Have you didn’t notice?Dr. Thomas walks in,

Have you ever heard of an organism from the phylum Platyhelminthes? Do you know how they reproduce? In this lesson you will learn the answers to these questions as well as some characteristics of Platyhelminthes and their relatives.

Surprise Reproduction

You are so excited to be working with Dr. Thomas, one of the best biologists on campus.

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Right now she is conducting experiments with organisms that she refers to as Platyhelminthes, or flatworms.Dr. Thomas has some of them in small containers in the lab. She has charged you to watch one particular container, container C, that has one tan colored flatworm inside.

The worm is in the container with all it needs for the next couple of days while Dr. Thomas is away, so it should be fine.You leave the lab turn off the lights and lock up for the weekend. When you come back on Monday, you notice that container C now has multiple flatworms floating around inside. How is that possible? Were there two flatworms inside the whole time, and you didn’t notice?Dr. Thomas walks in, notices the panic on your face and laughs.

She says, ‘Oh good, we have some new offspring!’ She offers you a seat and begins to explain flatworm reproduction to you.

What Are Platyhelminthes?

Flatworms are very unusual animals. Even though most people don’t know much about this phylum, Platyhelminthes are very plentiful, with around 20,000 different types. One of the most famous is the tapeworm, which can parasitize many animals including dogs, cats, and humans.

The typical flatworm is a soft, flat invertebrate, meaning it has no backbone. It’s also bilaterally symmetrical. Bilateral symmetry means that if a vertical line is placed in the middle of the organism, what is on the left side is mirrored on the right side.


Most flatworms are hermaphrodites, organisms that have both male and female sex organs. As a result of this characteristic, they are able to reproduce asexually and sexually.

Platyhelminthes have three methods by which they can reproduce:

  1. They can fertilize their own eggs
  2. They can regenerate a complete organism from a piece of themselves
  3. They can reproduce sexually with another flatworm

This is why the flatworm in container C was able to reproduce even though there was only one organism in the container.Although the Platyhelminthes can reproduce in three ways, it is always evolutionarily preferable for organisms to reproduce sexually. Sexual reproduction provides the species with more variety in offspring due to a new combination of genes.

Variety allows a species to adapt more quickly to changes in their environment and increases the chances of survival of the species as a whole.Some flatworms carry their own eggs, while other parasitic Platyhelminthes deposit their eggs in a host. Eggs deposited in the host often end up in the host’s fecal matter or bore themselves into the host’s tissues.If a non-infected organism comes in contact with the feces or eats the infected tissue, the new organism can become infected. Once in the new host the eggs can complete their development and begin the cycle all over again. This is the case with tapeworms, which can pass to humans who eat raw or insufficiently cooked meat.

Wow! Platyhelminthes are quite interesting organisms. You are happy that Dr. Thomas took the time to explain how they reproduce to you. The next time offspring appear overnight, you’ll know how they got there!

Lesson Summary

Platyhelminthes are flatworms that include nearly 20,000 different species.

Flatworms are soft invertebrates that are bilaterally symmetrical. Some Platyhelminthes are free living and others reside in a host organism.

  • Flatworms are hermaphrodites. Hermaphrodites are organisms that have both male and female sexual organs.
  • Platyhelminthes are able to reproduce asexually and sexually, but sexual reproduction is preferable.
  • Parasitic Platyhelminthes deposit their eggs in their host or bore themselves into the host tissues.

    Flatworms use these methods to make their way to new host targets. Once in the new host, the eggs can complete their development and begin the cycle all over again.


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