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Invertebrate Zoology
Lecture 9: Phylum Platyhelminthes, Part 1
I.
Phylum Platyhelminthes
A.
Phylogeny/Evolutionary relationships
B.
Diversity overview: Classes
C.
Bauplan Basics
D.
Feeding
E.
Gas Exchange
F.
Osmoregulation/Excretion
G.
Nervous System
H.
Movement/Attachment
I.
Phylogeny/Evolutionary relationships (briefly)
A.
Hypothesis 1: Cnidarian-like ancestor
1.
Arose from planula larva
B.
Hypothesis 2: Annelid-like ancestor
1.
Loss of coelom (by “filling in”)
C.
Hypothesis 3: Benthic ctenophore ancestor
Diversity: Classes
A.
Class Turbellaria
1.
Mostly free-living
2.
Carnivores and scavengers
3.
Head
a)
Sensory structures
4.
Found in diverse aquatic & moist habitats
B.
Class Monogenea
1.
Flukes
2.
Ectoparasitic
a)
Single host
b)
Mostly fish
3.
Prohaptor (anterior)
a)
Sucker or adhesive disc
4.
Opisthohaptor
a)
Hooked attachment
C.
Class Trematoda
1.
Flukes
2.
Endoparasitic
a)
1-3 hosts
b)
One always a snail
3.
Oral sucker
4.
Acetabulum (ventral sucker)
5.
Examples
a)
Liver flukes, i. e. Clonorchis
b)
Schistosoma
D.
Class Cestoda
1.
Tapeworms
2.
Endoparasitic
a)
Usually >1 host
3.
Scolex
a)
Anterior attachment
4.
Strobila
a)
Composed of proglottids
b)
Reproductive
5.
No digestive tract
II.
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III.
Bauplan Basics
A.
Triploblastic
1.
True mesoderm  muscles and mesenchyme
B.
Bilateral symmetry & cephalization
1.
What’s so great about a head?
2.
Compare movement & prey capture to radial phyla
C.
Share some protostome features
1.
Spiral cleavage
2.
Determinate cell fate
3.
Mesoderm from 4D cell
D.
No coelom! (=Acoelomates)
IV.
Feeding: Class Turbellaria focus
A.
Type 1: Simple, non-eversible pharynx
1.
Feeding
a)
Ciliary action
2.
Digestion
a)
Sac-like gastro-vascular cavity
b)
No gastrovascular cavity

How does digestion occur without a g.v. cavity?
B.
Type 2: Eversible pharynx
1.
Feeding (variations)
a)
Lasso prey
b)
Slimy secretions
c)
Penis with stylet
d)
Symbiotic algae
2.
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
V.
Feeding: Flukes (Class Monogenea, Class Trematoda)
A.
Feeding
1.
Pharynx expansion or
2.
Absorption via tegument
a)
What is tegument?
B.
Digestion
1.
Some extracorporeal
2.
Simple g.v. cavity (1-2 branches; blind ends)
VI.
Feeding (Class Cestoda)
A.
No mouth or digestive system
1.
Why not needed? (HINT: Location!)
2.
How do they obtain nutrients?
a)
Highly specialized tegument with microtriches
VII. Gas exchange
A.
Via body surface
1.
Why effective?
B.
Some distribution of gases via gastrovascular cavity
1.
Especially when highly branched
C.
Endoparasitic forms
1.
Anaerobic respiration (in many)
2.
Why effective for these animals?
VIII. Osmoregulation/excretion
A.
Protonephridia anatomy
1.
Flame bulb (flame = cilia)
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IX.
X.
2.
Collecting tubules (ciliated)
3.
Nephridiopore
4.
Bladder (flukes only)
B.
How does it work?
C.
Best developed for which habitat?
D.
Excretion
1.
Ammonia loss primarily via…?
Nervous system
A.
Organization
1.
True neurons
2.
Unidirectional conduction
3.
Ladder-like layout
4.
Cerebral ganglia
a)
No other ganglia
5.
Distinct sensory vs. motor pathways
B.
Variable role of nerve nets
1.
Acoels: predominates; Polyclads: in addition to CNS
C.
Multiple nerve cords in some
D.
Sensory structures
1.
Tactile receptors
a)
Thigmotaxis
2.
Chemoreceptors
a)
Ex: Planaria auricles
b)
Adaptive value of cilia?
3.
Rheoreceptors
a)
What are these?
4.
Statocysts
a)
In which types?
5.
Note direct connections with cerebral ganglia
6.
Ocelli
a)
Pigment cup (function?)
b)
Retinular cells (functions?)
Movement/attachment
A.
Mesoderm-derived muscles
1.
Longitudinal, circular, diagonal, dorso-ventral
a)
Movement: peristalsis, etc…
B.
Cilia-mucus
1.
Also have duo-gland system: attach/detach
C.
Parasitic groups: specialized attachment structures
1.
Class Monogenea
a)
Prohaptor (anterior)

Sucker or adhesive disc
b)
Opisthohaptor

Main attachment

Hooks or jaws
2.
Class Trematoda
a)
Oral sucker
b)
Acetabulum (ventral sucker)
3.
Class Cestoda
a)
Scolex

Hooks and suckers!
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