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.