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Flatworms - Class Turbellaria, Class Monogenea, Class Trematoda, Class Cestoidea

worm system animals organs

Flatworms are small, multicelled animals with elongated bodies that have clearly defined anterior (front) and posterior (rear) ends. These worms are bilaterally symmetrical, meaning that their two sides reflect each other. They usually have a recognizable head, which houses gravity and light-receptive organs, and eye spots. They lack circulatory and respiratory systems and have only one opening that serves both as their anus and mouth. Most flatworm species live in fresh and marine waters, although some live on land.

Their soft, flattened bodies are composed of three layers-the ectoderm, endoderm, and mesoderm. They may be covered by a protective cuticle or by microscopic hairs, called cilia. Their internal organs are comprised of a nervous system, usually hermaphrodite sexual organs, and an excretory system.

As members of the phylum Platyhelminthes, flatworms belong to four classes: Turbellaria, Monogenea, Trematoda, and Cestoidea. Within these four classes, there are hundreds of families and some 10,000 species, including animals with common names like free-living flatworms, parasitic flatworms, tapeworms, and flukes.


Probably the most familiar Turbellarians are the planarians, soft-bodied, aquatic, flattened worms that appear to have crossed eyes and ear lobes. In fact, the crossed eyes are eye spots with which the worm can detect light. The lobes to each side are sensory and also are equipped with glands to secrete an adhesive substance used in capturing prey.

The single opening on the ventral (bottom) surface of the worm serves as both mouth and anus. Internally the worm has a complex, branching gut that courses nearly the full length of the body. Since the worm has no circulatory system, the elongated gut brings food to nearly all areas of the worm's body. Planaria have no skeletal or respiratory systems.

These animals possess great powers of regeneration. If a planaria is cut in half, the front half will grow a new tail section and the rear half will generate a new head. If cut into thirds, the middle third will regrow a head and tail and the other two sections will regenerate as described.

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about 11 years ago

are all of these phylum part of the "flatworms" this is very confusing....<3

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over 5 years ago

Love the site dude..yall needed a comment

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over 8 years ago

Okay, First of all Turbellarians (or free-living flatworms) do not have ears. They are called auricles, and they do not hear. They are touch and chemical receptors. Also, they light-sensitive organs ARE the "eyes", but are actually termed orelli. And, those are ONLY light sensitive, they do not actually see. And in some turbellaria, the opening on the bottom through which the organism eats is called the pharynx.

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almost 2 years ago

this is great cheak youtube @romanatwoodvlogs get to 10m

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over 2 years ago

This is for MJ. I am working on a science project. I so happen to scroll down this page, why the heck would someone have time to correct the person who created this page. I know I am Commenting also, but I have a good excuse. My parents are making me work and I don't want to. I know no one is going to read this, because the last person to comment on this was over three years ago. Anyways I am going to go kill myself over this stupid project. Its a huge pain in my back. I need a break. Adios people who will never read this.

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over 5 years ago