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Lecture 11: Phylum Nemertea The Ribbon Worms

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1 Lecture 11: Phylum Nemertea The Ribbon Worms
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 Coelom?
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 Ocelli 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 Cilia-mucus
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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