Phylym Platyhelminthes features:

  • Triploblastic, accoelomate, bilaterally symmetrical
  • Protostome (lophotrochozoan)
  • Central nervous system, Cephalization
  • Digestive system with single opening (no anus)
  • Excretory system: protonephridia
  • Most are simultaneous hermaphrodites (monoecious)
  • Include both free-living and parasite species

Flatworms are Flat!

  • No specialized respiratory or circulatory systems, so gas exchange occurs by diffusion across the body surface
  • High surface area relative to volume

Phylum Platyhelminthes

  • Class Turbellaria
  • Class Cestoda
  • Class Trematoda

Protonephridia: Osmoregulation and waste elimination

  • Maintains osmotic balance and gets rid of nitrogenous waste

Turbellarian Sense Organs

Tubellarian epidermis and body walls

  • Diagonal muscles
  • Dorsoventral muscles
  • Longitudinal muscles
  • Rhabdoids
    • Packets of mucus for their bodies to glide and if they get startled
  • Ducts of glands
  • Basal bodies of cilia

Adaptions of a Parasitic Lifestyle

  • Evolution of holdfast organs
  • Reduction of nervous system, loss of cephalization, loss of sensory structures
  • Loss of locomotor organs
  • Reduction of digestive system; absorb nutrients through body wall
  • Greatly increased reproductive capacity, elaboration of reproductive organs; hermaphroditism, parthenogenesis
  • Primary, secondary and tertiary hosts
  • Mechanisms to avoid host immune system
  • Ability to live in anaerobic environment (host gut)