Invertebrate Zoology Lecture 11: Phylum Nemertea The Ribbon Worms

Invertebrate Zoology

Lecture 11: Phylum Nemertea

The Ribbon Worms

Lecture outline Phylum Nemertea

Phylogeny (briefly)DiversityBauplan BasicsFeedingCirculation/Gas ExchangeOsmoregulation/ExcretionNervous SystemMovementReproduction

Phylogeny (briefly)

Closely related to Platyhelminthes? Post Platyhelminthes?

Big question: Do they have a coelom?

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 parasiticOne type in

Dungeness crabs

Bauplan basics Similar to Platyhelminthes

Triploblastic (distinct mesoderm muscles)Bilateral symmetry & cephalizationShare similar features of protostome

development Coelom?

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

AdhesivesToxins

KNOW how proboscis everts & captures prey!

Food intake?

Diagram: Pearse/BuchsbaumLiving Invertebrates

Eversible proboscis: Class Anopla

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 proboscisAdditional neurotoxins

Central channel; toxins ejected next to stylet

Reserve stylet sac!Diagram: Pearse/BuchsbaumLiving Invertebrates

Focus: reserve stylet sac

Feeding/digestion Digestion

Complete digestive system Some specialization of

regionsExtra- and intracellular

digestion Eversible foregut

extracorporealMovement through tract

Ciliary Peristalsis of body wall NOTE: No muscle surrounds

digestive tract!

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

Circulation

Gas exchange Outer body surface

Shape favorable S/V ratio Some larger forms: gas exchange

via digestive tract Irrigation of foregut

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

Excretion

Ammonia loss primarily via…?

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

Sensory structures Adapted for active, predatory

lifestyle Sensory structures

concentrated at “head” Additional sensory

structures on entire body

Sensory structures Tactile receptors

Highly sensitive Ciliated epithelial

cells Sensory cells with

bristles

Sensory structures Chemoreceptors

Locate prey & mates

Cerebral organ Function of ciliated

canal? Other functions of

organ?

Sensory structures

Chemoreceptors Frontal sense organ

Chemosensory?

Ocelli Mostly pigment-cup 1 pair to many Some with lenses Negatively

phototaxic Nemerteans usually

active nocturnally

Movement

Mesoderm-derived musclesPrimarily longitudinal & circularPeristalsis, etc…

Fluid-infiltrated mesoderm & circulatory system allow for hydrostatic skeleton

Cilia-mucus

Reproduction

Asexual reproductionTransverse fission is commonSmall fragments new individuals

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

Reproduction Sexual reproduction

Early development Holoblastic Spiral cleavage Determinate cell fate Mesoderm usually from 4D cell

Indirect or direct developmentPilidium larvae