Download PowerPoint 9: Platyhelminthes 1

Invertebrate Zoology
Lecture 9: Phylum Platyhelminthes
Part 1
Lecture outline
 Phylum Platyhelminthes
Phylogeny/Evolutionary relationships
Diversity overview: Classes
Bauplan Basics
Feeding
Gas Exchange
Osmoregulation/Excretion
Nervous System
Movement/Attachment
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
Diversity: Class Turbellaria
 Mostly free-living
 Carnivores and
scavengers
 Head
Sensory structures
 Found in diverse
aquatic & moist
habitats
Diversity: Class Monogenea
 Flukes
 Ectoparasitic
Single host
Mostly fish
 Prohaptor (anterior)
Sucker or adhesive
disc
 Opisthohaptor
Hooked attachment
Diversity: Class Trematoda
 Flukes
 Endoparasitic
1-3 hosts
One always a snail
 Oral sucker
 Acetabulum (ventral
sucker)
 Examples
Liver flukes, i. e.
Clonorchis
Schistosoma
Diversity: Class Cestoda
 Tapeworms
 Endoparasitic
Usually >1 host
 Scolex
Anterior attachment
 Strobila
Composed of
proglottids
Reproductive
 No digestive tract
Photo: Kevin Mackenzie
Bauplan basics
 Triploblastic
True mesoderm 
muscles and
mesenchyme
 Bilateral symmetry &
cephalization
What’s so great about
a head?
Compare movement &
prey capture to radial
phyla
Bauplan basics
 Share some protostome features
Spiral cleavage
Determinate cell fate
Mesoderm from 4D cell
 No coelom! (=Acoelomates)
Spiral cleavage
Mesoderm from 4D cell
Feeding: Turbellaria focus
 Type 1: Simple, non-eversible pharynx
Feeding
 Ciliary action
Digestion
 Sac-like gastrovascular cavity
 No gastrovascular
cavity
How does digestion
occur without a g.v.
cavity?
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
Feeding: Flukes
 Feeding
Pharynx expansion or
Absorption via tegument
 What is tegument?
 Digestion
Some extracorporeal
Simple g.v. cavity (1-2
branches; blind ends)
Feeding: Cestodes
 No mouth or
digestive system
Why not needed?
(HINT: Location!)
How do they obtain
nutrients?
 Highly specialized
tegument with
microtriches
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?
Osmoregulation

Protonephridia
anatomy
 Flame bulb (flame =
cilia)
 Collecting tubules
(ciliated)
 Nephridiopore
 Bladder (flukes only)


How does it work?
Best developed for
which habitat?
Excretion
 Ammonia loss primarily via…?
Nervous system
 Organization
True neurons
Unidirectional
conduction
Ladder-like layout
Cerebral ganglia
 No other ganglia
Distinct sensory vs.
motor pathways
Nervous system (cont.)
 Variable role of nerve nets
Acoels: predominates; Polyclads: in addition to CNS
 Multiple nerve cords in some
Sensory structures

Adapted for bilateral symmetry, forward motion
 Sensory structures concentrated at “head”
 Additional sensory structures on entire body
Sensory structures

Tactile receptors
 Thigmotaxis

Chemoreceptors
 Ex: Planaria auricles
 Adaptive value of cilia?

Rheoreceptors
 What are these?

Statocysts
 In which types?
 Note direct connections
with cerebral ganglia
Sensory structures

Ocelli
 Pigment cup (function?)
 Retinular cells (functions?)
Movement/attachment
 Mesoderm-derived muscles
Longitudinal, circular, diagonal, dorsoventral
 Movement: peristalsis, etc…
 Cilia-mucus
Also have duo-gland system: attach/detach
 Parasitic groups: specialized attachment
structures
Parasitic attachment
 Class Monogenea
Prohaptor
(anterior)
 Sucker or
adhesive disc
Opisthohaptor
 Main
attachment
 Hooks or jaws
Parasitic attachment
 Class Trematoda
Oral sucker
Acetabulum
(ventral sucker)
Parasitic attachment
 Class Cestoda
Scolex
 Hooks and suckers!
Photo: Kevin Mackenzie
Photo: Dennis Kunkel