Earthworm

An earthworm is a soil-dwelling terrestrial invertebrate that belongs to the phylum Annelida. These worms are important for the soil because they help break down organic matter and make the soil healthier. You can often find them in moist, compost-rich soil, where they eat leaves and tiny organisms.

Earthworms have a simple body made of many segments, and they move by using tiny bristles on each segment. They breathe through their skin and have a system of fluid that helps them stay strong. Even though they don’t have a skeleton, their bodies stay firm because of this special fluid inside them.

Each earthworm has both male and female parts, and when two earthworms come together, they share special cells that help create new worms. These amazing creatures play a big role in nature by helping plants grow and keeping the ecosystem balanced.

Anatomy

Form and function

Adult earthworms range in size from very small—about 1 centimeter long—to very large—over 3 meters long. Most common earthworms grow to around 36 centimeters. An earthworm’s body is like a tube inside another tube, made up of many segments called metameres.

Each segment has tiny bristles called setae that help the worm move. Earthworms also have a special swelling called the clitellum, which is part of their reproductive system. The end of the worm, called the periproct, has the anus.

The outside of an earthworm’s body is covered by a thin layer called the cuticle. Under this layer are muscles and a fluid-filled space that helps give the worm its shape. Inside, earthworms have a digestive tract that runs from mouth to anus, blood vessels, and a nerve cord.

Nervous system

Earthworms have a simple brain made of two small lobes, located in the third body segment. Nerves connect the brain to the rest of the body through a nerve cord that runs along the bottom of the worm.

This nerve system helps earthworms react to touches and move quickly to escape danger. It also helps them sense light, temperature, and chemicals in their environment.

Sensory reception

Earthworms don’t have eyes, but they have special cells that can sense light. They also have many sensors all over their bodies that help them feel touch, temperature changes, and chemicals.

Digestive system

Earthworms eat by sucking in food with their pharynx. They grind their food in a gizzard and then digest it in their intestine, which has special folds to help absorb nutrients.

Circulatory system

Earthworms have two ways to move fluids around their bodies: one uses body fluids, and the other uses a closed system of blood vessels. Blood vessels pump blood forward and backward, helping carry nutrients and oxygen.

Excretory system

Earthworms remove waste using special organs called nephridia, found in most body segments. These organs collect waste and release it through pores on the worm’s sides.

Respiration

Earthworms breathe through their skin. Oxygen enters their blood, and carbon dioxide leaves their body, all through their moist skin.

Life and physiology

Earthworms are born small but fully formed, except for their reproductive parts, which grow in about 60 to 90 days. They reach full size in about one year. Scientists think that earthworms in the wild live four to eight years, but garden earthworms usually live only one to two years.

Earthworms can reproduce without mating through a process called parthenogenesis, where embryos develop without fertilization. Some earthworms can mate and exchange sperm, but they can also reproduce on their own. During mating, earthworms exchange sperm and then create a cocoon where eggs and sperm are placed. After three weeks, the cocoon hatches, and baby earthworms emerge.

Earthworms move by squeezing their bodies forward in a wave-like motion, using tiny bristles to grip the soil. They can regenerate lost body parts, but this ability depends on the species and the extent of the damage.

Taxonomy and distribution

Earthworms have many ways to organize their families, which makes it tricky to know how to group them. A system created by Blakemore in 2000 uses an older, trusted method that many people agree on.

Scientists group earthworms by looking at special parts of their bodies, like where certain features are and how their bodies are shaped. There are more than 6,000 known kinds of earthworms, but many might share the same name.

Some earthworm families live in certain parts of the world, like:

• Acanthodrilidae
• Ailoscolecidae – the Pyrenees and the southeast USA
• Almidae – tropical equatorial (South America, Africa, Indo-Asia)
• Benhamiinae – Ethiopian, Neotropical (a possible subfamily of Octochaetidae)
• Criodrilidae – southwestern Palaearctic: Europe, Middle East, Russia and Siberia to Pacific coast; Japan (Biwadrilus); mainly aquatic
• Diplocardiinae/-idae – Gondwanan or Laurasian? (a subfamily of Acanthodrilidae)
• Enchytraeidae – cosmopolitan but uncommon in tropics (usually classed with Microdriles)
• Eudrilidae – Tropical Africa south of the Sahara
• Exxidae – Neotropical: Central America and the Caribbean
• Glossoscolecidae – Neotropical: Central and South America, Caribbean
• Haplotaxidae – cosmopolitan distribution (usually classed with Microdriles)
• Hormogastridae – Mediterranean
• Kynotidae – Malagasian: Madagascar
• Lumbricidae – Holarctic: North America, Europe, Middle East, Central Asia to Japan
• Lutodrilidae – Louisiana, in the southeast USA
• Megascolecidae
• Microchaetidae – Terrestrial in Africa especially South African grasslands
• Moniligastridae – Oriental and Indian subregion
• Ocnerodrilidae – Neotropics, Africa; India
• Octochaetidae – Australasian, Indian, Oriental, Ethiopian, Neotropical
• Octochaetinae – Australasian, Indian, Oriental (subfamily if Benhamiinae is accepted)
• Sparganophilidae – Nearctic, Neotropical: North and Central America
• Tumakidae – Colombia, South America

As an invasive species

Out of about 7,000 earthworm species, only around 150 live all over the world. In the United States and Canada, about one-third of the earthworm species found are not native to those places.

Ecology

Earthworms are special creatures that live in soil and help keep it healthy. They come in three main types: those that live on top layers of soil and eat decaying leaves, those that burrow through the soil and mix it up, and those that dig deep holes to reach food.

Earthworms need the right conditions to thrive, like the proper temperature, moisture, and soil pH. Many animals, from birds to snakes to insects, see earthworms as food. Earthworms also help farmers and gardeners by mixing the soil, which lets air and water reach plant roots. Their waste, called casts, is rich in nutrients that plants love. Even though they are small, earthworms play a big role in keeping our planet healthy.

Threats

See also: Biodiversity loss § Earthworms

Some farming methods and chemicals can hurt earthworms. For example, nitrogen fertilizers can make the soil too acidic for them. In Australia, changes in farming, like using certain sprays on grass and switching from pastoral farming to other types of farming, greatly hurt the giant Gippsland earthworm. Because of this, it is now a protected species. Around the world, the use of some fertilizers and sprays has caused some earthworm species to disappear and many others to become endangered.

Economic impact

Earthworms are important in vermiculture, where they help break down organic waste. Common types used include Eisenia fetida and Eisenia andrei, also known as tiger worms or red wigglers. In warm regions, other worms like the African nightcrawler Eudrilus eugeniae and the Indian blue Perionyx excavatus are also used.

Earthworms are sold worldwide and are a big business. They are a good source of protein for fish, chickens, and pigs, and some people, like the Māori of New Zealand, eat them as a special food called Noke.