Platyhelminthes: The Flatworms
Platyhelminthes: The Flatworms
• What makes something a planarian (member of phylum platyhelminthes)?– Bilateral symmetry– cephalization– Single digestive opening (pharynx)– Respire via diffusion– Triploblastic body plan
Bilateral Symetry
Pharynx: Mouth on a tube
Pharynx
Mouth
Cephalization
Poor Flat-
worm! No Anus!
(single digestive opening)
Respiration: No Lungs… just diffusion
• That’s why they’re flat! More surface area for diffusion
Platyhelminthes have 3 cell layers
Human Germ Layers
• Ectoderm : skin + nerve system
• Endoderm : digestive & respiratory organs
• Mesoderm : everything else (heart, kidneys, muscles, etc.)
Cell Layers
Porifera Diploblast: Cnidaria
Triploblast:Platyhelminthes & other complex animals
Cell layers No cell layers, no tissues… just sponge
Ectoderm (outer layer & tentacles, etc.)
Ectoderm (skin, nerves, phalynx, etc.)
Endoderm (inner digestive layer – the mouth and gut)
Endoderm (mouth and gut)
Mesoderm (all other organs: eyes, muscles, etc.)
Anatomy of a Flatworm
Types of Platyhelminthes
• There are 3 groups of platyhelminthes– Planarians– Flukes– Tapeworms
Planarians: Free living nice guys
• Predators. Usually aquatic
Flukes: Parasites with Multiple Hosts• Fluke are
obligate pathogens.
• They have complex life cycles which require infection of multiple hosts.
Tapeworms • Parasites.