The earthworm: anatomy, species, role, and way of life

You come across it after a rain shower, when turning over a shovelful of soil, or at the bottom of a worm composter: the earthworm is one of those animals everyone knows… without really knowing it. No eyes, no lungs, several “hearts,” a body made of rings: this little soil dweller holds plenty of surprises.

Here are the essentials about the earthworm, explained simply — for the curious, gardeners, kids, and teachers alike. For each topic, we point you to the guide page that covers it in detail.

What is an earthworm?

The earthworm is a small invertebrate animal, meaning it has no skeleton, that lives in the soil or in decomposing organic matter. It belongs to the large group of annelids, the “ringed worms,” recognizable by their bodies made up of a series of segments.

In everyday language, people often say earthworm or nightcrawler interchangeably. Strictly speaking, “nightcrawler” mainly refers to the large worm found in our gardens, Lumbricus terrestris, but the word is often used loosely for other earthworms. The earthworm is also sometimes called a “rainworm,” because it comes up to the surface when the soil is waterlogged.

There is a huge variety of them: around 4,400 species of earthworms have been recorded worldwide to date, ranging from just a few inches (a few centimeters) to more than 3.3 ft (1 m) long for some giant species.

Anatomy: how is an earthworm built?

Behind its very simple appearance, the earthworm is a surprisingly well-organized animal. On the diagram: 1 the head and mouth, 2 the clitellum, 3 the rings, 4 the bristles (enlarged), 5 the tail.

• A ringed body. Its body is divided into dozens, or even more than a hundred, rings depending on the species. Each ring carries tiny bristles, invisible to the naked eye, which it uses to grip the soil as it moves forward.
• A head and a tail. The head, which is more pointed, is at the end closest to the clitellum; the mouth opens there, with no teeth or jaw.
• The clitellum. In adults, a lighter-colored swelling appears toward the front of the body: this is the clitellum, the organ that will form the cocoon during reproduction. Its presence shows that the worm is old enough to breed.
• No eyes, but sensitive to light. The earthworm has no eyes. Its skin is covered with light-sensitive cells, especially at the front of the body, which allow it to avoid light instinctively.
• Breathing through the skin. It has no lungs: oxygen passes directly through its skin, which must stay moist for this to work. That is why drought is so dangerous for it.
• Several “hearts.” It has several blood vessels that act as small hearts to pump its blood through its body.

To move forward, it contracts and stretches the different parts of its body in turn, a bit like an accordion, using its bristles to keep from slipping. More fun facts await you in the frequently asked questions at the bottom of the page.

Did you know?

When summer gets too dry, some earthworms go dormant: they curl up into a ball in a small chamber at the bottom of their burrow and wait for the rain to return.

The three main groups of earthworms

Not all earthworms live in the same place or in the same way. They are usually divided into three main groups, according to the depth at which they live (numbers on the illustration):

• 1. Epigeic worms live at the surface, in fallen leaves, manure, or compost. Small, red, and very hungry, they breed quickly. These are composting worms, such as Eisenia (including Eisenia fetida and Dendrobaena veneta), which are used in a worm composter.
• 2. Endogeic worms live in the top few inches of soil, where they dig mostly horizontal burrows and feed on soil rich in organic matter. Often pale or pinkish, they are rarely seen at the surface.
• 3. Anecic worms dig deep vertical burrows, sometimes several yards (several meters) deep. They come up at night to collect fallen leaves, which they pull down into the soil. This is the case with the large earthworm, Lumbricus terrestris, the one you find when digging in the garden.

Each group benefits the soil in different ways: we come back to this a little further down.

What does an earthworm eat?

The earthworm feeds on decomposing organic matter: fallen leaves, plant debris, manure, as well as the bacteria and fungi that grow on them. Having no teeth, it swallows small particles that have already softened, which its gizzard then grinds up with the help of tiny grains of soil. A composting worm can eat the equivalent of half its own weight every day. To find out what to feed it in a worm bin, and what to avoid, see our page What do earthworms eat?

How does it reproduce?

The earthworm is a hermaphrodite: each individual is both male and female. Yet it still needs a partner: two worms line up side by side, head to tail, and exchange sperm. A few days later, the clitellum forms a small cocoon, about the size of a grain of rice, from which young worms will hatch after about three weeks. The whole cycle is explained in our page How do earthworms reproduce?

How long does an earthworm live?

It all depends on its environment. In the wild, most worms live and die within a single year, faced with cold, drought, or predators (birds, moles, hedgehogs…). Sheltered from these dangers, in a well-maintained worm composter, their life expectancy increases considerably: Eisenia fetida composting worms have been monitored in captivity for up to four and a half years. We come back to these figures in our FAQ.

Its role in the soil and in the garden

The earthworm is one of the gardener’s best allies. By digging its burrows, it aerates the soil and helps water seep down to the roots. By digesting organic matter, it turns it into nutrients that plants can absorb. Its droppings, the little coils of soil you see on the surface, are an excellent natural fertilizer. Soil rich in earthworms is almost always living, fertile soil.

This quiet work is described in detail in our page on the ecological role of earthworms. And if your soil is short on them, find out how to reintroduce worms into your garden.

Key takeaways

Weed killers, pesticides, and tilling too deeply greatly reduce earthworm populations. A garden fed with compost and covered with mulch, on the other hand, gives them food and shelter.

Earthworm or composting worm: which one for a worm composter?

It is a common mix-up: the large earthworm you find when digging in the garden is not suitable for a worm composter. Used to burrowing deep and feeding on soil, it cannot survive in a bin filled with fresh food scraps.

To compost your food scraps, you need epigeic worms, the composting worms of the genus Eisenia: they naturally live in decomposing matter, process it quickly, and breed fast in a confined space. We introduce them in our page Which type of worm for your worm composter? To get started with the right worms, you can also buy live composting worms: our Eisenia composting worms are raised on the farm and shipped as a mix of three species.

Willy's tip

Don’t take worms from your garden to start a worm bin: they are not the right ones, and they are far more useful where they are, working your soil.

Common misconceptions about earthworms

• “The earthworm is an insect.” False. It has no legs, no shell, and no skeleton: it is an annelid, a group of animals that has nothing to do with insects.
• “Earthworms damage roots.” False. They feed on dead organic matter and the microorganisms that break it down, not on living roots. On the contrary, their presence benefits plants.
• “Earthworms are dirty or dangerous.” No: they don’t sting, don’t bite, and don’t transmit disease. You can handle them without worry, simply washing your hands afterward, just as you would after gardening.

Can you cut a worm in two to get two worms? Why does it come out of the ground when it rains? Can it swim? The answers are in our earthworm FAQ.

Frequently asked questions about earthworms

In summary

The earthworm is an annelid with no eyes or lungs that breathes through its skin and has several small hearts. There are around 4,400 species, divided into three main groups according to the depth at which they live. Everywhere, it transforms organic matter and maintains soil fertility. For a worm composter, it is not garden earthworms you need, but epigeic composting worms such as Eisenia.

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