1. Flatworms and Roundworms Section 28.3

2. 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

3. 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

4. Planaria

5. Marine Flatworm

6. Tapeworm

7. Fluke

8. Roundworms Also called nematodes Pseudocoelomates – fluid movement acts as a simple circulatory system Have a one-way digestive tract

9. Parasitic roundworms Three sources of human infection: 1. Ascaris – eggs in human waste end up in soil, enter through ingestion, end up in respiratory system 2. Trichinella – passed to humans through undercooked pork, causes a disease called trichinosis 3. Necator (hookworms) – larvae can penetrate the soles of bare feet and enter the bloodstream

10. Ascaris

11. Trichinella

12. Hookworms