Skip Navigation


Animals with bilateral symmetry, 3 germ layers, organ systems and distinct heads.

Atoms Practice
Estimated4 minsto complete
Practice Flatworms
This indicates how strong in your memory this concept is
Estimated4 minsto complete
Practice Now
Turn In
Phylum Platyhelminthes - Flatworms

Would you believe that this gold-dotted creature is a flatworm?

No? Well it is. There are more than 25,000 different types of flatworms, so they can be very different in how they appear. And many don't even look like your typical worm.


Flatworms belong to the phylum Platyhelminthes. Examples of flatworms are shown in Figure below. There are more than 25,000 species in the flatworm phylum. See Heads, Tails and Brains at http://www.pbs.org/kcet/shapeoflife/episodes/hunter.html for an introduction to flatworms.

Platyhelminthes: flatworms, tapeworms, flukes

Platyhelminthes. Platyhelminths include flatworms, tapeworms, and flukes.

Structure and Function of Flatworms

Flatworms range in length from about 1 millimeter (0.04 inches) to more than 20 meters (66 feet). They have a flat body because they do not have a coelom or even a pseudocoelom. They also lack a respiratory system. Instead, their cells exchange gases by diffusion directly with the environment. They have an incomplete digestive system.

Flatworms reflect several major evolutionary advances in invertebrates. They have three embryonic cell layers, including mesoderm. The mesoderm layer allows them to develop organ systems. For example, they have muscular and excretory systems. The muscular system allows them to move from place to place over solid surfaces. The excretory system lets them maintain a proper balance of water and salts. Flatworms also show cephalization and bilateral symmetry.

Flatworm Reproduction

Flatworms reproduce sexually. In most species, the same individuals produce both eggs and sperm. After fertilization occurs, the fertilized eggs pass out of the adult’s body and hatch into larvae. There may be several different larval stages. The final larval stage develops into the adult form, and the life cycle repeats.

Ecology of Flatworms

Both flukes and tapeworms are parasites with vertebrate hosts, including human hosts. Flukes live in the host’s circulatory system or liver. Tapeworms live in the host’s digestive system. Usually, more than one type of host is required to complete the parasite’s life cycle. Look at the life cycle of the liver fluke in Figure below. As an adult, the fluke has a vertebrate host. As a larva, it has an invertebrate host. If you follow the life cycle, you can see how each host becomes infected so the fluke can continue its life cycle.

Life cycle of the sheep liver fluke

Life Cycle of the Sheep Liver Fluke. The sheep liver fluke has a complicated life cycle with two hosts. How could such a complicated way of life evolve?

Tapeworms and flukes have suckers and other structures for feeding on a host. Tapeworms also have a scolex, a ring of hooks on their head to attach themselves to the host (see Figure below). Unlike other invertebrates, tapeworms lack a mouth and digestive system. Instead, they absorb nutrients directly from the host’s digestive system with their suckers.

Tapeworm suckers and hooks

Tapeworm Suckers and Hooks. The head of a tapeworm has several suckers. At the very top of the head is a “crown” of hooks called a scolex.

Not all flatworms are parasites. Some are free-living carnivores. They eat other small invertebrates and decaying animals. Most of the free-living species live in aquatic habitats, but some live in moist soil.

Immortal Worms

Gold-Dotted Flatworm, Thysanozoon sp.

That Ain't A Worm!

Flatworms are amazing creatures. What's a flatworm? Well, it's not a worm, but it is flat. It's flat because these creatures have figured out how to breath without lungs or gills! They skin breathe, which means they actually exchange gases (O2 in, CO2 out) by diffusion through their skin. Pretty slick, no need to develop fancy structures that are easily damaged like alveoli or gill rakers. The skin is good enough for a flatworm. Diffusion is also the basic process we use to exchange our gases, the thing is it only works for a short distance, and we're big (at least in gas diffusion terms). This is why we have so many alveoli in our lungs, it allows us to bring all our blood to within a few cell widths of our air spaces. Once this happens, oxygen diffuses in and carbon dioxide diffuses out. This is also why flatworms are flat, if they were thicker or rounder they'd have to develop another way to breath like we did, because some of their cells would be too far away from the air or water to breath through diffusion. But if you don't need to be big, skin breathing is a pretty smart way to go. Watch these videos to find out even more about these intriguing creatures and why scientists are studying them.

Explore More

Use the resources below to answer the following questions:

  1. How does Dr. Aboobaker hope to apply his work to human health?
  2. What are stem cells? What percentage of a flatworm's cells appear to be stem cells? How does this make them very resilient creatures?
  3. The smed-prep gene was identified as crucial for flatworms to regenerate their heads. What other organisms have been found to have this gene? What has this gene been shown to do in these organisms?
  4. What can flatworms do when food is scarce that other animals can't? What characteristic of flatworms let's them engage in this behavior?



  • Platyhelminths are flatworms such as tapeworms and flukes.
  • Flatworms have a mesoderm cell layer and simple organ systems. They also show cephalization and bilateral symmetry.
  • Many flatworms are parasites with vertebrate hosts. Some are free-living carnivores that live mainly in aquatic habitats.


Use this resource to answer the questions that follow.

  • http://www.hippocampus.org/Biology Non-Major Biology Search: Bilaterally Symmetric Invertebrates
  1. Describe the Platyhelminthes.
  2. Why are flatworms flat?
  3. What is a planarian?
  4. Describe issues associated with flukes and tapeworms.


1. Describe specialized feeding structures of parasitic platyhelminths.

2. Some parasitic flatworms have a very complicated life cycle with more than one host. Infer why this might be adaptive.

Notes/Highlights Having trouble? Report an issue.

Color Highlighted Text Notes
Show More

Image Attributions

Explore More

Sign in to explore more, including practice questions and solutions for Flatworms.
Please wait...
Please wait...