Objectives Explain the importance of a segmented worm's coelom. List worms that live as parasites in humans.
Segmented Worms Earthworms are common examples of a third worm group, the segmented worms. Segmented worms have bodies divided into small units or segments.
A segmented body is like a beaded necklace. Just as you could thread together a large red bead, five blue beads, and eight yellow beads, a segmented worm's body is built of similar and different segments.
The segmented worms are unlike flatworms and roundworms because they have a fluid-filled space, or cavity, between the gut and body wall. This cavity is called a coelom (SEE luhm).
A coelom provides the space needed for complex organs. Since it is fluidfilled, it also acts as a type of "water skeleton," supporting the worm's body so the worm can move easily.
The common earthworm is a good example of how organ systems are arranged in a segmented worm's body.
The crop and gizzard help the worm digest the soil it eats. They grind up the soil and remove organic matter from it. Notice also that the earthworm has five hearts and a brain. The brain is connected to a nerve cord that runs all the way down the worm's body.
When the earthworm moves, stiff bristles on the outside of the body act as anchoring points. They hold a part of the worm in place while another part pushes forward through the soil.
In these ways and others, an earthworm is well adapted to life in the soil. Its lifestyle is made possible by its segmented organization and its organ systems.
Earthworms and Soil Earthworms help enrich and improve the soil. They eat organic matter in the soil that plants can't use directly. They digest this matter and take nutrients from it.
What is left over are simpler substances, such as nitrates, that plants can use. Earthworms eliminate this waste from their bodies, adding it to the soil. Pellets of earthworm waste, called castings, are so full of plant nutrients that people use them as garden fertilizer.
Earthworms also improve the soil by burrowing in it. Their tunnels allow air and water to reach deep into the soil. They move organic matter from the surface down into deeper soil levels.
Many hundreds of earthworms can live under the surface of a square meter of ground. Each earthworm eats and discards its own weight in soil every day. Because it is full of living things like earthworms, soil is constantly changing.
Several kinds of worms live as parasites in human beings. They include tapeworms, a kind of flatworm, and roundworms, such as hookworm and Trichinella. Some of these worms spend part of their lives in the bodies of other animals. Trichinella worms live in pigs, for example, and can be present in the pork eaten by people.
Parasitic worms take nutrients from a person's body. As a result, people with worms do not have enough nutrients for their own needs.
Such people often feel weak and tired. They have lowered resistance to infections from microorganisms. They usually have digestive problems. Trichinella worms live in muscle tissue and cause muscle pains.
There are things you can do to prevent parasitic worms from entering your body. For example, if you wear shoes outside, hookworms can't bore into your feet. If you eat only well-cooked meat, you probably won't get tapeworms or trichinosis.
Few people in the United States today suffer from parasitic worms. These worms still infect people in other parts of the world, however.
They are especially problematic in tropical countries where, for example, most people can't afford sanitary ways to dispose of human waste. Medicine can get rid of the worms, but as long as sanitary conditions remain the same, people quickly become reinfected.
Check and Explain 1. How do flatworms differ from cnidarians? 2. Name the three main groups of worms. For each, describe how it differs from the others. 3. Predict Suppose you removed all the earthworms from an acre of soil. What would happen to the soil? Why? 4. Estimate You dig up a small area of moist soil 10 em wide, 10 cm long, and 10 cm deep and find five earthworms. Assuming that this number is average, how many earthworms would there be in a square plot of land 100 m on a side and 1 m deep?