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 Cephalization Pharynx Mouth

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.