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CHAPTER 14

CHAPTER 14. Flat Worms: Phylum Platyhelminthes. General Features. Animals that actively seek food, shelter, home sites, and mates require a different set of strategies and body organization than radially symmetrical sessile organisms Two major evolutionary advances in phylum Cephalization

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CHAPTER 14

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  1. CHAPTER 14 Flat Worms: Phylum Platyhelminthes

  2. General Features • Animals that actively seek food, shelter, home sites, and mates require a different set of strategies and body organization than radially symmetrical sessile organisms • Two major evolutionary advances in phylum • Cephalization • Concentrating sense organs in the head region • Bilateral symmetry • Body can be divided along only 1 plane of symmetry to yield 2 mirror images of each other • First phylum with Right and Left sides

  3. General Features • Acoelomates • Typical acoelomates have only one internal space, the digestive cavity • Without coelom (additional body cavity) • Triploblastic • Endoderm, Ectoderm, and Mesoderm • 1st phyla to have 3 germ layers • Protostomes • blastopore becomes the mouth • Incomplete Gut • One opening

  4. Diagram of an Acoelomate Body Plan

  5. Phylum Platyhelminthes Characteristics • Commonly called flatworms • Vary from a millimeter to many meters in length • Some free-living; others parasitic

  6. Stained Planaria Terrestrial flatworm

  7. Phylum Platyhelminthes • Platyhelminthes is divided into three classes:Turbellaria (Planaria), Trematoda (flukes), and Cestoda (tapeworm) • All members of Trematoda (flukes)and Cestoda (tapeworms) are parasitic • Class Turbellaria • Mostly free-living forms • Most are bottom dwellers in marine or freshwater • Freshwater planarians • Found in streams, pools, and hot springs • Terrestrial flatworms limited to moist places

  8. Phylum Platyhelminthes Form and Function • Epidermis and Muscles • Most have cellular, ciliated epidermis on a basement membrane • Most turbellarians have dual-gland adhesive organs • Viscid gland cells fasten microvilli of anchor cells to substrate • Secretions of gland cells provide a quick chemical detachment

  9. Cross Section Of Planaria Turbellaria

  10. Releasing and Attaching (Viscid) Glands

  11. Phylum Platyhelminthes Nutrition and Digestion • Some have a mouth, pharynx, and intestine • In planarians • Pharynx may extend through the ventral mouth • Intestine has three branches • One anterior and two posterior • Mouth of trematodes (parasitic flukes) • Opens near the anterior end • Pharynx is not extensible • Intestine ends blindly, varies in degree of branching

  12. Structure of Planarian

  13. Human Liver Fluke - trematode

  14. Phylum Platyhelminthes • Planaria (Turbellaria) • Carnivorous and detect food by chemoreceptors • Food trapped in mucous secretions from glands • Wrap themselves around prey • Extend the pharynx to suck up bits of food

  15. Phylum Platyhelminthes • Trematodes (parasitic flukes) • Feed on host cells, cellular debris, and body fluids • Enzymes from the intestine are secreted for extracellular digestion • Phagocytic cells in gastrodermis complete digestion at intracellular level • Undigested food egested out the pharynx • Cestodes (tapeworm) • Rely on the host’s digestive tract • Absorb digested nutrients

  16. Phylum Platyhelminthes Excretion and Osmoregulation • Flatworms have protonephridia (kidney) • Used for osmoregulation • Beating flagella drive fluids down collecting ducts • Wall of the duct beyond the flame cell bears folds or microvilli to resorb ions and molecules • Majority of metabolic wastes • Removed by diffusion through body wall • Collecting ducts join and empty at nephridiopores

  17. Phylum Platyhelminthes Nervous System • Subepidermal nerve plexus resembles nerve net of cnidarians • One to five pairs of longitudinal nerve cords lie under the muscle layer • Freshwater planarians • Brain is a bilobed cerebral ganglion (mass of nerve cells) anterior to the ventral nerve cords

  18. Phylum Platyhelminthes Sense Organs • Ocelli (light-sensitive eyespots) • Present in turbellarians, and larval trematodes • Tactile and chemoreceptive cells • Abundant • Statocysts (equilibrium) and rheoreceptors (sense direction of water currents)

  19. Phylum Platyhelminthes Reproduction and Regeneration • Fission • Many turbellarians constrict behind the pharynx and separate into two animals • Each half regenerates the missing parts • Provides for rapid population growth • Regeneration • If the head and tail are cut off • Each end grows the missing part; it retains polarity

  20. Phylum Platyhelminthes • Nearly all are monoecious (hermaphroditic) but cross-fertilize • Male Structures • One or more testes are connected to one vas deferens • The vas deferens runs to a seminal vesicle • A nipple-like penis or extensible tentacle is the copulatory organ

  21. Phylum Platyhelminthes • Turbellarians develop male and female organs opening at a common pore • After copulation, eggs and yolk cells enclosed in small cocoon • Attach by a stalk to plants • Embryos emerge and resemble little adults

  22. Phylum Platyhelminthes Classification of Phylum Platyhelminthes • Class Turbellaria - planaria • Class Trematoda - flukes • Class Cestoda - tapeworm 14-23

  23. Phylum Platyhelminthes Class Turbellaria • Mostly free-living • Range from 5 mm to 50 cm long • Very small planaria swim by cilia • Others move by cilia • Glide over a slime track secreted by adhesive glands • Rhythmical muscular waves pass backward from the head

  24. Different Intestinal Pattern of Turbellarians

  25. Marine tubellarian

  26. Phylum Platyhelminthes Classification of Phylum Platyhelminthes • Class Turbellaria - planaria • Class Trematoda - flukes • Class Cestoda - tapeworm 14-27

  27. Phylum Platyhelminthes Class Trematoda • All trematodes are parasitic flukes • Most adults are endoparasites (inside) of vertebrates • They resemble turbellaria but the tegument (skin) lacks cilia in adults • Sense organs are poorly developed • Adaptations for parasitism include: • Penetration glands • Hooks and suckers for adhesion • Increased reproductive capacity

  28. Phylum Platyhelminthes • General Trematoda Life Cycle • Egg passes from definitive host and must reach water • Hatches into a free-swimming ciliated larva, the miracidium • Miracidium penetrates tissues of a snail • Transforms into a sporocyst • Sporocyst reproduces asexually to form redia • Rediae reproduce asexually and form cercaria

  29. Phylum Platyhelminthes • Cercariae emerge from the snail • Penetrate a 2nd intermediate host (fish) • Develop into metacercariae (juvenile flukes) • Metacercaria develop into adults when eaten by definitive host • Some serious parasites of humans and domestic animals are trematodes • Example: sheep live fluke, human liver fluke

  30. Phylum Platyhelminthes Sheep Liver Fluke (Fasciola hepatica) • Adult fluke lives in bile passageways in the liver of sheep • Eggs are passed out in feces • Miracidia hatch and penetrate snails to become sporocysts • After two generations of rediae • Cercaria encyst on vegetation and await being eaten by sheep • When eaten, metacercariae develop into young flukes

  31. Phylum Platyhelminthes Clonorchis sinensis: Human Liver Fluke • Most important human liver fluke • Common in China, Japan, and Southeast Asia • Also infects cats, dogs, and pigs • Adult fluke is 10–20 mm long with an oral and ventral sucker

  32. Phylum Platyhelminthes Clonorchis Life Cycle (Liver Fluke) • Adults live in bile passageways of humans and other fish-eating mammals (sexual reproduction occurs here) • Eggs containing a complete miracidium are shed into water with feces • The eggs hatch only when ingested by snails of specific genera • Miracidium enters snail tissue and transforms into a sporocyst • Sporocyst produces one generation of rediae, which begin differentiation (asexual reproduction)

  33. Human Liver Fluke Life Cycle

  34. Phylum Platyhelminthes • Rediae pass into the snail liver • Turns into tadpole-like cercariae • Cercariae escape into water • Make contact with fish • Bore into fish muscles or under scales • Shed tail and turn into a metacercariae cyst • A mammal eats raw fish • Cyst dissolves and flukes migrate up bile duct • Heavy infection can destroy the liver and result in death • Control of parasites • Destroy snails and thoroughly cook fish

  35. Phylum Platyhelminthes Schistosoma: Blood Flukes • Over 200 million people infested with schistosomiasis • Common in Africa, South America, West Indies, and the Middle and Far East • Sexes are separate • 3 species with varied location: • large intestine • small intestine • urinary bladder

  36. Phylum Platyhelminthes Schistosoma Life Cycle • Eggs discharged in human feces or urine • In water, eggs hatch as ciliated miracidia • Contact with a particular species of snail to survive • In the snail, they transform to sporocysts • Sporocysts produce cercaria directly • Cercariae escape the snail and swim until they contact bare human skin or other host • Cercariae pierce the skin and shed their tails

  37. Phylum Platyhelminthes • Enter blood vessels and migrate to the hepatic portal blood vessels (blood vessel from digestive tract to liver) • Develop in the liver and they migrate to target sites • Eggs released by females are extruded through gut or bladder lining and exit with feces or urine • Eggs that remain behind become centers of inflammation

  38. Blood Fluke Life Cycle

  39. Cut Liver of individual who dies from hematemesis (vomiting blood). 180 adult flukes were found in autopsy

  40. Phylum Platyhelminthes • Control: proper disposal of human wastes • Schistosoma dermatitis (swimmer’s itch) • Occurs when cercariae penetrate an unsuitable host such as a human (our immune system fights them off leading to inflammation -itch) • Normal host many be a bird or other animal

  41. Lung Fluke - from uncooked crab meat

  42. Phylum Platyhelminthes Classification of Phylum Platyhelminthes • Class Turbellaria - planaria • Class Trematoda - flukes • Class Cestoda - tapeworm

  43. Phylum Platyhelminthes Class Cestoda • Tapeworms have long flat bodies with scolex • Holdfast structure with suckers and hooks • Scolex is followed by a linear series of reproductive units or proglottids • Lack a digestive system • Lack sensory organs except for modified cilia

  44. Tapeworm: Scolex is site of attachment.

  45. Tegument of a Tapeworm: Many microthriches help increase surface area for absorption.

  46. Phylum Platyhelminthes • Tegument is has no cilia • Entire surface of cestodes is covered with projections (microtriches) similar to microvilli seen in the vertebrate small intestine • Microtriches increase the surface area for food absorption

  47. Phylum Platyhelminthes • Nearly all cestodes require two hosts • Adult is parasitic in the digestive tract of the vertebrate • Over 1000 species of tapeworms known, infecting almost all vertebrates • Most tapeworms do little harm to host

  48. Phylum Platyhelminthes Taenia saginata: Beef Tapeworm • Lives as an adult in the digestive canals of humans • Juvenile form found in intermuscular tissue of cattle • Mature adults can reach over 10 meters in length with over 2000 proglottids (segments conaining reproductive organs) • Scolex has four suckers but no hooks • Gravid proglottids (with shelled, infective larvae) pass in feces, single • Proglottids rupture as they dry • Embryos are viable for five months and are picked up by grazing

  49. Phylum Platyhelminthes Beef Tapeworm Life Cycle - Continued • Cattle swallow shelled larvae that hatch as oncospheres • Oncospheres use hooks to burrow through the intestinal wall into blood or lymph vessels • When they reach voluntary muscle, they encyst to become “bladder worms” (cyst that resembles a bladder) • When the infected meat is eaten, the cyst wall dissolves and the scolex evaginates to attach to intestinal wall

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