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8:4 Worms
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Printed: January 6, 2014
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C ONCEPT
Concept 1. 8:4 Worms
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8:4 Worms
Lesson Objectives
• Describe the major features of the flatworms.
• Describe the major features of the roundworms.
• Describe the major features of the segmented worms.
Check Your Understanding
• In terms of body structure, what does segmentation refer to?
• What is a body cavity?
Vocabulary
• cephalization
• hydroskeleton
• tapeworms
What are Worms?
The word "worm" is not very scientific. But it is a word that informally describes animals 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.
Worms live in many different types of environments, including in the ocean, in fresh water, on land, and as parasites
of plants and animals.
Three types of worms with different body types will be explored in this lesson:
1. Flatworms, which have ribbon-like bodies with no body cavity.
2. Roundworms, which have a body cavity but no segments.
3. Segmented worms, which have both a body cavity and segmented bodies.
Flatworms
Worms in the phylum Platyhelminthes are called flatworms because they have flattened bodies. 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 (Figure 1.1 and Figure 1.2).
Other types of flatworms are parasitic and 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 1.3 and Figure 1.4).
Characteristics of Flatworms
The main characteristics of flatworms can be summarized as follows:
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FIGURE 1.1
Dugesia is a type of flatworm with a head
region and eyespots.
FIGURE 1.2
Marine flatworms can be brightly colored.
1. Flatworms have no true body cavity and an incomplete digestive system, meaning that the digestive tract has
only one opening.
2. Flatworms do not have a respiratory system, so they have pores that allow oxygen to enter through their body.
3. There are no blood vessels in the flatworms. Their gastrovascular cavity helps them to digest food and to send
nutrients throughout the body.
4. The flatworms have a ladder-like nervous system with 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.
Roundworms
The phylum Nematoda includes non-segmented worms known as nematodes or roundworms (Figure 1.5).
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Concept 1. 8:4 Worms
FIGURE 1.3
Tapeworms are parasitic flatworms that
live in the intestines of their hosts. They
can be very long.
FIGURE 1.4
Tapeworms attach to the intestinal wall
with a head region that has hooks and
suckers.
Characteristics of Flatworms
There are specific differences between the flatworms and the roundworms.
1. Unlike the flatworms, the roundworms have a body cavity with internal organs.
2. A roundworm has a complete digestive system, which includes both a mouth and an anus. They also include
a large digestive organ known as the gut.
3. Roundworms also have a simple nervous system with a primitive brain. The nerves are connected from the
top to the bottom of the body.
Roundworms can be free-living organisms, but they are probably best known for their role as significant plant and
animal parasites. Heartworms, which cause serious disease in dogs while living in the heart and blood vessels, are
a type of roundworm. Roundworms can also cause disease in humans. Elephantiasis, a disease characterized by the
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FIGURE 1.5
Nematodes can be parasites of plants
and animals.
extreme swelling of the limbs, is caused by infection with a type of roundworm (Figure 1.6).
FIGURE 1.6
One roundworm parasite causes elephantiasis, a disease characterized by
swelling of the limbs.
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Concept 1. 8:4 Worms
Segmented Worms
The phylum Annelida includes segmented worms, such as the common earthworm, some marine worms, and leeches
(Figure 1.7 and Figure 1.8). These worms are known as the segmented worms because their bodies are segmented,
or separated into repeating units. Most segmented worms feed on dead organic matter, while leeches can live in
freshwater and suck blood from host organisms.
Characteristics of Segmented Worms
1. Segmented worms have a well-developed body cavity filled with fluid, which serves as a hydroskeleton, a
supportive structure that helps move the worm’s muscles.
2. Segmented worms also tend to have organ systems that are more developed than the roundworms or flatworms.
Earthworms, for example, have a complete digestive tract, including an esophagus and intestines. The circulatory system consists of paired hearts and blood vessels, while the nervous system consists of the brain and a
ventral nerve cord.
FIGURE 1.7
Earthworms are segmented worms.
Table 1.1 compares the three worm phyla.
TABLE 1.1: Comparison of the three phyla containing worms.
Type of Worm
Flatworm
Roundworm
Segmented
Body Cavity
No
Yes
Yes
Segmented
No
No
Yes
Digestive System
Incomplete
Complete
Complete
Example
Tapeworm
Heartworm
Earthworm
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FIGURE 1.8
Leeches are parasitic segmented worms.
Lesson Summary
• The flatworms have no true body cavity and include free-living Dugesia and parasitic tapeworms.
• The roundworms, which can also be parasitic or free-living, are non-segmented worms with a complete
digestive tract and a primitive brain.
• The segmented worms include the common earthworm and leeches.
Review Questions
Recall
1. What is cephalization?
2. Name a parasitic flatworm.
3. Name a parasitic segmented worm.
4. Earthworms are in what phylum?
Apply Concepts
5. Are all worms classified into a single phylum?
6. Describe the respiratory system of the flatworms.
7. How does the body plan of the roundworms differ from that of the flatworms?
8. Describe the digestive system of roundworms.
9. What features distinguish Phylum Annelida from the other worms?
10. Describe the skeletal system of a segmented worm.
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Concept 1. 8:4 Worms
Critical Thinking
11. Which phylum includes worms with organs that are most similar to the organs found in humans? Support your
answer with three pieces of evidence.
Further Reading / Supplemental Links
•
•
•
•
•
•
•
•
http://animaldiversity.ummz.umich.edu/site/accounts/information/Annelida.html
http://animaldiversity.ummz.umich.edu/site/accounts/information/Nematoda.html
http://animaldiversity.ummz.umich.edu/site/accounts/information/Platyhelminthes.html
http://www.ucmp.berkeley.edu/platyhelminthes/platyhelminthes.html
http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html
http://www.ucmp.berkeley.edu/annelida/annelida.html
http://animaldiversity.ummz.umich.edu
http://en.wikipedia.org/wiki/Annelida
Points to Consider
Next we further our discussing of the invertebrates.
• Can you think of some invertebrates other than those discussed in this chapter?
• How would these other invertebrates be more advanced compared to worms?
References
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Mike6271. http://commons.wikimedia.org/wiki/File:Planariafull.jpg. Public Domain
Daryl H. Marine flatworms can be brightly colored.. Used under license from shutterstock.com
CDC. http://commons.wikimedia.org/wiki/Image:Taenia_saginata_adult_5260_lores.jpg. Public Domain
Inconnu. http://commons.wikimedia.org/wiki/File:Taenia_solium_Scolex.jpeg. Public Domain
ARS. Nematodes can be parasites of plants and animals.. Public Domain
prep4md. http://www.flickr.com/photos/27920559@N05/2778292954/. CC-BY 2.0
Squeezyboy. Earthworms are segmented worms.. CC-BY 2.0
Mircea BEZERGHEANU. Leeches are parasitic segmented worms.. Used under license from shutterstock.com
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