Phylum Platyhelminthes (flatworms) Summary

Phylum Platyhelminthes (flatworms)

  • Triploblastic: Possess three cell layers – ecto-, endo-, and mesoderm.
  • Mesoderm significance:
    • Allows increased size.
    • Enables organ system development (e.g., central nervous and digestive systems).
    • Improves muscular activity.
  • Characteristics:
    • Bilateral symmetry.
    • Triploblastic, with organs and organ systems developed from mesoderm.
    • Acoelomate: mesoderm fills the space between endoderm and ectoderm.
    • Flat bodies.
    • Unsegmented.
    • Well-developed organs (e.g., muscular pharynx, simple brain, eyespots).
    • Differentiated ‘head’ and posterior end.
    • Mouth, no anus.
    • Defined dorsal and ventral surfaces.
    • No developed transport system; diffusion for gaseous exchange.
    • Excretory system with branched tubes.

Classes of Platyhelminthes

  • Turbellaria: Free-living (e.g., Planarians).
  • Trematoda: Parasitic flukes.
  • Cestoda: Parasitic tapeworms.

Class characteristics

  • Turbellaria:
    • Free-living.
    • Marine.
    • Ciliated epidermis.
    • Carnivorous.
  • Trematoda (Flukes):
    • Parasites.
    • Suckers for attachment.
    • Complex life cycle with intermediate hosts.
  • Cestoda (Tapeworms):
    • Parasites of vertebrates.
    • Complex life cycle.
    • Suckers and hooks for attachment.
    • Proglottids produce eggs.
    • No digestive or nervous system.

Trematoda - Blood fluke

  • Schistosoma spp. cause Schistosomiasis (Bilharzia).
  • Life Cycle:
    • Eggs eliminated in feces/urine.
    • Miracidia hatch and penetrate snails.
    • Cercariae released, penetrate human skin, becoming schistosomulae.
    • Adults reside in mesenteric venules.
    • S. japonicum: superior mesenteric veins (small intestine).
    • S. mansoni: superior mesenteric veins (large intestine).
    • S. haematobium: venous plexus of bladder.

Beef Measles

  • Taenia saginata (tapeworm) causes beef measles (Cysticercus bovis) in cattle.
  • Cysts found in muscles of cattle.
  • Tapeworms grow to 4-10 meters.
  • Agricultural importance affecting beef industries.

Economical Importance

  • Dugesia used in regeneration experiments.
  • Fasciola hepatica damages liver and causes liver-rot disease.
  • Tapeworm causes taeniasis in humans; cysticercus larva causes cysticercosis in cattle.