Roundworms
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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Printed: August 3, 2012
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
www.ck12.org
C ONCEPT
Concept 1. Roundworms
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Roundworms
• Summarize traits of nematode invertebrates.
When most people picture a worm, do they picture a roundworm?
Actually, they do not. Whereas flatworms are flat, roundworms obviously appear round. With over 80,000 species,
there are plenty of different types of roundworms. But these are still not the types of worms most people picture
when they think of worms.
Roundworms
Roundworms make up the phylum Nematoda. This is a very diverse animal phyla. It has more than 80,000 known
species.
Structure and Function of Roundworms
Roundworms range in length from less than 1 millimeter to over 7 meters (23 feet) in length. As their name suggests,
they have a round body. This is because they have a pseudocoelom. This is one way they differ from flatworms.
Another way is their complete digestive system. It allows them to take in food, digest food, and eliminate wastes all
at the same time.
Roundworms have a tough covering of cuticle on the surface of their body. It prevents their body from expanding.
This allows the buildup of fluid pressure in the pseudocoelom. As a result, roundworms have a hydrostatic skeleton.
This provides a counterforce for the contraction of muscles lining the pseudocoelom. This allows the worms to move
efficiently along solid surfaces.
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Roundworm Reproduction
Roundworms reproduce sexually. Sperm and eggs are produced by separate male and female adults. Fertilization
takes place inside the female organism. Females lay huge numbers of eggs, sometimes as many as 100,000 per day!
The eggs hatch into larvae, which develop into adults. Then the cycle repeats.
Ecology of Roundworms
Roundworms may be free-living or parasitic. Free-living worms are found mainly in freshwater habitats. Some live
in soil. They generally feed on bacteria, fungi, protozoans, or decaying organic matter. By breaking down organic
matter, they play an important role in the carbon cycle.
Parasitic roundworms may have plant, vertebrate, or invertebrate hosts. Several species have human hosts. For
example, hookworms, like the one in Figure 1.1, are human parasites. They infect the human intestine. They are
named for the hooks they use to grab onto the host’s tissues. Hookworm larvae enter the host through the skin. They
migrate to the intestine, where they mature into adults. Adults lay eggs, which pass out of the host in feces. Then
the cycle repeats.
FIGURE 1.1
Hookworm Parasite.
Hookworms like
this one are common human parasites.
Tiny pinworms are the most common roundworm parasites of people in the U.S. In some areas, as many as one out
of three children are infected. Humans become infected when they ingest the nearly microscopic pinworm eggs.
The eggs hatch and develop into adults in the host’s digestive tract. Adults lay eggs that pass out of the host’s body
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Concept 1. Roundworms
to continue the cycle. Pinworms have a fairly simple life cycle with only one host.
Vocabulary
• Nematoda: Phylum of invertebrates called roundworms, which have a pseudocoelom and complete digestive
system.
• pseudocoelom: Partial, fluid-filled cavity inside the body of some invertebrates.
• roundworms: Nematodes (phylum Nematoda); the most diverse phylum of pseudocoelomates, and one of the
most diverse of all animals.
Summary
•
•
•
•
Roundworms make up the phylum Nematoda.
Roundworms have a pseudocoelom and hydrostatic skeleton. Their body is covered with tough cuticle.
Free-living roundworms are found mainly in freshwater habitats.
Parasitic roundworms have a variety of hosts, including humans.
Practice
Use this resource to answer the questions that follow.
• http://www.hippocampus.org/Biology → Non-Majors Biology → Search: Bilaterally Symmetric Invertebrates
1.
2.
3.
4.
5.
What is a nematode?
Do nematodes have a mouth and an anus?
Do nematodes have muscles?
What is the cuticle of a nematode?
Describe elephantiasis.
Review
1. How do free-living nematodes contribute to the carbon cycle?
2. Apply what you know about pinworms to develop one or more recommendations for preventing pinworm
infections in humans.
3. Platyhelminths and nematodes are both worms. Justify classifying them in different invertebrate phyla.
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