Lecture 9: Phylum Platyhelminthes Part 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 Bilateral symmetry & cephalization 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 How does it work? 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 Chemoreceptors Rheoreceptors 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 Cilia-mucus 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