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Invertebrate Zoology Lecture 11: Phylum Nemertea The Ribbon Worms.

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Presentation on theme: "Invertebrate Zoology Lecture 11: Phylum Nemertea The Ribbon Worms."— Presentation transcript:

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 Feeding/Digestion  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? Diagram: Pearse/Buchsbaum Living Invertebrates

7 Eversible proboscis: Class Anopla

8 Feeding/Digestion  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! 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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