Flatworms
Jessica Harwood
Douglas Wilkin, Ph.D.
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AUTHORS
Jessica Harwood
Douglas Wilkin, Ph.D.
EDITOR
Douglas Wilkin, Ph.D.
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Printed: January 15, 2015
CONTRIBUTORS
Doris Kraus, Ph.D.
Niamh Gray-Wilson
Jean Brainard, Ph.D.
Sarah Johnson
Jane Willan
Corliss Karasov
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C HAPTER
Chapter 1. Flatworms
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Flatworms
• Describe the major features of flatworms.
• Describe parasitic flatworms.
Do worms have eyes?
You might think that worms can’t see. But some worms, such as the above Dugesia flatworm, do have eyespots.
These are not exactly like your eyes, however. Eyespots can only detect light in their environment.
Flatworms
The word "worm" is not very scientific. But it is a word that informally describes animals (usually invertebrates)
that have long bodies with no arms or legs. (Snakes are vertebrates, so they are not usually described as worms.)
Worms show bilateral symmetry, meaning that the right side of their bodies is a mirror of the left.
One type of worm is the flatworm. Worms in the phylum Platyhelminthes are called flatworms because they have
flattened bodies. There are more than 18,500 known species of flatworms.
Features of Flatworms
The main characteristics of flatworms ( Figure 1.1) include:
1. Flatworms have no true body cavity, but they do have bilateral symmetry.
2. Flatworms have an incomplete digestive system. This means that the digestive tract has only one opening.
Digestion takes place in the gastrovascular cavity.
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3. Flatworms do not have a respiratory system. Instead, they have pores that allow oxygen to enter through their
body. Oxygen enters the pores by diffusion.
4. There are no blood vessels in the flatworms. Their gastrovascular cavity helps distribute nutrients throughout
the body.
5. Flatworms have a ladder-like nervous system; two interconnected parallel nerve cords run the length of the
body.
6. Most flatworms have a distinct head region that includes nerve cells and sensory organs, such as eyespots.
The development of a head region, called cephalization, evolved at the same time as bilateral symmetry in
animals.
FIGURE 1.1
Marine flatworms can be brightly colored,
such as this one from the class Turbellaria. These worms are mostly carnivores
or scavengers.
Flatworms in the Environment
Flatworms live in a variety of environments. Some species of flatworms are free-living organisms that feed on small
organisms and rotting matter. These types of flatworms include marine flatworms and freshwater flatworms, such as
Dugesia.
Other types of flatworms are parasitic. That means they live inside another organism, called a host, in order to get
the food and energy they need. For example, tapeworms have a head-like area with tiny hooks and suckers (known
as the scolex) that help the worm attach to the intestines of an animal host ( Figure 1.2). There are over 11,000
species of parasitic flatworms.
Summary
• Flatworms have no true body cavity and no blood vessels.
• Flatworms can be free-living or parasitic. Tapeworms are parasitic flatworms.
Explore More
Use the resources below to answer the questions that follow.
• Flatworms: The First Hunter at http://vimeo.com/37282961 (9:54)
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Chapter 1. Flatworms
FIGURE 1.2
Tapeworms are parasitic flatworms that
live in the intestines of their hosts. They
can be very long (left). Tapeworms attach
to the intestinal wall with a head region
that has hooks and suckers (right).
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/57276
1. Why was cephalization important for flatworms? What did this allow them to do effectively?
2. How is the nervous system of flatworms different from those of most cnidarians? What does this make them
better at than cnidarians?
3. What do flatworm eyes sense?
4. How do stereo senses help animals survive?
5. What is internal fertilization as opposed to external fertilization? Considering that there is an energetic cost to
making gametes, what advantage can internal fertilization have over external fertilization?
Review
1.
2.
3.
4.
5.
What is a flatworm?
What is cephalization?
How do flatworms transport oxygen and nutrients?
Describe the flatworm nervous system.
What is one example of a flatworm?
References
1. Steve Childs. Marine flatworms can be brightly colored . CC BY 2.0
2. Courtesy of the Centers for Disease Control and Prevention. Tapeworms are parasitic flatworms that live in t
he intestines of their hosts . Public Domain
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