Phylum Nemertea (Rhynchocoela)

Phylum Nemertea (Rhynchocoela)
Greek rhynchos, “snout”; coel, “cavity”
Triploblasts, bilateral, protostomes, probably coelomates, unsegmented worms.
Also called ribbon worms, about 1000 species.
Occur in marine, freshwater and terrestrial habitats.
With specialized proboscis that lies inside a
hydrostatic chamber (rhynchocoel). Eversible
proboscis is used to capture prey.
Complete digestive tract, with an anus. Protonephridia present.
With a closed circulatory system; hemoglobin present in some species
Bilobed cerebral ganglion, and 2 or more longitudinal nerve cords
The ectoderm is ciliated, and is dotted with gland cells and rhabdites.
With 2-3 layers of body wall muscles
Most are dioecious; asexual reproduction by
fragmentation
Cleavage holoblastic; early development typically spiralian,
either direct or indirect
Baseodiscus mexicanus (0.5 m)

Typical nemerteans
(From Brusca and Brusca 2002)
Probably the biggest evolutionary
advancement of nemerteans is the
ability to grow in great size (30m)
without segmentation or large body
cavity
Size range- less

Nemertean Bauplan
“Functionally” acoelomates
With true coelomic cavities (rhynchocoel, blood vessels) but with
relatively solid bodies
A number of Bauplan “improvements” solve surface/volume ratio
Circulatory system, protonephridia remove waste from blood, rather
than the mesenchyme tissues (flatworms).
a) No relying on diffusion for internal transport and exchange b)
complete gut, so anterior end is specialized on feeding and
ingestion, c) development of layered muscles after the reduced
reliability on diffusion for transport though mesenchyme
Result: Relatively large animals (up to 30 m), with complex
feeding and digestive activities

Key characters:
1) Mesenchyme thickness
2) Muscles
3) Placement of longitudinal nerve
cords
4) Major longit. blood vessels

Feeding and digestion
Proboscis is used to capture prey, - equipped in some species (Hoplonemertea) with piercing barbs or
stylets, which may contain a toxin to subdue the prey. It opens a) through the proboscis pore (in A&B)
or b) or associates with the foregut (C and D)
Stylets are continuously produced in reserve stylet sacs. Used when needed
Made from epithelial cells (styletocytes). Composed of organic core surrounded by crystalline Ca+P

Rhyncocoel
The proboscis pore leads to rhynchodeum (anterior
lumen), which is continuous with the epidermis, and to
proboscis canal
The rhynchocoel is a closed, fluid-filled space that is
squeezed by the surrounded muscles, causing the
eversible proboscis to be rapidly extended. Squeezing
increases the hydrostatic pressure of rhynchocoel.
Everted proboscis is retrieved by retractor muscle or
hydrostatically. The genus Gorgonorhynchus possess
an unusual, massively branched proboscis that takes
the appearance of a collection of medusa-like snakes,
called proboscides.
Most predators, some
scavengers, few ectoparasites
(e.g. Carcinonemertes errans
infects all Dungeness crabs)
a) A retracted and B) extended
proboscis of a hoplonemertean

Comparison of Proboscis Types
Credit: R. Ritger from adaptation in
Gibson, 1972 (Nemerteans, Hutchinson,
Univ Press)
Palaeo/Heteronemertea Heteronemertea Hoplonemertea

Digestive system
Anus is present (unlike flatworms), one way
traffic of food results in structure
specialization
Mouth opens to ectodermic foregut (buccal
cavity, esophagus, stomach)
Stomach leads to straight, elongate intestine
or midgut with numerous diverticula
Midgut leads to the ectodermic hindgut or
rectum which ends in the anus
Digestive tube is ciliated
In predatory nematodes, endopeptidases
break down the ingested proteins in the
lumen and then phago- and pinocytosis in
midgut. Food is stores mostly in fat.

Circulation and Gas Exchange
Nemerteans are the most primitive metazoans
with a true blood vascular system.
Ribbon worms have no heart, so blood is
squeezed through the vascular system by the
contraction of musculature. Blood is virtually
transparent, but some pigments resembling
hemoglobin have been observed.
The circulatory system consists of vessels and thin
wall spaces, lacunae
Great variation of circulation architecture
Simplest in palaeonemerteans (A).
In more complex forms, compartmentalization,
transverse, middorsal vessels.
Flow could be either way.
Gas exchange (respiration) is epidermal

A: Palaeonemertea Cephalothrix linearis
B: Palaeonemertea Tubulanus annulatus
C: Palaeonemertea Carinoma armandi
D: Heteronemertea Lineus sanguineus
E: Heteronemertea Cerebratulus lacteus
F: Hoplonemertea Amphiporus lactifloreus
Generalized Blood Vessel Systems
A B C D E F
Credit: R. Ritger from adaptation in
Gibson, 1972 (Nemerteans, Hutchinson,
Univ Press)

Excretion and Osmoregulation
2 to 1000s of protonephridia, similar to those of flatworms. Number depends on the
habitat. Some terrestrials may have up to 70,000 clusters of flame bulbs
Protonephridia associated with lateral blood vessels, sometimes immersed in blood (B)
They aid in osmoregulation in freshwater and terrestrial nemerteans
Terrestrial species (e.g. Geonemertes) inhabit moist environments to avoid desiccation.
Also some are covered in mucous. Minimal osmoregulation problems in open water or
benthic nemerteans
In some species,
nephridioducts are syncitial
and open up to 1000s
of epidermal pores

Nervous system
Nemerteans more cephalized than flatworms. Nervous system a simple net, dominated
by a brain derived from 4-5 ganglia. 5 longitudinal nerve processes stem from the brain
and innervate the mouth, proboscis, gut and ocelli. Eyes anteriorly (2-100s in FigureB)
Chemotactic; cephalic slits, cerebral organs, frontal glands (FigD) respond. Cephalic
slits are lined up with ciliated sensory epithilium. Water moved through the slits.
Frontal glands open to a pitlike-frontal sense organ (FigD). Some spp. have statocysts
Cerebratulus

Cerebral organs of nemerteans
Most nemerteans possess a pair of complex cerebral organs with a ciliated epidermal
invagination (cerebral canal)
Canal ends are surrounded by nervous tissue of the cerebral ganglion and glandular
tissue and also associated with lacunar blood spaces.
Cilia move water along and aid in prey detection
Overall the complete function of the cerebral organs
is not completely known.
Some pelagic hoplonemerteans and symbiotic
genera (e.g. Carcinonemertes and
Malacobdella do not have cerebral organs)

Reproduction
Most nemerteans are dioecious. Protandry and simultaneous hermaphroditism is known
System not as complex as flatworms. Numerous paired gonads (mesenchymal tissue)
located in pouches in the body wall between successive intestinal diverticuli.
During reproduction, a temporary gonoduct and gonopore develop for each gonad.
Gametes are shed to the sea through gonopores and body wall ruptures. Chemotaxis
plays a role in locating mates
External fertilization. Synchronous spawning with neighboring nemerteans? Several
nemerteans may form a mucous covered mass during spawning
Asexual reproduction by regeneration. Not all species can regenerate in the same
extreme fashion as Lineus

Epidermis
Frontal gland
Esophagus
Rhynchodaeum
Rhynchopore
Circular muscle
Lateral“green
” nerve cords
Frontal
organ
Excretory pores
Cephalic loop
Ovaries
Proboscis insertion
“yellow”
excretory system
Diagonal muscle
Longitudinal muscle
Lateral nerve
Mid dorsal nerve
Rhynchocoel (proboscis is
underneath)
Digestive
system cannot
be seen,
Underneath
rhynchocoel
NSF PEET grant (DEB 9712463) to
Jon Norenburg ( norenbur@lab.si.edu) and
Diana Lipscomb ( biodl@gwu.edu).

Reproduction
Internal fertilization. Sperm may be released in the mucous mating mass and move
to the ovaries of a female. The structure of spermatozoa is very different from that of
Turbellaria
Egg capsules may form within the body (Fig 11.13) where part of development takes
place.
Some terrestrial and deep pelagic nemerteans are ovoviviparous, direct development
(Fig. 11.14)
Cleavage holoblastic and spiral. Rhynchocoel is formed through schizocoely, (true
coelomic cavity)

Development
Paleonemertea, Hoplonemertea, Bdellonemertea – direct development within eggs
Heteronemerteans- produce free-swimming planktotrophic larva, pilidium. It is
somewhat similar to a trochophore but lacks anus.
Pilidium with incomplete gut, mouth between a pair of ciliated lobes (A). Ectodermal
invaginations pinch off internally to produce the adult ectoderm(B). Juvenile
metamorphoses in the larval skin during the planktonic stage(C). Skin is shed and
juvenile settles in the sediment
Desor larva is a creeping form. Iwata larva is a swimming form of the former.
These two larvae do not feed when they are in the larval stage.
Brusca and Brusca (2002)

1)
Emergence of juvenile nemerteans (Cerebratulus sp.)
2)
3) 4)
Adult Cerebratulus

Late-stage
Cerebratulus sp.
Pilidium larvae
Late-stage, side view
Cerebratulus larva
Cerebratulus larva Cerebratulus
Nemertean larva
Side view of a pilidium,
probably Lineus sp.
Bamfield, B.C

Ecological Notes
Nemerteans are found in and among seaweeds, rocks, mussel and barnacle beds, or
buried in mud, sand, or gravel substrates. Most are benthic and live in temperate
areas, but some terrestrial and freshwater species. Few fully pelagic species, that drift
or swim slowly in the open ocean.
Some ribbon worms live in burrows that have been constructed from slime secreted
from their mucus glands. They crawl, leaving a trail of slime similar to that of a
gastropod mollusc. Other species use their extended proboscis like an anchor around
the substrate, from which they ‘reel’ their bodies in. Alternative forms of locomotion
include burrowing and anguilliform swimming.
Some nemerteans actively track prey by following their chemical trails, - common prey
items are polychaetes, snails, small crustaceans, and molluscs. A few are parasitic.

Nemertean Taxonomy
Nemerteans are classified according to the relation of the nervous system to the
musculature, and the structure of the proboscis.
Class Anopla - ‘Unarmed’ Gr.
These animals are characterized by a mouth lying posterior to the brain, a CNS located
directly below the epidermis, and a proboscis that bears no stylets but instead works as
a wrap-around lasso. 2 Orders: Palaeonemertea, Heteronemertea
E.g. Baseodiscus
Class Enopla - ‘Armed’ Gr.
The enoplans have a mouth that lies anterior to the brain. The CNS is located within
the muscular layer, and the proboscis is armed in some species. Where an armed
proboscis does occur, a poison sac lies directly below the stylet, and poison is ejected
into wounds created by the stylet. 2 Orders: Hoplonemertea, Bdellonemertea
Ex. Prostoma - a fresh water armed nemertean. Geonemertes - a tropical terrestrial
genus of armed nemertean, which lives under rocks and stones.
Pelagic nemertean

Are Nemerteans sister taxa with flatworms?
First scenario
• Origin of triploblasty, spiralian condition, a= Acoelomate ancestor
b = ancestor and then flatworms retains all ancestral characters from 1
3. Proboscis, circulatory system, anus (synapomorphies) for Nemertea
4. Coelom
Alternative scenario
1. Origin of triploblasty, spiralian condition, schizocoely
2. Loss of coelom (so ancestor b and its descendants phyla are acoelomates)

Are Nemerteans sister taxa with flatworms?
First scenario
• Origin of triploblasty, spiralian condition, a= Acoelomate ancestor (characters
from ancestor a are retained in flatworms)
3. Circulatory system, anus
b = acoelomate ancestor
4. Nemertean synapomorphies (Characters from b are retained)
5. Coelom
Alternative scenario
• Coelom. If 1 is coelom, flatworms lost the coelom (2)
3. Coelom. If 3 is coelom, flatworms are primitively acoelomates and nemerteans
either lost the coelom (4) or kept it in the form of rhynchocoel
Coelomate ancestor b gives rise to coelomate phyla

1. Proboscis
2. Body wall architecture (3 types of muscles)
3. Indirect development, free-living larvae
4. Movement of the mouth anterior to the cerebral ganglion
5. Movement of the longitudinal nerve cords to a mesenchymal position
6. Fusion of the proboscis pore with the buccal region
7. Proboscis armature
8. Division of proboscis into distinct regions
9. Reduction of sensory organs
10. Appearance of posterior suckers
11. Elongation and coiling of the gut
12. Loss of intestinal diverticula
Nemertean Phylogeny