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