Transcript PowerPoint 11: Nemertea

Invertebrate Zoology
Lecture 11: Phylum Nemertea
The Ribbon Worms
Lecture outline
 Phylum Nemertea
Phylogeny (briefly)
Diversity
Bauplan Basics
Feeding
Circulation/Gas Exchange
Osmoregulation/Excretion
Nervous System
Movement
Reproduction
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 parasitic
One type in
Dungeness crabs
Bauplan basics
 Similar to Platyhelminthes
Triploblastic (distinct mesoderm  muscles)
Bilateral symmetry & cephalization
Share 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
Adhesives
Toxins
 KNOW how proboscis everts &
captures prey!
 Food intake?
Diagram: Pearse/Buchsbaum
Living 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 proboscis
Additional neurotoxins
 Central channel; toxins
ejected next to stylet
Reserve stylet sac!
Diagram: Pearse/Buchsbaum
Living Invertebrates
Focus: reserve stylet sac
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!
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 muscles
Primarily longitudinal & circular
Peristalsis, etc…
 Fluid-infiltrated mesoderm & circulatory system
allow for hydrostatic skeleton
 Cilia-mucus
Reproduction
 Asexual reproduction
Transverse fission is common
Small 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 development
Pilidium larvae