<img src="https://d5nxst8fruw4z.cloudfront.net/atrk.gif?account=iA1Pi1a8Dy00ym" style="display:none" height="1" width="1" alt="" />
Skip Navigation

11.7: Roundworms

Difficulty Level: At Grade Created by: CK-12
Atoms Practice
Estimated3 minsto complete
Practice Roundworms
Estimated3 minsto complete
Practice Now

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 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.

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 below, 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.

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 to continue the cycle. Pinworms have a fairly simple life cycle with only one host.


  • 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.


Use this resource to answer the questions that follow.

  • http://www.hippocampus.org/Biology \begin{align*}\rightarrow\end{align*} Non-Majors Biology \begin{align*}\rightarrow\end{align*} Search: Bilaterally Symmetric Invertebrates
  1. What is a nematode?
  2. Do nematodes have a mouth and an anus?
  3. Do nematodes have muscles?
  4. What is the cuticle of a nematode?
  5. Describe elephantiasis.


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.




Phylum of invertebrates called roundworms, which have a pseudocoelom and complete digestive system.


Partial, fluid-filled cavity inside the body of some invertebrates.


Nematodes (phylum Nematoda); the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals.

Image Attributions

Show Hide Details
Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
May 12, 2016
Files can only be attached to the latest version of Modality
100 % of people thought this content was helpful.
Loading reviews...
Please wait...
Please wait...
Image Detail
Sizes: Medium | Original

Original text