Platyhelminthes

• Platyhelminthes


  • Simplest organisms with consistently bilateral symmetry and triploblastic acoelomate organisation, cephalisation, ciliated ectoderm
• Platyhelminthes


  • Tapeworm (parasitic form)
  • Triclad (free-living form)
• Platyhelminthes


  • Bilaterally symmetrical
  • Cephalisation
  • Triploblastic and acoelomate
  • No respiratory or circulatory systems
  • Branched digestive system
  • Excretory system with flame cells
  • Most are hermaphrodites
• Classes of Platyhelminthes


  • Turbellaria
  • Trematoda
  • Cestoda
• Turbellaria


  • Free-living
  • Most are marine but many are freshwater and some are terrestrial
  • Broad, leaf-shaped and streamlined
  • Ciliated epidermis for locomotion and gaseous exchange
  • Rhabdoids produce mucous
  • Feed by secreting mucous to entangle prey
  • Mouth leads to branched gut
• Turbellaria


  • Triclads (e.g. Planaria)
• Cestoda (Tapeworms)


  • All endoparasites of vertebrates
  • Body has scolex (attachment organ), unsegmented neck, and proglottids (repetitive segments)
  • Each proglottid is hermaphroditic
  • Lack digestive system, rely on diffusion
  • Have thick cuticle, lack sensory organs
  • Respiration is anaerobic
• Tapeworm life cycle


  1. Proglottids containing embryos passed in faeces

  2. Embryos ingested by intermediate host (cattle or pigs)

  3. Juvenile form enters through mouth, migrates and establishes as cyst in host tissues

  4. Humans ingest infected meat, embryo develops into adult tapeworm in intestine