PLATYHELMINTHES: general characteristics

• Platyhelminthes


  Phylum of flatworms, the first animals to be called "flatworms"
• Platyhelminthes


  • Triploblastic
  • Acoelomate
  • Bilaterally symmetrical
  • Incomplete gut (lack anus)
  • Cephalization (have head with centralized nervous system)
• Nervous system of Platyhelminthes

  • Anterior cerebral ganglion ("basal animal brain")
  • Longitudinal nerve cords connected by transverse nerves
• Sensory organs of Platyhelminthes

  • Light-sensitive ocelli (eyespots)
  • Chemoreceptors
  • Sensory organs around oral suckers and holdfast organs (in parasitic forms)
• Platyhelminthes


  • Lack skeletal, respiratory, and circulatory systems
• Lack of circulatory system in Platyhelminthes

  • Cells can easily exchange molecules due to flattened body
  • Branched digestive system helps with circulation
  • Nutrients exchange through diffusion
• Environments of free-living Platyhelminthes

  Seawater, freshwater, moist terrestrial (leaf litter, soil)
• Free-living Platyhelminthes


  • Small, around 1mm in length, cylindrical
• Body structure of free-living Platyhelminthes

  • Ciliated epidermis
  • Outer circular and inner longitudinal muscles
  • Rod-shaped rhabdites forming protective mucus sheath
  • Parenchyma cells (mesoderm-derived) with pluripotent stem cells for regeneration
• Adhesive organs of free-living Platyhelminthes


  • Adhesive/viscid glands for chemical attachment
  • Releaser gland cells to dissolve adhesive
  • Anchor cells to hold apparatus in place
• Digestive system of Platyhelminthes


  1. Begins at mouth/pharynx

  2. Terminates at gastrovascular cavity or intestine

  3. Extracellular digestion with proteolytic enzymes

  4. Phagocytic cells complete intracellular digestion
• Digestive system of Platyhelminthes

  • Lack anus, regurgitate undigested material through mouth
  • Pharynx can extend out to allow feeding while moving head
• Excretory and osmoregulatory system of Platyhelminthes


  Protonephridia with flame cells that move fluid, excrete waste, and regulate osmosis
• Reproductive system of Platyhelminthes


  • Monoecious (hermaphroditic)
  • Capable of sexual and asexual reproduction
• Traditional 4-class division of Platyhelminthes (Turbellaria, Trematoda, Monogenea, Cestoda) is now considered artificial
• Modern classification of Platyhelminthes


  • Catenulida (unpaired protonephridia and testis, nonmobile sperm)
  • Rhabditophora (includes parasitic forms, have rod-like rhabdites and dual-gland adhesive organs)
• Rhabditophora contains at least 10 orders of free-living flatworms and several parasitic orders