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Classification/Diagnostic Characteristics

Earthworms are of the phylum Annelida (segmented worms), and are of the class Oligachaeta. Annelids have a segmented body plan, which allow the creatures to move parts of their bodies independently of one another, thus giving them more control of their movement. Oligachaeta have no parapodia, or thin outgrowths of the body wall that serve for gas exchange and movement. Nor do oligachaeta have eyes or anterior tentacles. Earthworms are also Coelomates, meaning they have a coelom, a body cavity that develops within the mesoderm and is lined with a layer of muscular tissue called the peritoneum, which also surrounds the internal organs. Coelomates have better control over the movement of the fluids in its body cavity. Additionally, Humans and Earthworms have mouths for nutrient intake and anuses for waste expulsion.

Relationship to Humans

In a phylogenetic sense, both Humans and Earthworms are of the Kingdom Animalia. Both have waste filtration systems, and the segmented body plan of the Earthworm corresponds to the segmentation of vertebrae in Humans, though it should be noted that worms are not vertebrates. Earthworms also have neurons that are clustered into ganglia, which coordinate movement, as well as more complex behavior. Both Earthworms and Humans are bilaterally symmetrical, meaning only the right and left sides of the organism, divided by a plane down the midline, are mirror images of each other.

Earthworms can play a key role in increasing the productivity of farms by increasing nutrient availability, producing a more stable soil structure, and improving the drainage of the soil. Because these worms feed on plant debris such as the dead roots, leaves, stems, grasses, and manure, their bodies have higher concentration of nutrients than the soil around. Worm castings, or the organic matter that is digested by earthworms, provide nutrients to the plants around and improve plant growth. In addition to their digestion of nutrients, earthworm bodies themselves also decompose easily, adding to the nutrient availability. Earthworms often leave their casts in tunnels, which allow plant roots to take in the nutrients in the casts and penetrate deeper into the soil where it is more moist. The tunneling and burrowing of earthworms also makes it easier for water to drain through the soil, and in some cases, water can drain 10 times faster in soil with earthworms than soil without. The nutrient availability and water drainage that are a result of earthworms has greatly improved productivity in farming, which allows more food to be produced for human consumption and can lead to a 25% long term increase in productivity. (5) (CM)

Habitat and Niche

Annelids must stay in moist environments, as they quickly lose water in dry conditions through their permeable exterior layers. This is why one often sees Earthworms on rainy days.

The Earthworms habitat consists primarily of moist soil which contains adequate but not excessive amounts of water. Within this soil, earthworms are classified as epigeic, endogeic, and aneic, or surface, topsoil, and subsoil dwellers. Epigeic species usually reside in areas which contain large amounts of organic matter, such as a leaf litter or a decaying plant. Epigeic species tend to be smaller and darker in pigmentation. Endogeic species inhabit in the topsoil, or the first 20 cm of soil. These earthworms tend to be slightly larger than epigeic worms and form semi-permanent burrows. The last habitat for this organism within the soil is filled by aneic species which can construct permanent burrow as far as three meters down. These species tend to be the largest and the palest due to lack of exposure to the sun. Rarely, earthworms can be found inside tree trunks, in trees, or living in aquatic environments.(8)
Earthworms usually fill the niche of decomposer, processing soil and decaying organic material to remove the necessary nutrients. Other organisms can then gain access to these nutrients by preying upon the earthworms. Earthworm tunneling also improves the drainage and filtration rate of the soil, increasing the amount of available nutrients in the soil and decreasing erosion. Earthworms can also break up hard soil and free previously inaccessible nutrients.(9) (SF)

Predator Avoidance

Earthworms tend to live in damp soil, which naturally protects them from landborne predators. Because Earthworms are rarely hunted underground, they are generally safe unless forced out of the soil by flooding or other disturbances. In these cases, Earthworms are generally defenseless to predators. (SM) [6]
Earthworms in Soil
Earthworms in Soil

Earthworms exposed to predators due to flooded conditions. They are unsafe and not doing a good job at avoidance. (SM) [7]
Earthworms burrow deep into the ground to avoid predators. Also, earthworms that are caught by predators will produce a powerful smell and twist about wildly in an attempt to escape. Their slippery bodies makes them difficult for predators such as birds to hold on to. Earthworms are also capable of regenerating if they are damaged by a predator. (JM 17)

Nutrient Acquisition

Many Earthworms are detrivores, meaning that they feed on decaying organic matter. Earthworms also eat the tiny organisms living in the soil, and one species is even known to eat other Earthworms. (Siddharth Singh-25).

Earthworms live in soil, which they ingest in order to extract nutrients. Earthworms have tubular guts, meaning nutrients are taken in through the mouth, absorbed while passing through the gut, with the remaining undigested nutrients expelled from the anus. Earthworms, like reptiles, birds, and some insects, have two stomach-like organs in a row. The crop is a storage sac that hold the food, while the gizzard is a muscular organ that grinds it up.
The earthworm can obtain nutrients from the soil as it burrows through and it can ingest its own weight in soil each day. The soil goes through the digestive tract that runs the length of their body that is called the alimentary canal. Food is taken in through the mouth and travels to the buccal cavity where taste cells are located. Then the food travels to the pharynx, which extends to around the sixth segment and acts as a suction pump that gets the food from the buccal cavity. The food travels through the gullet next and into the crop. Food is stored in the crop before it goes to the gizzard, and the gizzard along with other substances it ingested help grind up the food. After that the intestine begins and digestion and absorption occurs there. Similar to humans, the earthworm's intestine has many folds in the inner wall increasing surface area and gives the food more opportunities to be completely absorbed (18). (SM)
These folds are called typhlosoles. Also similar to humans, peristalsis- or the contracting of muscles- is used to carry food down the intestine.
Digestion in Earthworms is largely mostly extracellular, with cells that constitute the pharynx secreting proteases-enzymes that break down proteins, as well as mucus to ease the passage of food down the gut. In the intestine, enzymes like lipase, to break down lipids, and diastase, to break down starches into sugar, are secreted. (Siddharth Singh-25).
Earthworm anatomy (21) (SM)
Earthworm anatomy (21) (SM)

Reproduction and Life Cycle

As members of the class Oligachaeta, Earthworms are hermaphroditic, meaning that each individual is both male and female. When two individuals mate, both eggs and sperm are exchanged at the same time. Eggs and sperm are then deposited in the environment in a sac called a clitellum, in which fertilization occurs. When the offspring have developed, they leave the clitellum and begin independent life.
Earthworm Reproduction.gif
Earthworm life cycle
Earthworm life cycle

Since earthworms are hermaphrodites, every worm can mate with every other worm, which provides a large pool for selection (22). The fertilized eggs are then held in a protective cocoon. The hatchlings, baby worms, come out to burrow themselves into the soil where they become juvenile then mature worms (23). The average lifespan of the earthworm varies widely between different species. Gray worms live 1-2 years on average, red worms tend to live between 2-5 years, and the average life span of the night crawler is between 6-9 years but has been reported to live up to 20. (24) (SP)

Growth and Development

Earthworms start in cocoons for about 11 weeks where gestation occurs. The cocoon of hardened mucus and contains all the nutrients necessary for the survival of the earthworms. If conditions aren't good for survival, the cocoon will remain dormant. About three earthworms hatch from each egg and at that point they're referred to as hatchlings. At first they appear as white, but in a few hours they gain hemoglobin and turn their characteristic pink or red color. When they become sexually mature adults, diet plays a large role in their size. The maturity of earthworms is determined by the development of the clitellum, the thick ring around the worm. The life expectancy of earthworms is unknown. (1) (2) (LK)

earthworm_clitellum.jpg (3) (LK)


Earthworms, like all Annelids, have a thin, permeable body wall that allows for gas exchange. In earthworms, four pairs of setae, or bristles that prevent the worm from slipping backward, protrude from each body segment.

The earthworm's outermost layer is a thin, protective cuticle coated by a mucus which is secreted from the glands covering the epidermis. This skin setup keeps the earthworm's body moist at all times and thus provides for the moist canvas needed for gas exchange to occur; the earthworm can comfortably and efficiently absorb oxygen and expel carbon dioxide. Earthworms' skin is also sensitive to touch, light, and chemicals, which allows the organism to be in better understanding of its environment. Bristle-like structures called setae surround the earthworm's body, which is segmented for ease of movement, in close, wide, and separate pairs. Along with the muscles in the earthworm's various segments, setae ease locomotion and help the earthworm anchor and control itself in movement. (AC) (13)

Cross sectional view of setal pairs (AC) (14)
Cross sectional view of setal pairs (AC) (14)


Annelids have hydrostatic skeletons, meaning they have a quantity of fluid in a body cavity surrounded by muscle. When muscles contract, the fluid-filled cavity bulges out in the opposite direction. The hydrostatic skeleton enables earthworm to crawl, as its segments each contain compartment filled with extra-cellular fluid. The body wall surrounding each segment has two muscle layers, a circular layer and a longitudinal layer. The two layers alternately contract, the longitudinal layer creating lengthening and shortening movements, and the circular layer created narrowing and widening movements. Across the body, the bulging, shortened segments serve as anchors, while long, narrow segments move forward will longitudinal contractions pull the creature forward. Bristles stabilize the widest sections of the body against the substrate, or the surface in which the Earthworm moves.

Using their circular muscles, earthworms squeeze their muscle in each segment and then grip the surface with bristles, or setae, on its skin. Then, they contract their longitudinal muscles, which pulls their body to their anterior end. (15) (RS)
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Diagram of the movement of earthworms (16) (RS)

Sensing the Environment

Earthworms do not have eyes, ears, or a nose, so they must sense their environment in other ways. Because worms need to stay in wet environments, they have an aversion to light which could indicate a dry environment. Embedded in their skin are photoreceptors (light-sensitive cells) that analyze the amount of light shining on the worm. These photoreceptors are concentrated in an area toward the front of the worm called the prostomium. The prostomium is a lobe that hangs over the top of the mouth. When these cells are exposed to light, they send a chemical message to the brain indicating a dry environment. Earthworms can also feel vibrations in the ground, informing them of the approach of a possible predator (4). (RG)

Gas Exchange

Annelids do not have a study exterior. Instead, they are covered in a thin, permeable membrane that enables gas exchange with the environment.
Annelids therefore must stay in moist environments, as they quickly lose water in dry conditions. This is why one often sees Earthworms on rainy days. When oxygen diffuses across the earthworm's outer membrane, it enters capillaries where it binds to hemoglobin and is carried throughout the body. Carbon dioxide passively diffuses out of the earthworm's body through its skin as well, though it does not bind to hemoglobin. In order to maintain the surface moistness necessary for gas exchange to occur, earthworms secrete a special type of mucus, which is what gives them their hallmark slimy exterior. (YR) (19) (20)

Waste Removal

In Annelids, each body segment contains a fluid-filled cavity called a coelom. The closed circulatory system of the annelid moves blood via pressure. The coelom receives the blood pumped through the thin capillary walls, as well as other waste products that enter directly from the tissues. The coelomic fluid is then filtered through the metanephridia; just as vertebrates have kidneys for waste filtration, Annelids have a pair metanephridia, one pair per segment. The metanephridia are composed of 3 main structures: The nephrostome is a funnel-like opening, covered in tiny hairs called cilia, through which the coelomic fluid is swept; the tubule continues into the next segment, and the tubule cells reabsorb certain molecules and secrete others; finally, the end product-a urine containing nitrogenous and other wastes-leaves the body through the nephridopores.

Environmental Physiology (Temperature, Water, and Salt Regulation)

Earthworms are highly sensitive to salt stress. The salinity may reduce their growth at low salt concentrations or cause mortality at high salt concentrations. They cannot tolerate high ionic levels since high salt concentration can destroy their sensitive skin, and thus cannot control osmotic regulation. Additionally, the neurosecretory cells in earthworms play a vital role in water balance as well as ionic and osmotic regulation.The salt stress and desiccation significantly interfere with the functions of these neurosecretory cells. Therefore, salts such as sodium chloride are extremely toxic to most of the earthworm species. (12) (NU) Earthworms are extremely sensitive to external conditions because they regulate oxygen and water levels through diffusion. They cannot self-regulate their salt content, so earthworms will dry up if they are exposed to excess salt without water.

Internal Circulation

Earthworms have closed circulatory systems, in which blood vessels separate the circulating fluid-or blood- from the interstitial fluid, or the fluid found around the cells. Blood is composed of blood plasma, a fluid which contains dissolved solutes, and blood cells. The blood cells and macro-molecules remain in circulation, but water and less-heavy solids leak out of the capillaries, the the smallest kind of blood vessel, whose surface is quite permeable. The extra cellular fluid is composed of the blood plasma and interstitial+ fluid.
In the Earthworm, a large ventral blood vessel (ventral meaning located on the bottom of the worm) runs from the worm's front (anterior) end to its back (posterior) end. Across the worm, smaller blood vessels branching from the ventral vessel carry blood to capillaries, where gases, nutrients, and wastes pass between the blood and interstitial fluid. The blood then flows through larger vessels which carry it to the the main doral vessel (dorsal meaning located on the top of the worm) which carries the blood to the front end of the body. The dorsal and ventral vessels are connected by five muscular vessels, which, like hearts, contract in order to keep the blood circulating. There are also one-way valves to prevent blood from flowing the wrong direction.

Siddharth Singh
Siddharth Singh

Chemical Control (Endocrine System)

During waste removal, tubule cells reabsorb certain compounds and secrete others, maintaining a chemical balance in the organism. The Earthworm uses calciferous glands to regulate the pH of its coelomic fluid. The glands line the oseophagus, located next to the gizzard, and secrete carbonic anhydrase, which fixes carbon dioxide, an acidic compound, into calcium carbonate, a base. This is essential in regulating the pH of the organism. (Siddharth Singh-25)

Cross Section, Showing Calciferous Glands
Cross Section, Showing Calciferous Glands

Review Questions

1. What determines the maturity of an earthworm? (SM)
2. Why are earthworms essential to our ecosystem? (SP)
3. Without having eyes, ears, or a nose to sense their environment like humans do, how do earthworms sense and react to their environments? (AG)
4. Which aspects of earthworm physiology could be used to prove that earthworms and humans have a common ancestor? (MT)
5. How does the Earthworm regulate the acidity of its coelomic fluid? (Siddharth Singh)

Sources Cited

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21. Earthworm anatomy picture
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