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Platyhelminthes The Flatworms

Discover the fascinating world of Platyhelminthes, or flatworms, encompassing over 34,000 species with unique traits and life cycles. These triploblastic organisms exhibit bilateral symmetry, with most being parasitic and hermaphroditic. Learn about the classes within this phylum and their diverse adaptations for survival.

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Platyhelminthes The Flatworms

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  1. PlatyhelminthesThe Flatworms • Pechenik: 34,000 spp (Brusca & Brusca: 20,000 spp) Characteristics: • Triploblastic • Ectoderm, mesoderm, endoderm • Acoelomate • Bilateral symmetry (w some cephalization) • Protonephridia = Organs for excretion/osmoregulation • Flatness allows diffusion for gas exchange • 80% are parasitic • Simultaneous hermaphrodites (monoecious)

  2. Opisthaptor Ectoparasite One host Trematoda Acetabulum endoparasite Mollusc/vertebrate life cycle Cestoda Monogenea Turbellaria Free living Microtriches Scolex Proglottids Cilia lost from ectoderm (parasitic lifestyle) Phylum Platyhelminthes

  3. Class Turbellaria

  4. Class Turbellaria

  5. Class Turbellaria

  6. Class Turbellaria

  7. Class Turbellaria

  8. Class Turbellaria • Most are free living • Marine & FW • Sensory receptors • Chemical (auricles) • Tactile • Light Dugesia sp.

  9. Class Turbellaria • Most are free living • Marine & FW • Sensory receptors • Chemical • Tactile • Light • Cephalized • Crawl on substrate using cilia Dugesia sp.

  10. Class Turbellaria • Incomplete digestive tract Pharynx

  11. Class Monogenea haptor opisthaptor

  12. Class Monogenea Ectoparasites of fish skin and gills Only 1 host

  13. Monogenea Reproduction: Eggs → oncomiracidium larva → attach to fish Larva similar to cestode oncosphere Adults resemble digene trematoda (convergent evolution – life cycles very different)

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