1. Phylum Platyhelmenthes
The Flatworms

2. Phylum Level Characteristics
Bilateral symmetry showing cephalization
3 tissue layers (MESODERM!)
Acoelom
Incomplete digestive system (gastrovascular cavity)
Hydrostatic skeleton
Free-living or parasitic
Flattened body
No segmentation

3. Planaria (free-living)
Nervous System
Nerve network (2 nerve cords run length of body)
Concentrated in head called ganglia “brain”
Respond to light and chemicals “eyespot”
Connected to several muscle layers so can change shape and move

4. Planaria (free-living)
Reproductive System
Hermaphroditic
When 2 worms mate each transfers sperm to the other
Asexual by fragmentation (regenerative capabilities) due to unspecialization / simplicity of tissue

5. Planaria (free-living)
Digestive System
Development of pharynx to extend out of mouth, brings food in to gastrovascular cavity which is highly branched
Enzymes digest food in cavity (extracellular digestion)
Nutrients absorbed into all cells directly from gastrovascular cavity so no circulatory system needed

6. Planaria (free-living)
Excretory System
Like cnidarians, flatworms expel undigested material through the mouth (pharynx)
Excess water eliminated through a network of tubes and specialized cells called flame cells
Propel wastes into and out of body through pores

7. Tapeworm (Parasitic) Specialization
Hooks to attach to intestines of hosts
Loss of sensory organs
No digestive system, simply absorb food through body wall
Highly efficient reproductive system
Hundreds of proglottids filled with eggs and sperm
Fertilized eggs mature and are released when the proglottid is excreted with the feces of the host
Egg is ingested by secondary host where it grows and forms cysts in muscle
Meat is consumed by primary host and life cycle continue

9. Assignment:
Read pages 570 – 575
Complete planaria handout