which then branch off into even smaller capillaries, where molecules and nutrients diffuse between the blood and the interstitial fluid (which is between cells). The blood then flows back into larger vessels which eventually join into the large dorsal blood vessel that carries blood from the posterior end back to the anterior end. The dorsal vessel along with five connecting hearts contract to pump the blood and keep it flowing in one direction.[25]

Chemical Control

Although earthworms have very simple brains, they can secrete hormones, such as during reproduction.

The endocrine system in an earthworm is simple compared to the complex human endocrine system. It contains neurosecretory cells in the ganglia, the nervous system of invertebrates, that regulate growth, reproduction, and regeneration.[26] (BB-V)

Review Questions

1. Describe the integument of the earthworm and the advantages of having a soft external covering and having a moist surface. (AY)2. How do earthworms lengthen and shorten segments of their body? (WSS)3. How does the environment an earthworm lives in help it survive? (Shwetha)4. Explain the mechanism the earthworm uses to get its necessary nutrients. (?)5. Describe the 4 stages of growth the earth worm undergoes in its life. (DA)


Hillis, David M., et al. Principles of Life. Sunderland, MA: Sinauer Associates, Inc., 2012. Print.
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    pp. 469–470
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    pp. 771–772
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    pp. 470, 641–642
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    pp. 470
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    pp. 469–470
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    pp. 723
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    pp. 674
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    pp. 469–470
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    pp. 788
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    pp. 747
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2. "Earthworm: Respiration System." JVCs Science Fair Projects. N.p., 1 Mar. 2011. Web. 13 Dec. 2012. <>.

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