1. Platyhelminthes! Bilateral Symmetry!
- Collection of sensory organs in head
- Better directional sense
- Quicker nervous system (development of CNS)
Triploblastic – three body layers! A first!
- Ectoderm, endoderm, and mesoderm – the mesoderm gives rise to complex tissues that can form muscles and bones and some organs. In Platyhelminthes, it forms a VERY basic gut = a pseudocoel.
Because they do not form complex layers, they are restricted to being flat! Hence, flatworms!

2. NEW STUFF!! Flame cells! Neoblasts!
Brains! (sort of new – similar to Rotifera)
Three Embryonic Layers! Endoderm, *Mesoderm*, and Ectoderm.
Parasitism! Extendable Pharynx! (similar to Rotidera)
Yolk glands – Produce eggs (similar to Rotifera)
Complex Reproductive Organs! They actually have sex!
Complex, Multi-stage Life Cycles

3. Regeneration! More on the Neoblast… What is it?
How does it “know”? (oh my…) Consider going on vacation…
Why do humans not regenerate like some of their simpler ancestors?

4. CLASSES: So… why the confusion? Turbellaria – Planarian (dugesia)
Trematoda – Flukes
Parasitic
Cestoda – Tapeworms
1 & 2 are paraphyletic (containing all descendants of a common ancestor)
Recent studies show the entire phylum may be polyphyletic (have arisen as two independent groups… meaning they may be more related to the earthworms/mollusks!)
So… why the confusion?

5. Trematode Life Cycle: Flukes
Depending on the habitat in the infected host, flukes can be classified as blood flukes, liver flukes, lung flukes, or intestinal flukes.

6. Cestoda Life Cycle: Feces… Feces…
Feces can be ingested OR more likely is attached externally and deposited during processing.