Phylum Nemertea Phylogeny (briefly)




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Invertebrate Zoology

Lecture 11: Phylum Nemertea




  1. Phylum Nemertea

    1. Phylogeny (briefly)

    2. Diversity

    3. Bauplan Basics

    4. Feeding

    5. Circulation/Gas Exchange

    6. Osmoregulation/Excretion

    7. Nervous System

    8. Movement

    9. Reproduction




  1. Phylogeny/Evolutionary relationships (briefly)

    1. Closely related to Platyhelminthes?

    2. Post Platyhelminthes?

      1. Big question: Do they have a coelom?

  2. Diversity

    1. Mostly free-living

    2. Primarily predators

    3. Habitats

      1. Benthic marine

        1. Ex. 1: Mussel beds

        2. Ex. 2: Antarctic (big!)

      2. Some in freshwater and terrestrial

    4. A few are parasitic

      1. One type in Dungeness crabs

  3. Bauplan Basics

    1. Similar to Platyhelminthes

      1. Triploblastic (distinct mesoderm à muscles)

      2. Bilateral symmetry & cephalization

      3. Share similar features of protostome development

    2. Coelom?

  4. Feeding/Digestion

    1. Eversible proboscis: Class Anopla

      1. Stored inverted within the rhynchocoel

        1. Rhynchocoel from coelom?

        2. Separate from digestive system

      2. Associated muscles

        1. Location/Functions

      3. Surface glands secrete

        1. Adhesives

        2. Toxins

      4. KNOW how proboscis everts & captures prey!

      5. Food intake?

    2. Eversible proboscis: Class Enopla

      1. Differences from Anopla

        1. Common opening with digestive system = mouth

          • Proboscis eversion

          • Eversion of foregut. Why?

        2. Stylet at end of proboscis

        3. Additional neurotoxins

        4. Reserve stylet sac!

    3. Digestion

      1. Complete digestive system

      2. Some specialization of regions

      3. Extra- and intracellular digestion

        1. Eversible foregut à extracorporeal

      4. Movement through tract

        1. Ciliary

        2. Peristalsis of body wall

        3. NOTE: No muscle surrounds digestive tract!

  5. Circulation

    1. Closed circulatory system

      1. Partly coelomic in origin?

      2. Blood vessels and lacunae

      3. Circulation primarily via peristalsis of body wall.

      4. Blood cells with respiratory pigments (including Hb)

      5. Functions

        1. Transport of nutrients, gases, hormones, waste;

        2. Additional metabolism

        3. Hydrostatic skeleton

  6. Gas exchange

    1. Outer body surface

      1. Shape à favorable S/V ratio

    2. Some larger forms: gas exchange via digestive tract

      1. Irrigation of foregut

  7. Osmoregulation/excretion

    1. Protonephridia

      1. Most important in which habitats?

      2. Absent in deep-sea, pelagic forms

      3. Role in excretion?

        1. Associated with blood vessels

    2. Behavioral osmoregulation

      1. Mucus covering

      2. Burrowing

    3. Excretion

      1. Primarily via…?

  8. Nervous system/sensory structures

    1. Organization (briefly)

      1. Somewhat similar to Platyhelminthes

        1. Two lateral nerve cords

        2. Often an additional dorsal nerve cord

      2. Cerebral ganglia more developed

        1. Extensive innervation from anterior sensory structures

    2. Sensory structures

      1. Adapted for active, predatory lifestyle

        1. Sensory structures concentrated at “head”

        2. Additional sensory structures on entire body

      2. Tactile receptors

        1. Highly sensitive

        2. Ciliated epithelial cells

        3. Sensory cells with bristles

      3. Chemoreceptors

        1. Locate prey & mates

        2. Cerebral organ

          • Function of ciliated canal?

          • Other functions of organ?

        3. Frontal sense organ

          • Chemosensory?

      4. Ocelli

        1. Mostly pigment-cup

        2. 1 pair to many

        3. Some with lenses

        4. Negatively phototaxic

          • Nemerteans usually active nocturnally

  9. Movement

    1. Mesoderm-derived muscles

      1. Primarily longitudinal & circular

      2. Peristalsis, etc…

        1. Fluid-infiltrated mesoderm & circulatory system allow for hydrostatic skeleton

    2. Cilia-mucus

  10. Reproduction

    1. Asexual reproduction

      1. Transverse fission is common

      2. Small fragments à new individuals

    2. Sexual reproduction

      1. Gonads develop within mesenchyme

        1. Along body length

      2. Mass mating

      3. Mostly external fertilization

        1. May occur in mucus sacs

        2. Spawning via temporary pores or body wall rupture

      4. Sometimes internal fertilization

        1. If so, males with claspers or penis

    3. Early development

      1. Holoblastic

      2. Spiral cleavage

      3. Determinate cell fate

      4. Mesoderm usually from 4D cell

    4. Indirect or direct development

      1. What do these terms mean?

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