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Flatworms belong to the phylum Platyhelminthes, which includes planarians, flukes, and tapeworms.

Learn more about these animals as you explore the contents of this lesson.

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Phylum Platyhelminthes

Can you say Platyhelminthes three times fast? It is a mouthful, but all it really means is flat worm. It should now come as no surprise to you that all of the animals in the Platyhelminthes phylum are flatworms! There are three main types of flatworms: flukes, planarians, and tapeworms. Some flatworms are free living organisms, but most are parasitic. This means that they live off of a host organism, causing harm to the host.

Since most flatworms are parasitic in nature, they are mostly responsible for sickness or disease. However, some free-living flatworms do help the environment by acting as decomposers.

Flatworm Body Structure & Systems

All flatworms have bilateral symmetry. This means that if you draw a line down the center of their bodies it will divide the flatworm into matching halves.

Flatworms have an incomplete digestive system, with only one opening. They have no anus, so they eat and expel waste from the same opening. They also have a simple brain and nervous system, which creates a head area and eyespots that are able to sense light. They move by contracting muscle cells or the movement of tiny hairlike structures on their body called cilia.

Flatworms do not have a respiratory or circulatory system. Instead, these functions take place through the process of absorption. Most flatworms are hermaphroditic and posses reproductive organs of both sexes. This means they have the ability to self-reproduce or reproduce sexually.

Flukes and Tapeworms

Most flatworms fall into the category of flukes or tapeworms, which are harmful parasites. You may have heard someone make a joke about someone having a tapeworm if he is a skinny person, but in reality the sickness that can result from having a parasite is no joking matter!Tapeworms can grow to be quite long. They have a special head called a scolex that has hooks and suckers that help them stay attached to their host and make them difficult to remove.

If an animal has tapeworms, they can contaminate the meat. If you eat this meat and it is not cooked well enough, you will likely gain a tapeworm infection.You can also get a fluke infection from contaminated food, but it is more common to get a fluke infection if you come into contact with contaminated water or soil, and the parasite enters your body through the skin. Blood flukes are the most common type to infect humans. Flukes pose a serious health concern in underdeveloped parts of the world where there is poor sewage disposal.

Planarians

Planarians make up a smaller group of flatworms. You could call them the black sheep of the flatworm world.

They are different because they are not parasitic. Instead, planarians are carnivorous, free-living flatworms. Planarians may eat other small invertebrates or decaying organic matter. They can be found living in moist environments beneath rocks or logs, in wetland areas, or in streams and lakes.

Lesson Summary

Platyhelminthes means flat worm. This is a phylum that contains three main types of flatworms: flukes, planarians, and tapeworms. Most flatworms are parasitic, meaning that they live off a host organism causing harm to the host, but some are free living organisms.

All flatworms have bilateral symmetry, which means if you draw a line down the center of their bodies it will divide the flatworm into matching halves. They also have an incomplete digestive system and a simple brain and nervous system. They also lack a true respiratory or circulatory system. They move around using tiny hair-like structures on their body called cilia. They are also mostly hermaphroditic, which means they possess reproductive organs of both sexes.

Most flatworms are tapeworms and flukes, parasites that cause disease. You can get these parasites from under-cooked meats or a contaminated environment. Tapeworms have a specialized head with suckers and hooks, called a scolex, which helps them stay attached to their host.

Some flatworms are planarians. Planarians are carnivorous, free-living flatworms that feed on small invertebrates or decaying organic matter.

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