9.4 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 are the first significant group of animals with 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 below) include:

1. Flatworms have no true body cavity, but they do have bilateral symmetry. Due to the lack of a body cavity,flatworms are known as acoelomates.
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. 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.  This process does not occur in cnidarians, which evolved prior to flatworms and have radial symmetry.

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 below). There are over 11,000 species of parasitic flatworms.

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). The majority of flatworms are parasites.

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)

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. What is a flatworm?
2. What is cephalization?
3. How do flatworms transport oxygen and nutrients?
4. Describe the flatworm nervous system.
5. What is one example of a flatworm?