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Understanding Unsegmented Worms: Phylum Platyhelminthes Overview

Learn about characteristics, classes, and behaviors of unsegmented worms known as Platyhelminthes, including their unique body structure, reproduction, feeding habits, and ecological roles.

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Understanding Unsegmented Worms: Phylum Platyhelminthes Overview

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  1. Unsegmented Worms Phylum Platyhelminthes

  2. I. Unsegmented worms A. = Worms that are not divided into sections externally & internally B. E.g. 1. Phylum Platyhelminthes aka flatworms 2. Phylum Nematoda aka Roundworms

  3. C. What is a coelom? = fluid-filled body cavity between body wall & digestive tract

  4. Kinds of coelom • Acoelomate= no coelom • i.e. body cavity is • NOT fluid filled

  5. Kinds of coelom 2. Pseudocoelomate= Fluid filled body cavity BUT No mesentary or peritoneum

  6. Kinds of coelom 3. Eucoelomate = True coelom w/ mesentary (holds gut in place) & peritoneum (mesodermal lining of body cavity

  7. D. Phylum Platyhelminthes • Class : Turbellaria – planarians 2. Class : Trematoda – flukes 3. Class : Cestoda – tapeworms

  8. D. Phylum Platyhelminthes Also Acoelomate: • Phylum Nemertea – ribbon worms • Phylum Gnathostomulida- jaw worms

  9. E. General Info re: Flatworms 1. = “platy” = flat “helminth” = worm 2. Body structure a. Size range 1 mm ------► many meters (Cestoda) b. Shape – flattened dorso-ventrally c. Only several cell layers thick

  10. E. General Info re: Flatworms  Cells fill body cavity (not fluid)

  11. F. Level of Organization = organ system

  12. G. Symmetry • bilateral • (has forward movement & cephalization) I ‘m a little bilateral animal too!

  13. H. Habitat • aquatic (freshwater or marine) • moist soil • endoparasites of vertebrates

  14. Planarians- mostly free-living carnivore or scavengers I. Feeding - Class Turbellaria

  15. Incomplete digestive tract – one opening (no anus) Mouth on ventral side Protrusible pharynx Gastrovascular cavity (GVC) “Intestine” very branched Enzymes digest food I. Feeding - Class Turbellaria

  16. I. Feeding - Class Trematoda • Parasitic flukes • endoparasites of vertebrates • need only minimal digestive tract • E.g. Human Liver fluke

  17. I. Feeding – Class Cestoda tapeworms • Endo-parasites • Scolex w/ hooks & suckers • NO need for dig. tract – absorbs nutrients from gut of host scolex suckers

  18. J. Respiration • - via diffusion • (only a few cell layers thick)

  19. K. Internal transport • via diffusion Marine flatworm

  20. L. Excretion – via diffusion • EXCEPTION: planarians have • flame cells ( w/ flagella • that help move waste to • excretory pores, then out of the body

  21. M. Response – (Nervous system) • Primitive brain – anterior ganglia ganglion = cluster of nerve cells 2. Two Longitudinal nerves (the length of the body) 3. Ladder-like cross-bridges of nerves

  22. M. Response – (Nervous system)

  23. M. Response – (Nervous system) 4. Sense organs: (flatworms) a. Ocelli – eyespots / photodetectors b. Auricles - contain chemoreceptors (chemicals) & thigmoreceptors (touch) c. Statocysts – balance d. Rheoreceptors – sense direction of water current

  24. N. Locomotion - Planaria – use cilia, slime, circular and longitudinal muscles to GLIDE • Trematoda, Cestoda – little motion cilia

  25. O. Reproduction 1. Asexual – Regeneration (Planaria only) 2. Sexual – a. pattern = monoecious (hermaphrodites) b. cross- fertilization (swap sperm)

  26. P. Ecological Roles 1. Scavengers/ predators – recycle nutrients to ecosystem (Class Turbellaria) 2. Prey for fish & birds (Class Turbellaria) 3. Endo-parasites (cause disease) (Class Trematoda & Class Cestoda)

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