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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
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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 • Free-living • Mostly marine • Planarians – freshwater turbellarians • Cestoda • 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
Roundworms • Also called nematodes • Pseudocoelomates – fluid movement acts as a simple circulatory system • Have a one-way digestive tract
Parasitic roundworms • 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
