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Phylum Platyhelminthes (flatworms)

Phylum Platyhelminthes (flatworms). A phylogeny of Animalia. Platyhelminthes. Bilateria. Phylum Platyhelminthes. The Flatworms Part 1: Free-living Flatworms. Emerging Patterns in Evolution. Bilateral symmetry Dorsal & ventral, anterior & posterior

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Phylum Platyhelminthes (flatworms)

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  1. Phylum Platyhelminthes (flatworms)

  2. A phylogeny of Animalia Platyhelminthes Bilateria

  3. Phylum Platyhelminthes The Flatworms Part 1: Free-living Flatworms

  4. Emerging Patterns in Evolution • Bilateral symmetry • Dorsal & ventral, anterior & posterior • Associated with locomotion on a solid surface • Evolution of a head end: cephalization • Possess a head (leading end with sense organs) • Internal fertilization

  5. Bauplan • Dorsoventrally flattened • Lack circulatory system for transport‑ must rely on diffusion for gas exchange • Triploblastic • Embryo has 3 cell layers • Ectoderm (cnidarians) • Endoderm (cnidarians) • AND NOW, introducing THEMESODERM! • Acoelomate - Hey, where’s my cavity?

  6. Phylum Platyhelminthes – tissue organization • Triploblastic = 3 tissue layers • Ectoderm (outer)Endoderm (inner)Mesoderm (between)

  7. Muscles and Movement Hydrostatic Skeleton

  8. Bauplan • Nervous system • Anterior brain • Paired ganglia and nerves • Pairedventral nerve cords • Longitudinal • Lateral • Incomplete gut • No anus • Food exits via mouth

  9. Phylum Platyhelminthes - digestive system • Gastrovascular cavity • Protrusible pharynx in some taxa • Digestion is both intracellular and extracellular

  10. Bauplan • Excretory system & osmoregulation • Diffusion across body wall • Protonephridia • Specialized excretory organs • Probably mainly for water balance - produces urine- a filtrate of extracellular fluid • May also help osmoregulate

  11. Phylum Platyhelminthes- excretory system • “Protonephridial” system: blind‑ending tubes with flame cells & fenestrae at ends, connected with excretory pores • Probably mainly for water balance produces urine- a filtrate ofextracellular fluid • Flatworms have extracellular fluid compartmentcells protected from external environment • Cnidaria lack this- cells are in contact with water (in freshwater cnidaria, each cell has prominent contractile vacuoles)

  12. Protonephridial osmoregulatory/excretory system “flame cell” fenestrae duct

  13. Bauplan • Lack cuticle or other rigid skeletal support • Lack fluid-filled body cavity • Spermatozoa possess a pair of flagella that arise from the paired centrioles • Epidermal cells are typically multiciliated. • Neoblasts---stem cells that give rise to the other cell types in the body.

  14. endoderm

  15. Life History • Most are simultaneous hermaphrodites • Function as male and female simultaneously • Transfer sperm and receive sperm simultaneously • Monoecious • Most are not self-fertile • (A few exceptions) • Most are oviparous (= produce shelled eggs that are released and hatch outside the body). • Most can also reproduce asexually by budding or transverse fission

  16. Asexual reproduction by transverse fission (budding) • Interesting in 2 ways: • Another flatworm class (cestoda) is segmented ‑ they grow by serial reproduction of body parts • If you cut a turbellarian in half, it can regenerate • Regeneration involves undifferentiated cells in mesoderm called neoblasts

  17. Class Cestoda(tapeworms) all parasitic Class Monogenea(monogenetic trematodes, gill flukes) all parasitic Class Trematoda(digenetic trematodes, flukes) all parasitic Class Turbellaria(turbellarians)mostly free-living Platyhelminthes- 4 classes traditional

  18. Platyhelminthes • Inability of flatworms to synthesize fatty acids and sterols may explain their need for establishing symbiotic relationships • The parasitic classes of flatworms are linked by a synapomorphy- the tegument(see next slide) • Clade Neodermata

  19. Turbellaria “Neodermata” The parasitic flatworms) (Tegument)

  20. Classification • Phylum Platyhelminthes • Classes • Acoelomorpha: • Acoels (free-living) + Neodermata (parasites) • Turbellaria: free-living flatworms • Trematoda: flukes • All are parasites • Cestoda: tapeworms • All are parasites • Monogenea? (Maybe belong with cestoda) • Ectoparasites of fish

  21. Turbellarians were traditionally classified based on the form of the gastrovascular cavity

  22. Acoels are not Platyhelminthes??? • rRNA analysis (see cladograms) shows that Acoela is basal to other bilateral clades • Digestive syncytium instead of a gut • Netlike nervous system, lack brain • Kinked cilia • Acoel eggs cleave only once and the two resulting cells immediately generate many small cells. • Link to more info on Acoela: http://devbio.umesci.maine.edu/styler/globalworming/

  23. 18S rDNA-based maximum-likelihood tree of 61 metazoan species From:Ruiz-Trillo et al. (1999) Acoel flatworms: earliest extant bilaterian metazoans, not members of Platyhelminthes. Science 283:1919-1923 Platyhelminthes

  24. Class Turbellaria • Order Acoela • Small flatworms with no permanent gut cavity. • Free-living, marine and brackish water.

  25. Rhabodocoel turbellarians • Diverse and important predators on meiofauna in freshwater ecosystems • Recent research on predation on freshwater bivalves

  26. Mesostoma – a viviparous rhabdocoel – note the embryos within the uterus

  27. C. Barnhart Mesostoma – close-up showing the babies Mom’s oral sucker Babies Mom’s eyes

  28. Macrostomum – predator on juvenile bivalves

  29. Eggs of Macrostomum

  30. Platyhelminthes • Turbellaria • Tricladida Bipalium A land planarian common in greenhouses world-wide – causing problems in Britain

  31. Bipalium feeding on an earthworm

  32. Class Turbellaria • Order Tricladida • Gut with three branches • Free living • Freshwater • Planarians such as Dugesia • A few inhabit terrestrial (moist) habitats • Neodermata • Exclusively parasitic

  33. Class Turbellaria • Marine species (beautiful colors) • Significant members of coral reef ecosystems • Some are major predators of colonial ascidians (sea squirts) • Others are pests of commercial clams and oysters • Some live symbiotically with/on a variety of reef invertebrates

  34. Bdelloura- a marine triclad that is commensal with horseshoe crabs (most turbellarians are not parasites)

  35. Class Turbellaria • Locomotion • Usually by cilia on a layer of mucus • Dugesia can move at about 1.5 mm/sec • Peristaltic waves can achieve higher velocities • Marine flatworms can use this to swim gracefully • Also use cilia same way as protists do

  36. Rhabdites secrete mucus

  37. Class Turbellaria • Locomotion • Terrestrial planarians • Glide smoothly on the substrate by the action of powerful, closely spaced cilia in a special medial ventral strip (creeping sole), on a thin coat of mucus secreted on the substrate by glands opening into the creeping sole • Planarians that migrate on plants or objects above the ground sometimes lower themselves to the ground on a string of mucus.

  38. Class Turbellaria • Body construction • Lab notes • Live Dugesia • Slides of whole planaria and cross sections

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