Phylum Nemertea (Rhynchocoela)
Phylum Nemertea (Rhynchocoela) 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)
- Page 2 and 3: Typical nemerteans (From Brusca and
- Page 5 and 6: Key characters: 1) Mesenchyme thick
- Page 7 and 8: Rhyncocoel The proboscis pore leads
- Page 9 and 10: Digestive system Anus is present (u
- Page 11 and 12: A: Palaeonemertea Cephalothrix line
- Page 13 and 14: Nervous system Nemerteans more ceph
- Page 15 and 16: Reproduction Most nemerteans are di
- Page 17 and 18: Reproduction Internal fertilization
- Page 19 and 20: 1) Emergence of juvenile nemerteans
- Page 21 and 22: Ecological Notes Nemerteans are fou
- Page 23 and 24: Are Nemerteans sister taxa with fla
- Page 25: 1. Proboscis 2. Body wall architect
<strong>Phylum</strong> <strong>Nemertea</strong> (<strong>Rhynchocoela</strong>)<br />
Greek rhynchos, “snout”; coel, “cavity”<br />
Triploblasts, bilateral, protostomes, probably coelomates, unsegmented worms.<br />
Also called ribbon worms, about 1000 species.<br />
Occur in marine, freshwater and terrestrial habitats.<br />
With specialized proboscis that lies inside a<br />
hydrostatic chamber (rhynchocoel). Eversible<br />
proboscis is used to capture prey.<br />
Complete digestive tract, with an anus. Protonephridia present.<br />
With a closed circulatory system; hemoglobin present in some species<br />
Bilobed cerebral ganglion, and 2 or more longitudinal nerve cords<br />
The ectoderm is ciliated, and is dotted with gland cells and rhabdites.<br />
With 2-3 layers of body wall muscles<br />
Most are dioecious; asexual reproduction by<br />
fragmentation<br />
Cleavage holoblastic; early development typically spiralian,<br />
either direct or indirect<br />
Baseodiscus mexicanus (0.5 m)
Typical nemerteans<br />
(From Brusca and Brusca 2002)<br />
Probably the biggest evolutionary<br />
advancement of nemerteans is the<br />
ability to grow in great size (30m)<br />
without segmentation or large body<br />
cavity<br />
Size range- less
<strong>Nemertea</strong>n Bauplan<br />
“Functionally” acoelomates<br />
With true coelomic cavities (rhynchocoel, blood vessels) but with<br />
relatively solid bodies<br />
A number of Bauplan “improvements” solve surface/volume ratio<br />
Circulatory system, protonephridia remove waste from blood, rather<br />
than the mesenchyme tissues (flatworms).<br />
a) No relying on diffusion for internal transport and exchange b)<br />
complete gut, so anterior end is specialized on feeding and<br />
ingestion, c) development of layered muscles after the reduced<br />
reliability on diffusion for transport though mesenchyme<br />
Result: Relatively large animals (up to 30 m), with complex<br />
feeding and digestive activities
Key characters:<br />
1) Mesenchyme thickness<br />
2) Muscles<br />
3) Placement of longitudinal nerve<br />
cords<br />
4) Major longit. blood vessels
Feeding and digestion<br />
Proboscis is used to capture prey, - equipped in some species (Hoplonemertea) with piercing barbs or<br />
stylets, which may contain a toxin to subdue the prey. It opens a) through the proboscis pore (in A&B)<br />
or b) or associates with the foregut (C and D)<br />
Stylets are continuously produced in reserve stylet sacs. Used when needed<br />
Made from epithelial cells (styletocytes). Composed of organic core surrounded by crystalline Ca+P
Rhyncocoel<br />
The proboscis pore leads to rhynchodeum (anterior<br />
lumen), which is continuous with the epidermis, and to<br />
proboscis canal<br />
The rhynchocoel is a closed, fluid-filled space that is<br />
squeezed by the surrounded muscles, causing the<br />
eversible proboscis to be rapidly extended. Squeezing<br />
increases the hydrostatic pressure of rhynchocoel.<br />
Everted proboscis is retrieved by retractor muscle or<br />
hydrostatically. The genus Gorgonorhynchus possess<br />
an unusual, massively branched proboscis that takes<br />
the appearance of a collection of medusa-like snakes,<br />
called proboscides.<br />
Most predators, some<br />
scavengers, few ectoparasites<br />
(e.g. Carcinonemertes errans<br />
infects all Dungeness crabs)<br />
a) A retracted and B) extended<br />
proboscis of a hoplonemertean
Comparison of Proboscis Types<br />
Credit: R. Ritger from adaptation in<br />
Gibson, 1972 (<strong>Nemertea</strong>ns, Hutchinson,<br />
Univ Press)<br />
Palaeo/Heteronemertea Heteronemertea Hoplonemertea
Digestive system<br />
Anus is present (unlike flatworms), one way<br />
traffic of food results in structure<br />
specialization<br />
Mouth opens to ectodermic foregut (buccal<br />
cavity, esophagus, stomach)<br />
Stomach leads to straight, elongate intestine<br />
or midgut with numerous diverticula<br />
Midgut leads to the ectodermic hindgut or<br />
rectum which ends in the anus<br />
Digestive tube is ciliated<br />
In predatory nematodes, endopeptidases<br />
break down the ingested proteins in the<br />
lumen and then phago- and pinocytosis in<br />
midgut. Food is stores mostly in fat.
Circulation and Gas Exchange<br />
<strong>Nemertea</strong>ns are the most primitive metazoans<br />
with a true blood vascular system.<br />
Ribbon worms have no heart, so blood is<br />
squeezed through the vascular system by the<br />
contraction of musculature. Blood is virtually<br />
transparent, but some pigments resembling<br />
hemoglobin have been observed.<br />
The circulatory system consists of vessels and thin<br />
wall spaces, lacunae<br />
Great variation of circulation architecture<br />
Simplest in palaeonemerteans (A).<br />
In more complex forms, compartmentalization,<br />
transverse, middorsal vessels.<br />
Flow could be either way.<br />
Gas exchange (respiration) is epidermal
A: Palaeonemertea Cephalothrix linearis<br />
B: Palaeonemertea Tubulanus annulatus<br />
C: Palaeonemertea Carinoma armandi<br />
D: Heteronemertea Lineus sanguineus<br />
E: Heteronemertea Cerebratulus lacteus<br />
F: Hoplonemertea Amphiporus lactifloreus<br />
Generalized Blood Vessel Systems<br />
A B C D E F<br />
Credit: R. Ritger from adaptation in<br />
Gibson, 1972 (<strong>Nemertea</strong>ns, Hutchinson,<br />
Univ Press)
Excretion and Osmoregulation<br />
2 to 1000s of protonephridia, similar to those of flatworms. Number depends on the<br />
habitat. Some terrestrials may have up to 70,000 clusters of flame bulbs<br />
Protonephridia associated with lateral blood vessels, sometimes immersed in blood (B)<br />
They aid in osmoregulation in freshwater and terrestrial nemerteans<br />
Terrestrial species (e.g. Geonemertes) inhabit moist environments to avoid desiccation.<br />
Also some are covered in mucous. Minimal osmoregulation problems in open water or<br />
benthic nemerteans<br />
In some species,<br />
nephridioducts are syncitial<br />
and open up to 1000s<br />
of epidermal pores
Nervous system<br />
<strong>Nemertea</strong>ns more cephalized than flatworms. Nervous system a simple net, dominated<br />
by a brain derived from 4-5 ganglia. 5 longitudinal nerve processes stem from the brain<br />
and innervate the mouth, proboscis, gut and ocelli. Eyes anteriorly (2-100s in FigureB)<br />
Chemotactic; cephalic slits, cerebral organs, frontal glands (FigD) respond. Cephalic<br />
slits are lined up with ciliated sensory epithilium. Water moved through the slits.<br />
Frontal glands open to a pitlike-frontal sense organ (FigD). Some spp. have statocysts<br />
Cerebratulus
Cerebral organs of nemerteans<br />
Most nemerteans possess a pair of complex cerebral organs with a ciliated epidermal<br />
invagination (cerebral canal)<br />
Canal ends are surrounded by nervous tissue of the cerebral ganglion and glandular<br />
tissue and also associated with lacunar blood spaces.<br />
Cilia move water along and aid in prey detection<br />
Overall the complete function of the cerebral organs<br />
is not completely known.<br />
Some pelagic hoplonemerteans and symbiotic<br />
genera (e.g. Carcinonemertes and<br />
Malacobdella do not have cerebral organs)
Reproduction<br />
Most nemerteans are dioecious. Protandry and simultaneous hermaphroditism is known<br />
System not as complex as flatworms. Numerous paired gonads (mesenchymal tissue)<br />
located in pouches in the body wall between successive intestinal diverticuli.<br />
During reproduction, a temporary gonoduct and gonopore develop for each gonad.<br />
Gametes are shed to the sea through gonopores and body wall ruptures. Chemotaxis<br />
plays a role in locating mates<br />
External fertilization. Synchronous spawning with neighboring nemerteans? Several<br />
nemerteans may form a mucous covered mass during spawning<br />
Asexual reproduction by regeneration. Not all species can regenerate in the same<br />
extreme fashion as Lineus
Epidermis<br />
Frontal gland<br />
Esophagus<br />
Rhynchodaeum<br />
Rhynchopore<br />
Circular muscle<br />
Lateral“green<br />
” nerve cords<br />
Frontal<br />
organ<br />
Excretory pores<br />
Cephalic loop<br />
Ovaries<br />
Proboscis insertion<br />
“yellow”<br />
excretory system<br />
Diagonal muscle<br />
Longitudinal muscle<br />
Lateral nerve<br />
Mid dorsal nerve<br />
Rhynchocoel (proboscis is<br />
underneath)<br />
Digestive<br />
system cannot<br />
be seen,<br />
Underneath<br />
rhynchocoel<br />
NSF PEET grant (DEB 9712463) to<br />
Jon Norenburg ( norenbur@lab.si.edu) and<br />
Diana Lipscomb ( biodl@gwu.edu).
Reproduction<br />
Internal fertilization. Sperm may be released in the mucous mating mass and move<br />
to the ovaries of a female. The structure of spermatozoa is very different from that of<br />
Turbellaria<br />
Egg capsules may form within the body (Fig 11.13) where part of development takes<br />
place.<br />
Some terrestrial and deep pelagic nemerteans are ovoviviparous, direct development<br />
(Fig. 11.14)<br />
Cleavage holoblastic and spiral. Rhynchocoel is formed through schizocoely, (true<br />
coelomic cavity)
Development<br />
Paleonemertea, Hoplonemertea, Bdellonemertea – direct development within eggs<br />
Heteronemerteans- produce free-swimming planktotrophic larva, pilidium. It is<br />
somewhat similar to a trochophore but lacks anus.<br />
Pilidium with incomplete gut, mouth between a pair of ciliated lobes (A). Ectodermal<br />
invaginations pinch off internally to produce the adult ectoderm(B). Juvenile<br />
metamorphoses in the larval skin during the planktonic stage(C). Skin is shed and<br />
juvenile settles in the sediment<br />
Desor larva is a creeping form. Iwata larva is a swimming form of the former.<br />
These two larvae do not feed when they are in the larval stage.<br />
Brusca and Brusca (2002)
1)<br />
Emergence of juvenile nemerteans (Cerebratulus sp.)<br />
2)<br />
3) 4)<br />
Adult Cerebratulus
Late-stage<br />
Cerebratulus sp.<br />
Pilidium larvae<br />
Late-stage, side view<br />
Cerebratulus larva<br />
Cerebratulus larva Cerebratulus<br />
<strong>Nemertea</strong>n larva<br />
Side view of a pilidium,<br />
probably Lineus sp.<br />
Bamfield, B.C
Ecological Notes<br />
<strong>Nemertea</strong>ns are found in and among seaweeds, rocks, mussel and barnacle beds, or<br />
buried in mud, sand, or gravel substrates. Most are benthic and live in temperate<br />
areas, but some terrestrial and freshwater species. Few fully pelagic species, that drift<br />
or swim slowly in the open ocean.<br />
Some ribbon worms live in burrows that have been constructed from slime secreted<br />
from their mucus glands. They crawl, leaving a trail of slime similar to that of a<br />
gastropod mollusc. Other species use their extended proboscis like an anchor around<br />
the substrate, from which they ‘reel’ their bodies in. Alternative forms of locomotion<br />
include burrowing and anguilliform swimming.<br />
Some nemerteans actively track prey by following their chemical trails, - common prey<br />
items are polychaetes, snails, small crustaceans, and molluscs. A few are parasitic.
<strong>Nemertea</strong>n Taxonomy<br />
<strong>Nemertea</strong>ns are classified according to the relation of the nervous system to the<br />
musculature, and the structure of the proboscis.<br />
Class Anopla - ‘Unarmed’ Gr.<br />
These animals are characterized by a mouth lying posterior to the brain, a CNS located<br />
directly below the epidermis, and a proboscis that bears no stylets but instead works as<br />
a wrap-around lasso. 2 Orders: Palaeonemertea, Heteronemertea<br />
E.g. Baseodiscus<br />
Class Enopla - ‘Armed’ Gr.<br />
The enoplans have a mouth that lies anterior to the brain. The CNS is located within<br />
the muscular layer, and the proboscis is armed in some species. Where an armed<br />
proboscis does occur, a poison sac lies directly below the stylet, and poison is ejected<br />
into wounds created by the stylet. 2 Orders: Hoplonemertea, Bdellonemertea<br />
Ex. Prostoma - a fresh water armed nemertean. Geonemertes - a tropical terrestrial<br />
genus of armed nemertean, which lives under rocks and stones.<br />
Pelagic nemertean
Are <strong>Nemertea</strong>ns sister taxa with flatworms?<br />
First scenario<br />
• Origin of triploblasty, spiralian condition, a= Acoelomate ancestor<br />
b = ancestor and then flatworms retains all ancestral characters from 1<br />
3. Proboscis, circulatory system, anus (synapomorphies) for <strong>Nemertea</strong><br />
4. Coelom<br />
Alternative scenario<br />
1. Origin of triploblasty, spiralian condition, schizocoely<br />
2. Loss of coelom (so ancestor b and its descendants phyla are acoelomates)
Are <strong>Nemertea</strong>ns sister taxa with flatworms?<br />
First scenario<br />
• Origin of triploblasty, spiralian condition, a= Acoelomate ancestor (characters<br />
from ancestor a are retained in flatworms)<br />
3. Circulatory system, anus<br />
b = acoelomate ancestor<br />
4. <strong>Nemertea</strong>n synapomorphies (Characters from b are retained)<br />
5. Coelom<br />
Alternative scenario<br />
• Coelom. If 1 is coelom, flatworms lost the coelom (2)<br />
3. Coelom. If 3 is coelom, flatworms are primitively acoelomates and nemerteans<br />
either lost the coelom (4) or kept it in the form of rhynchocoel<br />
Coelomate ancestor b gives rise to coelomate phyla
1. Proboscis<br />
2. Body wall architecture (3 types of muscles)<br />
3. Indirect development, free-living larvae<br />
4. Movement of the mouth anterior to the cerebral ganglion<br />
5. Movement of the longitudinal nerve cords to a mesenchymal position<br />
6. Fusion of the proboscis pore with the buccal region<br />
7. Proboscis armature<br />
8. Division of proboscis into distinct regions<br />
9. Reduction of sensory organs<br />
10. Appearance of posterior suckers<br />
11. Elongation and coiling of the gut<br />
12. Loss of intestinal diverticula<br />
<strong>Nemertea</strong>n Phylogeny