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Bilaterally Symmetrical Worms

Bilaterally Symmetrical Worms. Bilateralism allows more active pursuit of prey and sophisticated behaviors. Flatworms (Phylum Platyhelminthes). Simplest bilateral body plan Flat back & belly Central nervous system Simple brain (nerve cells) Nerve cords Coordinates movement 3 cell layers

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Bilaterally Symmetrical Worms

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  1. Bilaterally Symmetrical Worms • Bilateralism allows more active pursuit of prey and sophisticated behaviors

  2. Flatworms (Phylum Platyhelminthes) • Simplest bilateral body plan • Flat back & belly • Central nervous system • Simple brain (nerve cells) • Nerve cords • Coordinates movement • 3 cell layers • Epidermis (external) • Mesoderm (middle – muscles, reproductive system) • Internal

  3. Flatworm examples: • Turbellarians – free living carnivores • Flukes (trematodes) – parasites (common in fish, seabirds,whales) • Adults always live in vertebrates/larvae inhabit invertebreates • Tapeworms (cestodes) – parasites with long body of repeating units (attached to intestine wall) • Lack gut or mouth • Absorb nutrients directly across body wall • Largest was found in a whale (40m)

  4. Ribbon Worms (Nemerta) • More complex than flatworms: • Complete digestive tract (gut with 2 openings) • Circulatory system (blood transports nutrients and oxygen to tissues) • Proboscis – long tube from mouth used to capture prey (Most distinctive feature) • All are predators – feed on worms/crustaceans

  5. Nematodes (Roundworms) • Slender cylindrical bodies with pointed ends • Hydrostatic skeleton - Muscle/fluid system • Provides support • Aids in locomotion • Diverse lifestyles: • Parasites • Bottom dwellers

  6. Estimates between 10,000 – 25,000 species • Anisakis – can inhabit intestines of seals and dolphins • Larvae found in flesh of fish • May infect humans who eat raw/undercooked fish

  7. Segmented Worms (Phylum Annelida) • Segmentation: series of similar compartments • Coelom: true body cavity • Longitudinal muscles – lengthen/shorten segments • Circular muscles – increase/reduce diameter • Effective crawlers/burrowers Closed circulatory system • Blood remains in vessels to transport nutrients, oxygen, and carbon dioxide

  8. Polychaetes Almost all marine annelids are polychaetes • Parapodia – flattened extensions from each segment and have stiff, sharp setae (bristles) • Closed circulatory system • Excretory organs • Gills – absorb oxygen, release carbon dioxide

  9. BENTHIC DWELLERS • Nereis(sandworm) • Glycera(bloodworm) • Arenicola(deposit feeders)

  10. Tube dwellers: • Can be temporary or permanent • Live singly or in aggregations • Tubes are made of mucuc, protein, seaweed, mud particles, sand grains, or shell fragments Terebella - suspension feeders - tentacles w/cilia Sabella - feather dusters - use feathery tentacles covered w/cilia

  11. Planktonic for Life • Tomopteris • Flat, expanded parapodia for swimming • Eunice (Palolo worm) • Tropical Pacific • Posterior half swims to the surface to spawn • Swarming – timed with the phases of the moon

  12. Lifestyles (continued): • Hydrothermal Vents – heat & hydrogen sulfide are toxic to most organisms • Lack mouth and gut • Chemosynthetic bacteria in body use sulfide to make food which is used by worms

  13. Hydrothermal Tubeworm Structure • Plume – captures oxygen, carbon dioxide, and hydrogen sulfide • Hemoglobin – binds sulfide and carries thru bloodstream to bacteria • Trophosome – feeding body which contains bacteria (285 billion per oz.)

  14. Annelid example: Leeches • Bloodsuckers attach to fish & invertebrates • Lack parapodia

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