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 Photo: Kevin Mackenzie
Anterior attachment
Strobila
Composed of
proglottids
Reproductive
No digestive tract
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 gastro-
vascular 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, dorso-
ventral
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