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Acoelomates: Platyhelminthes, Nemertea & Gnathostomulida

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  1. Acoelomates Phylum Platyhelminthes Phylum Nemertea Phylum Gnathostomulida

  2. Taxonomy

  3. Acoelomate Phyla • Platyhelminthes • Flat worms • Nemertea • Ribbon worms • Gnathostomulida • Jaw worms

  4. Why bilateral symmetry? • Movement toward prey or host • Cephalization • Directional sense organs • Chemoreceptors • Ocelli (light sensing eyespots) • Rheoreceptors (sense water currents)

  5. Acoelomates Key Features • Three germ layers • Ectoderm, endoderm, mesoderm • Mesoderm forms muscle and mesenchyme • Organ-system level of organization • Cephalization • Excretory system • Some have circulatory and one-way alimentary canal

  6. Acoelomates Body Plan

  7. Phylum Platyhelminthes • Specialized Cells • Rhabdites - secrete mucous sheath • Tegument - outer covering of syncytial cells • Flame cells – Osmoregulation • Some have endolecithal egg cells – yolk contained within egg cell

  8. Platyhelminthes Cross Section

  9. Tegument • Syncytium • Apical cell membrane missing

  10. Flame Cell • Osmoregulation • Beating flagella creates negative pressure to draw fluid into chamber • Water is directed through channels then through an external pore

  11. Major Classes of Platyhelminthes • Class Turbellaria • Planaria – free living • Class Trematoda • Liver flukes – endoparasites • Class Monogenea • Fish ectoparasites • Class Cestoda • Tape worms – endoparasites

  12. Class Turbellaria • Key Features • Ladder like nervous system • Eye spots (Ocelli) • Auricles - chemoreceptors • Free living flatworms • Muscular and ciliary movement • Carnivorous

  13. Planarian Body Plan

  14. Class Trematoda • Similar in structure to Turbellaria • Special adaptation for endoparasitic life style • Cyst glands • Suckers and hooks for griping host • Digenetic – complex life cycle with multiple hosts

  15. Liver Fluke

  16. Liver Damage Caused By Flukes • Scar tissue • Blocked bile ducts

  17. Flukes in Liver Section

  18. Fasciolopsis - Anterior End

  19. Schistosoma mansoni • Blood fluke • Male and female are dioecious

  20. Schistosoma cercaria

  21. Class Cestoda • Tapeworms - endoparasites • Key Features • Lack a digestive system • Composed of reproductive units called proglottids • Adult tegument covered in microvilli-like projections – increases surface area, non-ciliated

  22. Tapeworm • New proglottids are added just behind the scolex

  23. Tapeworm Tegument • Microvilli help with food adsorption • Sensory cells – no other sense organs

  24. Mature Proglottid • Reproductive unit • Mature proglottids are either shed or produce shelled embryos • Each proglottid has both male and female organs

  25. Tape WormLife Cycle

  26. Taenia soliumPork tapeworm

  27. Taenia Cyst in Muscle

  28. Phylum Nemertea • Ribbon worms • Ciliated epidermis • Locomotion – gliding over slime track and muscular contraction • Complete digestive tract – mouth and anus • Extracellular digestion • Proboscis to capture prey

  29. Amphiporus

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