Site hosted by Build your free website today!
Phyla Platyhelminthes and Nematoda: Flatworms and Roundworms
As was seen in Porifera and Cnidaria, the flatworms and roundworms are also some of our planet's earliest invertebrates. Although a little more neurologically complex, these organisms still work with very rudimentary devices to obtain food and gather information about their environment. As you shall also learn, many are parasitic with fairly complex life cycles.


Phylum Platyhelminthes

A) Class Turbellaria (Planarians)

The most common of all members in this class is the simple planarian. View below…

The most interesting evolutionary feature of this organism, as with all members of these phyla, is the presence of cephalization. Read about the nervous system here…

Planarians are capable of very simple conditioning tasks, such as avoidance of electrical shocks (due to the memory of earlier ones). These "memories" are biochemical in nature. View the following site and scroll down to the section titled "Biochemical Memory in Flatworms"

1. What can be said about the flatworm which ate the other? What did it gain? Can this happen in human beings? Why or why not?

2. Bonus question: A baby is fitted with a leather strap around its forehead. There is a tiny hammer (very tiny) which has the capability to swing down and tap the baby between the eyes. Note: There is a reflexive response in humans to blink when this happens. A researcher fits this on a baby's head, rings a bell, then the baby is tapped between the eyes. This is done for 5-10 minutes. The device is taken off. The bell is then rung and the baby blinks.

Another baby is taken in. The same procedure is followed as with the first baby. However, the baby does not blink. What happened?


Also of importance are the digestive and excretory systems. Again, these systems are crude by human standards but reflect important biological advances…


5. What does cephalization refer to? Why is it important for the survival of the organism that the concentration of nervous material be concentrated in the anterior portion of the body?

6. It was mentioned before that the presence of a digestive and excretory system was an important biological advancement. Why is that so? What systematic benefit does it provide to the organism?

Reproduction, as in many of the organisms we have studied, is varied in planarians. Go back to this site and summarize the ways planaria reproduce.


B) Class Trematoda (Flukes)

This class is fairly similar to Turbellaria, however their life cycles differ.

Following an explanation of the life cycle of a common liver fluke…


6. What does this parasite usually infect?

7. Assume that you have a herd of cattle which are infect by the liver fluke. How could you avoid contamination in the future?

8. How could you break the life cycle of the liver fluke? (This is a different question than above, read carefully!)


Schistosomiasis is a serious blood born disease in Third World counties caused by a species of fluke.


9. Is the worm itself the primary agent of the disease? Why or why not?

10. What are some ways the eggs of the worm can infect a human?


C) Class Cestoda

An overview of the Cestodes…commonly known as tapeworms….

Some pictures…


10. Since these parasites do not have a developed digestive system, where can you surmise these organisms live (in a host)?

11. Abdominal pain is one symptom of the presence of a tapeworm. What could another symptom possibly be?

And now a very odd, poetic view of this parasite…

Phylum Nematoda

Common refered to as the roundworms these differ in morphology many still lead a parasitic lifestyle. The most common roundworm, Ascaris, is found below..

Read the overview of nematodes…


A general link to these roundworms….


12. Name one digestive morphological difference between Ascaris and the tapeworm.

13. Name one parasitic similarity between Ascaris and a tapeworm.

Members of these phyla probably get a bad reputation (maybe deservedly so). However, with the knowledge of their biology and function, they can be used to kill crop eating insects.



14. How are these nematodes introduced into the insects?

15. How are the insects killed?

16. What are the possible adverse ramifications of the use of nematodes as a pesticide?