Earthworms: An Introduction
In Amy Stewart’s engrossing book on worms The Earth Moved: On the Remarkable Achievements of Earthworms, she states:
“Worms are ruminators; they sift through whatever surrounds them, turn it over, explore it, move through it. They are deliberate creatures, in no great hurry, but always in motion, twisting and burrowing, shrinking and contracting, and eating. They spend their lives in a kind of active meditation, working through the detritus in which they live, the bits of leaves and grass and particles of soil. For a being with such a simple brain, a worm seems, in this way, almost thoughtful.” (148)
How incredible – that earthworms are constantly working through and improving the earth beneath our feet, yet most of us know very little about worms. How many kinds are there? What do they all eat? How do they reproduce? What are they doing when they throw themselves in harm’s way on the sidewalk during a rain? Read on to learn…
Types of Worms
There are about 7,000 species of earthworms on our planet and they are present in soil everywhere except for the harshest desert and arctic regions. Many of these species originated in the Middle East, Europe, and Asia and have since been introduced to the Americas and islands around the world.
Earthworms can be separated into three major groups based on their burrowing and feeding habits: Endogeic, Anecic, and Epigeic worms. All play important roles in the soil food web.
Endogeic worms live in the upper layers of soil. Their complex horizontal burrow systems are not permanent but they are very important for aerating soil and allowing moisture and nutrients to move through the soil. Endogeic worms are probably the least recognizable to most people since they very rarely come to the surface. Many endogeic worms live in the rhizosphere, the area immediately around plant roots, and they help with the exchange of nutrients there. These worms are medium-sized and pale in color.
Anecic worms build permanent, deep vertical burrows (up to 6 feet deep!) in the soil, and rise to the surface at night to search for food. They often pull leaves or other decaying matter into their burrows to feed but they also eat soil. Anecic worms often leave little mounds of castings alongside or atop the opening of their burrows and are important for aerating the soil and helping with water retention. These large worms have long lives – sometimes up to six years – and they mature and reproduce slowly. They require stable burrows and low population density to survive. A typical anecic worm is the Lumbricus terrestris, or common nightcrawler. Anecic worms are great for fishing bait.
Epigeic worms are surface dwellers and feed only on decaying organic material, not soil. They are not powerful burrowers, instead preferring to live in loose organic litter (like fallen leaves) or very loose topsoil rich in organic matter. They will not survive in most garden soils unless there is a good layer of organic matter on top. They are small and usually darkly colored, and reproduce at a high rate in ideal conditions. Because epigeic worms live on the very surface of soil, they are better able to withstand temperature and moisture fluctuations than other worms. Epigeic worms create castings that are many times higher in nutrients than the material they originally consumed, and are a vital part of the soil food web. The epigeic worm Eisenia fetida, or red wiggler, is great for vermicomposting (composting with worms).
A wonderful resource for worm biology is Jerry Minnich’s The Earthworm Book: How to Raise and Use Earthworms for Your Farm and Garden, pages 5-16.
Structure – Earthworms are cold-blooded and they remain cool because they are always moist (think of it like humans sweating to stay cool). They breathe through their skins through osmosis – oxygen is taken in through the body wall, introduced into the blood, and circulated throughout the body. Staying moist is vital to this process, but this is also why they leave the ground en masse when it is saturated during a rain – otherwise they’d drown! There is a film of mucus on the worm’s skin that helps it to hold moisture, lubricates its pathways through soil, is left behind in the soil to cement the walls of burrows, and is vital to the reproductive process.
Because worms live below ground, or at least underneath a layer of litter, they have no need for eyes. They have no ears or teeth or limbs either. Earthworms are streamlined to move beneath our feet, and have been for millions of years. They senses vibration, light, and movement and can react accordingly.
Worms have segments called somites, with larger worms having more segments than smaller ones. Each segment is heavily muscled, and the segmented structure of a worm allows it to push through dense soil, lift sticks and rocks, and move with agility. Along each segment, there are tiny bristles called seta, and these help the worm to move, to stay put in the ground when a robin or person is trying to capture it, and to hold onto another worm while mating.
Worms have five “hearts” that are vessels between the intestine and the body wall. These vessels direct blood through two major blood vessels that run on the top and the bottom of the worm’s body. There are minor vessels and capillaries that carry blood to all parts of the worm’s body, much as ours do.
Digestion – Because worms have no teeth and very few digestive fluids, they depend on bacteria, fungi, molds, and other organisms to predigest their food. Worms can survive on any organic matter, including cellulose, wood fibers and starches, but they need microorganisms, bacteria, fungi, nematodes and protozoa to break down the fibers. Worms are separated into two main parts – the anterior (the front of the worm where the “head” is), and the posterior (the rear portion of a worm). The mouth of a worm is in the first section of the anterior, and is overhung by a “lip” called the prostomium. This helps to guide food into the mouth, or push aside objects that are too big to eat. After a worm brings a piece of food or soil into its mouth, it enters the buccal cavity, which is basically the inside of its mouth. Next, food enters the pharynx, which is kind of a suction pump, drawing food from the mouth into the esophagus. From the esophagus, the food moves through the crop to the gizzard. Worms have gizzards like birds, which powerfully contract to grind food for further digestion. After the gizzard comes the intestine, and this is where most of the digestion and absorption takes place. Micro-organisms in the worm’s gut help it to break down food into water-soluble nutrients, which, when they are excreted, make for excellent plant food. The end product of the digestive process is a worm cast.
Nervous System – In each segment of the worm’s body, a nerve cord enlarges into nerve cells and forms a ganglion. Pairs of these ganglia are basically the worm’s “brain”, from which impulses are given off to other parts of the body. The central nervous system then transmits impulses on nerves that coordinate muscle action, reaction to light, tough or moisture sensation. Worms have cells scattered on their skin, particularly toward the anterior, which detect light. These cells are particularly sensitive to blue light but have trouble detecting red light. These cells help worms stay out of UV light rays, which can kill them very quickly. Worms have a very sensitive sense of touch, which helps them to find other worms to mate with, avoid danger, and find food.
Earthworms are hermaphrodites, which means that each worm possesses both male and female sexual parts. However, they are not self-fertilizing—two worms are needed in order to reproduce, and they can’t mate cross-species. The sexual organs are located in the anterior part of the worm. Two worms of equal length lie with the undersides of their bodies together, with their heads pointing in opposite directions and their clitellums touching. This way, the male organs on one worm line up with the female organs on the other, and vice versa. A large amount of mucus is secreted around the clitellum of each worm and this makes for the safe transfer of sperm between them. It also helps the worms cling to one another, along with the setae. Worms separate from each other after a few hours, but fertilization of the eggs does not happen immediately. The eggs and sperm are held together in mucus formed on the clitellum, and the mucus begins to harden. The worm wriggles backwards, with the mucus in a ring around it, until the mucus containing the eggs and sperm finally slides off the front of the worm, and in doing so, seals itself. This is the worm cocoon. Sizes vary based on the species of worm, but it will not be much wider than the worm that produced it. It is lemon shaped and brownish or dirty yellow in color. When the cocoon is separate from the worm, fertilization finally takes place. That is, if conditions are ideal. If it is too hot or too cold or too wet or too dry, the sperm and eggs can stay separate inside the cocoon for months before fertilization. A few days, weeks, or months after fertilization (depending on the species of worm), the cocoon hatches one to six baby worms. Worms do not have a larval stage; when the baby worms hatch, they are already fully formed worms, albeit small. Parent worms do not nurse their young. The babies are on their own as soon as they are born.
Find Amy Stewart’s “The Earth Moved” here on Amazon.com