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Flatworms and Roundworms Section 28.3 Flatworms The largest group of acoelomate worms Contain a mesoderm Has tissues organized into organs Bilaterally symmetrical Flat, ribbon-like body Have a highly branched gastrovascular cavity Dissolved substances move through the body by diffusion Most do not have a respiratory or circulatory system Flatworms, continued… Turbellaria Cestoda Free-living Mostly marine Planarians – freshwater turbellarians Parasitic tapeworms Anterior end contains suckers and hooks Body increases in length by producing segments called proglottids that break off during reproduction Trematoda Flukes Endoparasites (inside) or ectoparasites (outside) May have complex life cycles with more than 1 host Planaria Marine Flatworm Tapeworm Fluke Roundworms Also called nematodes Pseudocoelomates – fluid movement acts as a simple circulatory system Have a one-way digestive tract Parasitic roundworms 1. 2. 3. Three sources of human infection: Ascaris – eggs in human waste end up in soil, enter through ingestion, end up in respiratory system Trichinella – passed to humans through undercooked pork, causes a disease called trichinosis Necator (hookworms) – larvae can penetrate the soles of bare feet and enter the bloodstream Ascaris Trichinella Hookworms
