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27.1 Flatworms : Phylum Platyhelminthes (to remember: “Platy” means “Flaty”) Soft, flattened worms that have tissues and internal organ systems. Ex) planaria, tubellarians, flukes, tapeworm First animals to have: three germ layers bilateral symmetry cephalization Recap: Coleom Body Plan Flatworms are acoloemates, meaning they don’t have a true body cavity. -Body cavity lined with tissue made from mesoderm. - No coelom forms between the tissues of flatworms -Flatworms have bilateral symmetry Two well-formed sides that can be Identified as left and right -Cephalized: have a distinct head Feeding Some are free-living, some are parasitic To eat, flatworms have a muscular tube called a pharynx, that they extend out of the mouth, and use as a pump, to move food into their digestive cavity. Once digested food diffuses to the rest of the body tissue. Undigested food is ejected from the pharynx. (one opening—no anus) http://www.youtube.com/watch?v=A6N8KZqGAJg Planaria eat a worm segment, observation of pharynx Parasitic worms live on the tissue of other animals (suck blood and nutrients); tapeworms, blood flukes Excretion Flatworms have flame cells that remove excess water and cellular waste (such as ammonia) from the body. Flame cells join together to form an internal network of tubes that empty through pores on the worms’ skin. Respiration and Circulation Flatworms are thin, and most of the cells are close to the outside—they get oxygen by diffusion. They have no respiratory system or circulatory system. Movement Free-living flatworms usually move by using cilia to glide through water, and muscle cells, which are controlled by their nervous system to twist and turn in response to stimuli. Parasitic worms usually require a host to be moved. Response Most flatworms have a concentration of nerve cells called ganglia at their head. They are not a true brain, however, they control the nerve cords that run along both sides of the body. Many free-living worms have eyespots, that detect changes in light. Free-living worms are more complex and use their nervous system to find food and hiding spots. (In lab, you will see that planaria avoid light) Reproduction Most free living flatworms are hermaphrodites, meaning they have both testes and ovaries. During sexual reproduction, two worms join and exchange sperm. Eggs are laid after fertilization. Can also reproduce asexually by splitting in two (called fission). Planaria are able to regenerate their tissue and for two new identical planaria. Video explaining stem cells and regeneration part 1: http://www.youtube.com/watch?v=J9EuFuJF9N0 Part 2: http://www.youtube.com/watch?v=YBLDFI4OhWY http://www.youtube.com/watch?v=8Ew3yupNMF8&feature=related : Planaria with 8 heads Turbellarians – Free-living flatworms that live in fresh and salt water. Flukes – parasitic flatworms that infect internal organs of their host. They infect multiple hosts. Blood flukes reproduce sexually in humans by living in the intestine. They release embryos into fecal matter, which go on to contaminate another host. Tapeworms—have no digestive tract—they eat food that has already been digested by their host. Their sperm fertilizes their own eggs, or the eggs of another, the proglottids break off and are passed out in fecal matter.