Flatworms - Ummm. They're Flat.We have to run out of steam there. But that's the one obvious connection between all of the worms in this group. Sure we can tell you that there are over 20,000 known species. Most of the species are parasitic. They are super-primitive organisms that were the first to develop mesoderm. The big thing you should remember is that they are flat. There may be one more fact to remember. They are found in every environment that has water. Many scientists believe there are hundreds of species we will still discover.
Basic TypesYou'll learn about a few classic examples of flatworms in your classes. Tapeworms are always a nasty example. They live in intestinal tracts of many species. Cats, dogs, and even you could wind up with tapeworms. You'll also hear about flukes. It's another parasitic species that has a close relationship with man and other mammals. They are suckers in the flatworm family. Planaria are harmless flatworms that are often used in your biology labs. They are freshwater flatworms that have a head like an arrow. They're black with two white spots on their heads. Those spots are not eyes, but they do sense light. Flatworms have very simple nervous systems.
Where Will You Find Them?If you find water, there's a good chance flatworms will be around. Most are very small and not obvious immediately. Flatworms get most of their oxygen through diffusion. Since they have no specialized circulatory system, their flatness gives them a greater surface area to absorb more oxygen. There are species in many freshwater and saltwater environments as well as inside larger organisms.
While we made them seem easy to find, it's sometimes very hard to track them down. Since many flatworms are parasites, they have lifecycles in different hosts. Eggs many be deposited in the feces of one species. Another organism might eat those eggs and development begins in the digestive system. The flatworm might then mature in the muscle tissue. Flatworms can get around.
Some Structure StuffAlthough really simple, flatworms have some advanced structures compared to some other animals. They were the first species to develop mesoderm. That mesoderm tissue develops into organs and muscles as the organism grows. They also have simple nervous systems and sensory organs.
While they don't have a respiratory system or circulatory system to speak of, they do have a neat little digestive system. Most species of flatworms have no anus and a cavity with only one opening. While not all species do, many flatworm species use a mouth to eat. The only problem with no anus is that the stuff you don't digest has to go out the mouth when you're done. Other materials that need to be removed from the system can be excreted through specialized cells called flame cells. Flame cells are the first appearance of excretory ducts (passages) in organisms. Flame cells have cilia that beat and flap to filter out materials the flatworm wants to remove.
Flatworm Eats Amphipod (Smithsonian SERC Video)