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What is Platyhelminthes? In this lesson, you will learn about this phylum as well as some interesting interesting characteristics of Platyhelminthes organisms. You can read about some example organisms before taking a brief quiz.

Platyhelminthes: A Big Word for Flatworms

Let’s say you’re on a trip at the nature center. As you and your friends explore, you come across a big tank with many ocean organisms inside.

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You see what looks like a colorful piece of paper moving along slowly at the bottom of the tank. ‘What is that?’ You ask your guide Marlon.Marlon tells you that the creature is a flatworm, a member of the phylum Platyhelminthes.

‘Wow,’ You say. ‘What a weird looking animal!’ Marlon goes on to explain that this little guy and his relatives are the stars of a new exhibit dedicated to the phylum as he leads you through your exploration of the Platyhelminthes exhibit.

General Characteristics of Platyhelminthes

Platyhelminthes is a very interesting phylum that contains nearly 20,000 species. These organisms are generally hermaphrodites, meaning they can have both male and female sexual organs and can produce offspring on their own. They’re also soft-bodied invertebrates (they have no spine), and much like humans, flatworms have bilateral symmetry, which means that if you drew a vertical line down the middle of the animal, the left side of the organism would mirror the right side.

Most flatworms also do not have a body cavity! This means most flatworms really don’t have anything inside but a small gut, and some don’t even have that. They have a small opening through which they take in food and use the same opening to expel waste. Lastly, Platyhelminthes species range in size from microscopic to a few feet long, and they breathe across the whole body!

Examples of Platyhelminthes

As you go deeper into the nature center’s Platyhelminthes exhibit, you see four different areas, each dedicated to the four classes of flatworms:

  • Turbellaria: Prey on smaller animals and live in salt and freshwater.

    They are not parasitic. Examples are the New Zealand flatworm and Symsagittifera roscoffensis, also known as the mint sauce worm due to its bright green color.

  • Trematoda: Often referred to as flukes, are parasitic.

    They live inside and off of host cells and tissues. They’re very widespread and serious human parasites. These flatworms often start out in an invertebrate host and end up in a vertebrate target or primary host. Examples include the Chinese liver fluke and the human lung fluke. If untreated, some of these organisms can cause death in their target host.

  • Cestoda: Consists of tapeworms.

    They are intestinal parasites in vertebrates. An example of a Cestoda is Taenia saginata, also called the beef tapeworm.

  • Monogenea: Spend their whole lives as parasites to a single host, typically in and on fish gills and skin.

    They do not infect humans. An example is the Gyrodactylus salaris, which typically lives on freshwater fish.

As you exit the Platyhelminthes exhibit, Marlon reminds you that flatworms can be very harmful to humans. For that reason, it’s important to clean and cook food properly, avoid contact with animal feces and follow other hygiene measures to avoid these parasites.

Lesson Summary

Platyhelminthes, also referred to as flatworms, are soft-bodied invertebrates, meaning they have no backbone. These organisms are hermaphrodites – they have both male and female sexual organs and can produce offspring individually.

They’re bilaterally symmetrical, which means one of these organism’s two halves reflect each other, and they have no body cavity and only one opening (through which they take in food and expel waste). These creatures breathe across the entire body, and they can be microscopic or a few feet long. Platyhelminthes are broken into four classes: Turbellaria, Trematoda, Cestoda, and Monogenea.


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