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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
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