1. Lecture 9: Phylum Platyhelminthes Part 1
Invertebrate Zoology
Lecture 9: Phylum Platyhelminthes
Part 1

2. Lecture outline Phylum Platyhelminthes
Phylogeny/Evolutionary relationships
Diversity overview: Classes
Bauplan Basics
Feeding
Gas Exchange
Osmoregulation/Excretion
Nervous System
Movement/Attachment

3. Phylogeny (briefly) Hypothesis 1: Cnidarian-like ancestor
Arose from planula larva
Hypothesis 2: Annelid-like ancestor
Loss of coelom (by “filling in”)
Hypothesis 3: Benthic ctenophore ancestor

4. Diversity: Class Turbellaria
Mostly free-living
Carnivores and scavengers
Head
Sensory structures
Found in diverse aquatic & moist habitats

5. Diversity: Class Monogenea
Flukes
Ectoparasitic
Single host
Mostly fish
Prohaptor (anterior)
Sucker or adhesive disc
Opisthohaptor
Hooked attachment

6. Diversity: Class Trematoda
Flukes
Endoparasitic
1-3 hosts
One always a snail
Oral sucker
Acetabulum (ventral sucker)
Examples
Liver flukes, i. e. Clonorchis
Schistosoma

7. Diversity: Class Cestoda
Tapeworms
Endoparasitic
Usually >1 host
Scolex
Anterior attachment
Strobila
Composed of proglottids
Reproductive
No digestive tract
Photo: Kevin Mackenzie

8. Bauplan basics Triploblastic Bilateral symmetry & cephalization
True mesoderm  muscles and mesenchyme
Bilateral symmetry & cephalization
What’s so great about a head?
Compare movement & prey capture to radial phyla

9. Bauplan basics Share some protostome features
Spiral cleavage
Determinate cell fate
Mesoderm from 4D cell
No coelom! (=Acoelomates)

10. Spiral cleavage

11. Mesoderm from 4D cell

12. Feeding: Turbellaria focus
Type 1: Simple, non-eversible pharynx
Feeding
Ciliary action
Digestion
Sac-like gastro-vascular cavity
No gastrovascular cavity
How does digestion occur without a g.v. cavity?

13. Feeding: Turbellaria focus
Type 2: Eversible pharynx
Feeding (variations)
Lasso prey
Slimy secretions
Penis with stylet
Symbiotic algae
Digestion (variations)
Extracorporeal or prey ingested whole
Extracellular: often highly branched g.v. cavity
Intracellular (finish)
Waste exits via mouth; some with small anus

14. Feeding: Flukes Feeding Pharynx expansion or Absorption via tegument
What is tegument?
Digestion
Some extracorporeal
Simple g.v. cavity (1-2 branches; blind ends)

15. Feeding: Cestodes No mouth or digestive system
Why not needed? (HINT: Location!)
How do they obtain nutrients?
Highly specialized tegument with microtriches

16. Gas exchange Via body surface
Why effective?
Some distribution of gases via gastrovascular cavity
Especially when highly branched
Endoparasitic forms
Anaerobic respiration (in many)
Why effective for these animals?

17. Osmoregulation Protonephridia anatomy How does it work?
Flame bulb (flame = cilia)
Collecting tubules (ciliated)
Nephridiopore
Bladder (flukes only)
How does it work?
Best developed for which habitat?

18. Excretion
Ammonia loss primarily via…?

19. Nervous system Organization True neurons Unidirectional conduction
Ladder-like layout
Cerebral ganglia
No other ganglia
Distinct sensory vs. motor pathways

20. Nervous system (cont.) Variable role of nerve nets
Acoels: predominates; Polyclads: in addition to CNS
Multiple nerve cords in some

21. Sensory structures Adapted for bilateral symmetry, forward motion
Sensory structures concentrated at “head”
Additional sensory structures on entire body

22. Sensory structures Tactile receptors Chemoreceptors Rheoreceptors
Thigmotaxis
Chemoreceptors
Ex: Planaria auricles
Adaptive value of cilia?
Rheoreceptors
What are these?
Statocysts
In which types?
Note direct connections with cerebral ganglia

23. Sensory structures Ocelli Pigment cup (function?)
Retinular cells (functions?)

24. Movement/attachment Mesoderm-derived muscles Cilia-mucus
Longitudinal, circular, diagonal, dorso-ventral
Movement: peristalsis, etc…
Cilia-mucus
Also have duo-gland system: attach/detach
Parasitic groups: specialized attachment structures

25. Parasitic attachment Class Monogenea Prohaptor (anterior)
Sucker or adhesive disc
Opisthohaptor
Main attachment
Hooks or jaws

26. Parasitic attachment Class Trematoda Oral sucker
Acetabulum (ventral sucker)

27. Parasitic attachment Class Cestoda Scolex Hooks and suckers!
Photo: Kevin Mackenzie
Photo: Dennis Kunkel