Flatworms and Section 3 Section 3 Roundworms FocusFocus Overview Flatworms Objectives Before beginning this section review with your students the When you think of a worm, you probably visualize a creature with a ● Compare the three classes objectives listed in the Student long, tubular body, such as an earthworm. You might be less famil- of flatworms. 8B 8C TAKS 2 Edition. This section explains the iar with flatworms and roundworms. The flatworms are the largest ● Summarize the life cycle characteristics and life histories of group of acoelomate worms. Although the flatworm body plan is of a blood fluke. 8C TAKS 2 relatively simple, it is a great deal more complex than that of a flatworms and roundworms. ● Describe the body plan of sponge or cnidarian. Flatworms have a middle tissue layer, the Students will explore the three dif- a roundworm. 8C TAKS 2 mesoderm. And unlike sponges and cnidarians, the flatworm has ferent classes of flatworms, as well tissues that are organized into organs. ● Summarize the life as the anatomy of both flatworms The flatworm’s body is bilaterally symmetrical and flat, like a piece cycle of the roundworm and roundworms. Ascaris. 8C of tape or ribbon. As a result, each cell in the animal’s body lies very TAKS 2 close to the exterior environment. This permits dissolved substances, Key Terms such as oxygen and carbon dioxide, to pass efficiently through the Bellringer flatworm’s solid body by diffusion. In addition, portions of the flat- proglottid The word worm can refer to a wide worm’s highly-branched gastrovascular cavity run close to practically fluke variety of organisms. Ask students all of its tissues. This gives each cell ready access to food molecules. tegument to think about what a worm is, and Most flatworms have no respiratory or circulatory system. ask them to list on paper the names Flatworms belong to phylum Platyhelminthes, which contains or describe the types of worms they three major classes: Turbellaria, Cestoda, and Trematoda. They can remember. Ask students where range in size from free-living forms less than 1 mm (0.04 in.) in these worms live and what they eat. length to parasitic intestinal tapeworms several meters long. Have volunteers share their descrip- tions with the rest of the class. Turbellaria LS Verbal Almost all members of class Turbellaria are free-living marine flatworms, such as the one shown in Figure 14. However, marine flatworms are rarely studied by MotivateMotivate students because they are difficult to raise in captivity. Instead, students usu- Discussion/ ally study a freshwater turbellarian such Question as Dugesia, one of a group of flatworms Ask students what they might do if commonly called planarians. Dugesia is they were in a rowboat that had a shown in Up Close: Planarian, on the small leak. (Students may answer following page. Figure 14 Marine flatworm. Most free-living flatworms are marine species that swim with that they would use a small container graceful wavelike movements. to bail out the water as it seeps in.) Explain to students that all organ- Evolutionary Milestone isms have mechanisms that help 3 Bilateral Symmetry them maintain proper water bal- Flatworms were likely the first bilaterally symmetrical animals, with left ance. Tell them that water seeps and right halves that mirror each other. Like all bilaterally symmetrical into Dugesia, the flatworm they animals, flatworms have a distinct anterior (cephalic) end. will study on the next page, like it 629 seeps into a leaky boat. Ask them to suggest ways in which Dugesia might maintain water balance. Chapter Resource File Planner CD-ROM (The excretory system expels water • Reading Organizers so that the animal does not swell up.) • Lesson Plans GENERAL LS Logical Bio 3E • Directed Reading • Reading Strategies • Supplemental Reading Guide • Active Reading GENERAL Journey to the Ants: A Story of • Data Sheet for Quick Lab GENERAL Scientific Exploration Transparencies TT Bellringer TT Planarian Chapter 28 • Simple Invertebrates 629 Up Close Up Close Planarian TAKS 2; TAKS 3 Planarian TAKS 2 Bio 8C, 10A; TAKS 3 Bio 7B ● Scientific name: Dugesia sp. Discussion Guide the discussion ● Size: Average length of 3–15 mm (0.1–0.6 in.) by posing the following questions: ● Range: Worldwide •Instead of a circulatory system ● Habitat: Cool, clear, permanent lakes and streams that delivers nutrients to tissues, ● Diet: Protozoans and dead and dying animals what does a planarian have? Dugesia feeding (Branches of the digestive tract reach the tissues directly.) Characteristics •How do planarians reproduce Nervous System Sensory information gathered by Feeding Dugesia, a free-living flatworm, must extend asexually? (They tear themselves the brain is sent to the muscles by two main nerve cords its muscular pharynx out of its centrally located mouth in two, and each half regenerates that are connected by cross branches. Light-sensitive in order to feed. to form a complete worm.) structures called eyespots are connected to the brain. Reproduction Dugesia reproduces asexually in the The eyespots are close to each other, giving Dugesia •How do planarians reproduce summer by attaching its posterior end to a stationary across-eyed appearance. sexually? (They are hermaphro- object and stretching until it breaks in two, each of which dites that fertilize each other’s Brain will become a complete animal. Sexual reproduction eggs. Protective capsules surround also occurs. Individuals are hermaphrodites, and two groups of fertilized eggs, which individuals simultaneously hatch in 2–3 weeks.) Female transfer sperm to each other. reproductive Eggs of both individuals are system Eyespot Male fertilized and are released in reproductive clusters enclosed in a protective system capsule. Several capsules are TTeacheach Nerve cord laid at a time, and the eggs inside hatch in 2 to 3 weeks. GENERAL Pore Activity Pharynx Writing Life of a Planarian Mouth Have your students imagine a Reproductive day in the life of a planarian and pore write a story about the planarian’s adventures. Their story should include information about the planarian’s nervous system, water Tubule balance, reproduction, feeding, Excretory system Intestine digestion, and excretion. Caution Flame cell them to avoid anthropomorphic Water Balance Because Dugesia’s body cells contain more solutes Digestion The highly branched descriptions of the planarian, than fresh water does, water continually enters its body by osmosis. intestine enables nutrients to pass though a little creative license Excess water moves into a network of tiny tubules that run the length of close to all of the flatworm’s tissues. should be allowed. Have students Dugesia’s body. Side branches are lined with many flame cells, specialized Nutrients are absorbed through the volunteer to read their stories to cells with beating tufts of cilia that resemble a candle flame. The beating intestinal wall. Undigested food is the class. LS Verbal TAKS 2 Bio 10A cilia draw water through pores to the outside of the worm’s body. expelled through the mouth. 630 Trends in Neurology Impulsive Invertebrates Scientists have used better understand the neurological bases of pp. 630–631 many types of invertebrates, including planari- learning. Also, researchers in Germany have Student Edition ans and other flatworms, to try and under- recently made a computer chip that can send TAKS Obj 1 Bio/IPC 2B, 2C TAKS Obj 2 Bio 8C stand more about nerve cells and the nervous signals to a single nerve cell in a living leech. TAKS Obj 2 Bio 10A system. More than 50 years ago, biologists The leech’s nerve cell can return the signal TAKS Obj 3 Bio 7B used giant axons from squids to learn how back to the computer chip. The researchers TEKS Bio 7B, 8C, 10A nerve impulses travel along nerve-cell are hoping that this technology will help them TEKS Bio/IPC 2B, 2C processes. More recent experiments with develop computers that can communicate with Teacher Edition Aplysia, a large sea slug, have helped scientists nerve cells in a human body. Bio 3F TAKS Obj 1 Bio/IPC 2B, 2C TAKS Obj 2 Bio 8C, 10A TAKS Obj 3 Bio 7B, 12B TEKS Bio 3F, 7B, 8C, 10A, 12B TEKS Bio/IPC 2B, 2C 630 Chapter 28 • Simple Invertebrates Observing Planarian Behavior Most bilaterally symmetrical organisms have sense organs con- centrated in one end of the animal. You can observe how this Observing arrangement affects the way they explore their environment. Planarian 2B 2C Materials TAKS 1 Behavior TAKS 1 Bio/IPC 2B, 2C eyedropper, live culture of planaria, small culture dish with pond water, hand lens or dissecting microscope, forceps, and small Skills Acquired piece of raw liver (3–7 cm) Observing, comparing, Procedure evaluating conclusions 1. Using the tip of the eyedrop- 4. Observe the planarian’s 3. Contrast the feeding behav- Teacher’s Notes per, place a planarian in the response. If the planarian ior of planarians with that of culture dish with pond water. approaches the liver, move hydras, described earlier in Before students begin, demon- the liver to a different position. this chapter. strate how to place planarian in 2. Using the hand lens or dis- secting microscope, observe 5. Continue observing the pla- 4. Critical Thinking a culture. Planarians are pho- the planarian as it adjusts to narian for 5 minutes, moving Evaluating an Argument tonegative and should be kept its environment. Determine the liver frequently. Evaluate this statement: in the dark as much as possible. which end of the planarian Bilateral symmetry gives pla- Analysis Use opaque pans, and keep the contains sensory apparatus naria an advantage when for exploring the environment. 1. Describe the planarian’s feeding because sensory water temperature close to means of locomotion. organs are concentrated in 18˚C. Prior to the lab, avoid 3.