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9.6: Segmented Worms

Difficulty Level: At Grade Created by: CK-12
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Does an earthworm have a brain?

Just like you, earthworms do have a brain. Their brains are much simpler than yours, however. Earthworms’ brains allow them to sense and respond to light and touch.

Segmented Worms

When you think of worms, you probably picture earthworms. There are actually many types of worms, including flatworms, roundworms, and segmented worms. Earthworms are segmented worms.

Segmented worms are in the phylum Annelida. These worms are known as the segmented worms because their bodies are segmented, or separated into repeating units. Besides the earthworm, the segmented worms also include leeches and some marine worms. Most segmented worms feed on dead organic matter, like the earthworm. Leeches (Figure below), however, can live in freshwater and suck blood from their animal host.

Leeches are parasitic segmented worms

Leeches are parasitic worms. Notice the presence of segments.

Features of Segmented Worms

Segmented worms have a number of characteristic features.

  1. Segmented worms have a well-developed body cavity filled with fluid. This fluid-filled cavity 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. And the nervous system consists of the brain and a ventral nerve cord.

Comparison of Worms

The following table compares the three worm phyla (Table below).

Type of Worm Body Cavity Segmented Digestive System Example
Flatworm No No Incomplete Tapeworm
Roundworm Yes No Complete Heartworm
Segmented Yes Yes Complete Earthworm


  • hydroskeleton: Fluid-filled cavity used as support for contracting muscles.
  • leech: Segmented worm that feeds on the blood of animals.
  • segmented worm: Worm with a body plan of repeating segments.



Use the resources below to answer the questions that follow.

  1. What new realms did animals like Abarenicola open up for other animals?
  2. Where are the eyes of feather duster worms (Sabellidae) located?
  3. What effect do tube dwelling worms have on mudflat ecosystems?
  4. How much can a giant tube dwelling worm from a hydrothermal vent grow in a year? Given that some of these vent communities last less than a decade, how is this growth advantageous to the animal?
  5. How do earthworms help breakdown leaf litter? Be as specific as you can in your answer.
  1. What are some ways a doctor might use a leech?
  2. Do leeches eat often?


  1. What features distinguish Phylum Annelida from the other worms?
  2. Describe the skeletal system of the segmented worms.

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Fluid-filled cavity used as support for contracting muscles.


Segmented worm that feeds on the blood of animals.


Paired, un-jointed lateral outgrowths used for locomotion.

segmented worm

Worm with a body plan of repeating segments.

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Difficulty Level:
At Grade
7 , 8
Date Created:
Nov 29, 2012
Last Modified:
Dec 01, 2016
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