Phylum Platyhelminthes

Phylum Platyhelminthes

  1. Phylum Platyhelminthes
  2. Phylogeny/Evolutionary relationships
  3. Diversity overview: Classes
  4. Bauplan Basics
  5. Feeding
  6. Gas Exchange
  7. Osmoregulation/Excretion
  8. Nervous System
  9. Movement/Attachment

  1. Phylogeny/Evolutionary relationships (briefly)
  2. Hypothesis 1: Cnidarian-like ancestor
  3. Arose from planula larva
  4. Hypothesis 2: Annelid-like ancestor
  5. Loss of coelom (by “filling in”)
  6. Hypothesis 3: Benthic ctenophore ancestor
  7. Diversity: Classes
  8. Class Turbellaria
  9. Mostly free-living
  10. Carnivores and scavengers
  11. Head

a)Sensory structures

  1. Found in diverse aquatic & moist habitats
  1. Class Monogenea
  2. Flukes
  3. Ectoparasitic

a)Single host

b)Mostly fish

  1. Prohaptor (anterior)

a)Sucker or adhesive disc

  1. Opisthohaptor

a)Hooked attachment

  1. Class Trematoda
  2. Flukes
  3. Endoparasitic

a)1-3 hosts

b)One always a snail

  1. Oral sucker
  2. Acetabulum (ventral sucker)
  3. Examples

a)Liver flukes, i. e. Clonorchis

b)Schistosoma

  1. Class Cestoda
  2. Tapeworms
  3. Endoparasitic

a)Usually >1 host

  1. Scolex

a)Anterior attachment

  1. Strobila

a)Composed of proglottids

b)Reproductive

  1. No digestive tract
  1. Bauplan Basics
  2. Triploblastic
  3. True mesoderm  muscles and mesenchyme
  4. Bilateral symmetry & cephalization
  5. What’s so great about a head?
  6. Compare movement & prey capture to radial phyla
  7. Share some protostome features
  8. Spiral cleavage
  9. Determinate cell fate
  10. Mesoderm from 4D cell
  11. No coelom! (=Acoelomates)
  12. Feeding: Class Turbellaria focus
  13. Type 1: Simple, non-eversible pharynx
  14. Feeding

a)Ciliary action

  1. Digestion

a)Sac-like gastro-vascular cavity

b)No gastrovascular cavity

How does digestion occur without a g.v. cavity?

  1. Type 2: Eversible pharynx
  2. Feeding (variations)

a)Lasso prey

b)Slimy secretions

c)Penis with stylet

d)Symbiotic algae

  1. Digestion (variations)

a)Extracorporeal or prey ingested whole

b)Extracellular: often highly branched g.v. cavity

c)Intracellular (finish)

d)Waste exits via mouth; some with small anus

  1. Feeding: Flukes (Class Monogenea, Class Trematoda)
  2. Feeding
  3. Pharynx expansion or
  4. Absorption via tegument

a)What is tegument?

  1. Digestion
  2. Some extracorporeal
  3. Simple g.v. cavity (1-2 branches; blind ends)
  1. Feeding (Class Cestoda)
  2. No mouth or digestive system
  3. Why not needed? (HINT: Location!)
  4. How do they obtain nutrients?

a)Highly specialized tegument with microtriches

  1. Gas exchange
  2. Via body surface
  3. Why effective?
  4. Some distribution of gases via gastrovascular cavity
  5. Especially when highly branched
  6. Endoparasitic forms
  7. Anaerobic respiration (in many)
  8. Why effective for these animals?
  9. Osmoregulation/excretion
  10. Protonephridia anatomy
  11. Flame bulb (flame = cilia)
  12. Collecting tubules (ciliated)
  13. Nephridiopore
  14. Bladder (flukes only)
  15. How does it work?
  16. Best developed for which habitat?
  17. Excretion
  18. Ammonia loss primarily via…?
  19. Nervous system
  20. Organization
  21. True neurons
  22. Unidirectional conduction
  23. Ladder-like layout
  24. Cerebral ganglia

a)No other ganglia

  1. Distinct sensory vs. motor pathways
  1. Variable role of nerve nets
  2. Acoels: predominates; Polyclads: in addition to CNS
  3. Multiple nerve cords in some
  4. Sensory structures
  5. Tactile receptors

a)Thigmotaxis

  1. Chemoreceptors

a)Ex: Planaria auricles

b)Adaptive value of cilia?

  1. Rheoreceptors

a)What are these?

  1. Statocysts

a)In which types?

  1. Note direct connections with cerebral ganglia
  2. Ocelli

a)Pigment cup (function?)

b)Retinular cells (functions?)

  1. Movement/attachment
  2. Mesoderm-derived muscles
  3. Longitudinal, circular, diagonal, dorso-ventral

a)Movement: peristalsis, etc…

  1. Cilia-mucus
  2. Also have duo-gland system: attach/detach
  3. Parasitic groups: specialized attachment structures
  4. Class Monogenea

a)Prohaptor (anterior)

Sucker or adhesive disc

b)Opisthohaptor

Main attachment

Hooks or jaws

  1. Class Trematoda

a)Oral sucker

b)Acetabulum (ventral sucker)

  1. Class Cestoda

a)Scolex

Hooks and suckers!

Page 1 of 3

Top View