Phylum: Platyhelminthes


  • Triploblastic, Acoelomate, Bilarterally symmetric
  • Protostome (lophotrochozoan)
  • Central Nervous System, Cephalization
  • Digestive system with single opening no anus
  • Excretory system: protonephridia
  • Most are simultaneous hermaphrodites
  • Include both free living and parasite species

Flat Worms:

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

Three classes: Turbellaria, Cestoda, Trematoda

Turbellaria- The Planarian

Anatomy: Pharynx, Eyespots, Ganglia, Ventral nerve cords, GV Cavity

Screen Shot 2017-10-08 at 10.54.00 PM.png

“Planarian” by Wikimedia Commons under CC by 2.0

Protonephridia: Osmoregulation and Waste elimination

Sense Organs: Cup Ocellus



Adaptations 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)

Class Cestoda





Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s