invertebrate zoology lecture 9: phylum platyhelminthes part 1
Post on 22-Dec-2015
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TRANSCRIPT
Invertebrate Zoology
Lecture 9: Phylum Platyhelminthes
Part 1
Lecture outline
Phylum PlatyhelminthesPhylogeny/Evolutionary relationshipsDiversity overview: ClassesBauplan BasicsFeedingGas ExchangeOsmoregulation/ExcretionNervous SystemMovement/Attachment
Phylogeny (briefly)
Hypothesis 1: Cnidarian-like ancestorArose from planula larva
Hypothesis 2: Annelid-like ancestorLoss 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 hostMostly fish
Prohaptor (anterior)Sucker or adhesive
disc
OpisthohaptorHooked attachment
Diversity: Class Trematoda Flukes Endoparasitic
1-3 hostsOne always a snail
Oral sucker Acetabulum (ventral
sucker) Examples
Liver flukes, i. e. Clonorchis
Schistosoma
Diversity: Class Cestoda
Tapeworms Endoparasitic
Usually >1 host
ScolexAnterior attachment
StrobilaComposed of
proglottidsReproductive
No digestive tract
Photo: Kevin Mackenzie
Bauplan basics Triploblastic
True mesoderm muscles and mesenchyme
Bilateral symmetry & cephalizationWhat’s so great about
a head?Compare movement &
prey capture to radial phyla
Bauplan basics Share some protostome features
Spiral cleavageDeterminate cell fateMesoderm 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
cavityHow 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 orAbsorption via tegument
What is tegument?
DigestionSome extracorporealSimple g.v. cavity (1-2
branches; blind ends)
Feeding: Cestodes
No mouth or digestive systemWhy 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
OrganizationTrue neuronsUnidirectional
conductionLadder-like layoutCerebral 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 musclesLongitudinal, circular, diagonal, dorso-
ventral Movement: peristalsis, etc…
Cilia-mucusAlso have duo-gland system: attach/detach
Parasitic groups: specialized attachment structures
Parasitic attachment
Class MonogeneaProhaptor
(anterior) Sucker or
adhesive discOpisthohaptor
Main attachment
Hooks or jaws
Parasitic attachment
Class TrematodaOral suckerAcetabulum
(ventral sucker)
Parasitic attachment
Class CestodaScolex
Hooks and suckers!
Photo: Kevin Mackenzie
Photo: Dennis Kunkel