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Invertebrate Zoology

Invertebrate Zoology. Lecture 11: Phylum Nemertea The Ribbon Worms. Lecture outline. Phylum Nemertea Phylogeny (briefly) Diversity Bauplan Basics Feeding Circulation/Gas Exchange Osmoregulation/Excretion Nervous System Movement Reproduction. Phylogeny (briefly).

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Invertebrate Zoology

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  1. Invertebrate Zoology Lecture 11: Phylum Nemertea The Ribbon Worms

  2. Lecture outline • Phylum Nemertea • Phylogeny (briefly) • Diversity • Bauplan Basics • Feeding • Circulation/Gas Exchange • Osmoregulation/Excretion • Nervous System • Movement • Reproduction

  3. Phylogeny (briefly) • Closely related to Platyhelminthes? • Post Platyhelminthes? • Big question: Do they have a coelom?

  4. Diversity • Mostly free-living • Primarily predators • Habitats • Benthic marine • Ex. 1: Mussel beds • Ex. 2: Antarctic (big!) • Some in freshwater and terrestrial • A few are parasitic • One type in Dungeness crabs

  5. Bauplan basics • Similar to Platyhelminthes • Triploblastic (distinct mesoderm  muscles) • Bilateral symmetry & cephalization • Share similar features of protostome development • Coelom?

  6. Eversible proboscis: Class Anopla Stored inverted within the rhynchocoel Rhynchocoel from coelom? Separate from digestive system Associated muscles Location/Functions Surface glands secrete Adhesives Toxins KNOW how proboscis everts & captures prey! Food intake? Feeding/Digestion Diagram: Pearse/Buchsbaum Living Invertebrates

  7. Eversible proboscis: Class Anopla

  8. Eversible proboscis: Class Enopla Differences from Anopla Common opening with digestive system = mouth Proboscis eversion Eversion of foregut. Why? Stylet at end of proboscis Additional neurotoxins Central channel; toxins ejected next to stylet Reserve stylet sac! Feeding/Digestion Diagram: Pearse/Buchsbaum Living Invertebrates

  9. Focus: reserve stylet sac

  10. Feeding/digestion • Digestion • Complete digestive system • Some specialization of regions • Extra- and intracellular digestion • Eversible foregut  extracorporeal • Movement through tract • Ciliary • Peristalsis of body wall • NOTE: No muscle surrounds digestive tract!

  11. Circulation • Closed circulatory system • Partly coelomic in origin? • Blood vessels and lacunae • Circulation primarily via peristalsis of body wall. • Blood cells with respiratory pigments (including Hb) • Functions • Transport of nutrients, gases, hormones, waste; • Additional metabolism • Hydrostatic skeleton

  12. Circulation

  13. Gas exchange • Outer body surface • Shape  favorable S/V ratio • Some larger forms: gas exchange via digestive tract • Irrigation of foregut

  14. Osmoregulation/excretion • Protonephridia • Most important in which habitats? • Absent in deep-sea, pelagic forms • Role in excretion? • Associated with blood vessels • Behavioral osmoregulation • Mucus covering • Burrowing

  15. Excretion • Ammonia loss primarily via…?

  16. Nervous system • Organization (briefly) • Somewhat similar to Platyhelminthes • Two lateral nerve cords • Often an additional dorsal nerve cord • Cerebral ganglia more developed • Extensive innervation from anterior sensory structures

  17. Sensory structures • Adapted for active, predatory lifestyle • Sensory structures concentrated at “head” • Additional sensory structures on entire body

  18. Sensory structures • Tactile receptors • Highly sensitive • Ciliated epithelial cells • Sensory cells with bristles

  19. Sensory structures • Chemoreceptors • Locate prey & mates • Cerebral organ • Function of ciliated canal? • Other functions of organ?

  20. Sensory structures • Chemoreceptors • Frontal sense organ • Chemosensory? • Ocelli • Mostly pigment-cup • 1 pair to many • Some with lenses • Negatively phototaxic • Nemerteans usually active nocturnally

  21. Movement • Mesoderm-derived muscles • Primarily longitudinal & circular • Peristalsis, etc… • Fluid-infiltrated mesoderm & circulatory system allow for hydrostatic skeleton • Cilia-mucus

  22. Reproduction • Asexual reproduction • Transverse fission is common • Small fragments  new individuals

  23. Reproduction • Sexual reproduction • Gonads develop within mesenchyme • Along body length • Mass mating • Mostly external fertilization • May occur in mucus sacs • Spawning via temporary pores or body wall rupture • Sometimes internal fertilization • Males with claspers or penis

  24. Reproduction • Sexual reproduction • Early development • Holoblastic • Spiral cleavage • Determinate cell fate • Mesoderm usually from 4D cell • Indirect or direct development Pilidium larvae

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