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