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9.4: Flatworms

Difficulty Level: At Grade Created by: CK-12
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Do worms have eyes?

You might think that worms can't see. But some worms, such as the above Dugesia, do have eyespots. These are not exactly like your eyes, however. Eyespots can only detect light in their environment.


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. 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.

Features of Flatworms

The main characteristics of flatworms (Figure below) 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.
  3. Flatworms do not have a respiratory system. Instead, they have pores that allow oxygen to enter through their body.
  4. There are no blood vessels in the flatworms. Their gastrovascular cavity also helps distribute nutrients throughout the body.
  5. Flatworms have a ladder-like nervous system.
  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.

Marine flatworms can be brightly colored

Marine flatworms can be brightly colored.

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 fresh-water 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 that help the worm attach to the intestines of an animal host (Figure below)

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).


  • cephalization: Concentration of the sense organs and nervous tissue in a head region.
  • flatworm: Invertebrate animals with bilateral symmetry and an elongated, flattened body.
  • tapeworm: Parasitic flatworm, which lives in the intestines of vertebrates.


  • Flatworms have no true body cavity and no blood vessels.
  • Flatworms can be free-living or parasitic. Tapeworms are parasitic flatworms.


Use the resources below to answer the questions that follow.

  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? How is this useful to the animal?
  4. How do stereo senses help animals survive? Be as complete as you can be in your answer.
  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?


  1. How do flatworms transport oxygen and nutrients?
  2. What are two examples of flatworms?

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Concentration of the sense organs and nervous tissue in a head region.


Invertebrate animals with bilateral symmetry and an elongated, flattened body.

gastrovascular cavity

Internal space that is the site of digestion and distribution of nutrients.


The front end of a tapeworm, bearing suckers and hooks for attachment.


Parasitic flatworm, which lives in the intestines of vertebrates.

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Difficulty Level:
At Grade

Concept Nodes:

7 , 8
Date Created:
Nov 29, 2012
Last Modified:
Aug 30, 2016
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