Invertebrate Anatomy OnLine
Copyright 2001 by
is one of many exercises available from Invertebrate
Anatomy OnLine , an
Internet laboratory manual for courses in Invertebrate Zoology. Additional
exercises can be accessed by clicking on the links on the left. A
glossary and chapters on supplies and laboratory techniques are also available. Terminology
and phylogeny used in these exercises correspond to usage in the Invertebrate
Zoology textbook by Ruppert, Fox, and Barnes (2004). Hyphenated
figure callouts refer to figures in the textbook. Callouts
that are not hyphenated refer to figures embedded in the exercise. The glossary
includes terms from this textbook as well as the laboratory exercises.
Anopla C, Hetronemertea O,
Lineidae F (Fig
worms are active, benthic predators that use an eversible, sticky or barbed, and
sometimes poisonous proboscis to capture prey (Fig 11-4). Nemerteans are long
and slender, aptly known as ribbon worms, or rubber band worms. The longest is
about 50 m but most are much less than that, usually no more than 20 cm. Many
are brightly colored. Most of the 1150 species are marine but a few live in
freshwater or terrestrial habitats.
gut is complete and extends from anterior mouth to posterior anus. The long
proboscis is housed in a cavity, the rhynchocoel, from which it is everted by
hydrostatic pressure generated by surrounding muscles. The tubular proboscis
everts, by turning inside out, from an anterior proboscis pore which may or may
not be independent of the mouth.
anterior brain and a pair of longitudinal lateral nerve cords are present. Excretion
is via protonephridia. Nemerteans
are gonochoric. The body wall is complex with numerous layers of muscles,
epithelia, spaces, and connective tissue, and varies with order. There
is no cuticle.
have traditionally been thought of as having a compact body plan because the
viscera do not appear to be enclosed in a body cavity. Recent
studies however have revealed the presence of a coelom and necessitate the
reevaluation of the phylogenetic affinities of these animals. The fluid
transport system has been shown to be a coelomic space. The
rhynchocoel is also a coelomic space.
are the unarmed nemerteans whose proboscis lacks stylets. The
mouth is posterior to the brain. Anopla
is a paraphyletic taxon.
The heteronemertean body wall comprises four muscles
layers with the lateral nerve cords located in the innermost circular layer.
be large enough to be suitable for dissection in teaching laboratories although
it is rarely available in numbers sufficient to support such use. It
can be collected in coastal sediments or purchased living or preserved from
biological supply companies.
represented by over 100 species, many of which are very large, some reaching
many meters. On
the east coast of North America the only large species is Cerebratulus
lacteus but the west coast has
several large species suitable for dissection. The dissection is best performed
using a living specimen but preserved material will serve if necessary.
specimens should be observed in aquaria or dishes of seawater before being
anesthetized. Specimens should be handled carefully as many large species tend
to break into fragments or evert their proboscis when handled. Should
the proboscis be everted during the exercise, note the extreme stickiness of its
is nearly impossible for you to free it from a glass surface, for example, and
very difficult to detach from your finger should it be touched. Touch
worm, however, can release its sticky hold at will and retract the proboscis
back into the body.
of the distinguishing characteristics of Cerebratulus is
its ability to swim propelled by undulations of the body. To facilitate swimming
the body is flattened dorsoventrally and has wide, flat, lateral margins. Watch
a Cerebratulus swim
in an aquarium.
Normally Cerebratulus lives
in soft sediments where it burrows using retrograde peristaltic contractions of
the body wall muscles. Place
a specimen in a large dish of fine sand and seawater and watch it burrow.
place a specimen in a 4" culture dish of clean seawater and take it to your
a gram or two of magnesium chloride crystals to the center of the dish and let
the motion of the worm gradually mix the chemical with the water. Continue
observing the worm and its movements.
for retrograde (anterior to posterior) peristaltic contractions as the animal
responds to the relaxant. Normally
these waves are used for burrowing (rapidly). Watch
also for production of a copious mucus. Mucus
from the body surface is much less sticky than that from the proboscis. Set
the dish aside and do something else until the worm is relaxed.
your specimen is completely relaxed and no longer responds to stimuli (1-2
hours), place the dish on the stage of the dissecting microscope.
long and dorsoventrally flattened. The
body surface is ciliated. The
weakly developed head is at the anterior end. The
posterior end is often missing but if your worm is complete, there will be a
short, filamentous, tail-like caudal cirrus extending
from the posterior end of the body. The
body is wide and has thin flattened lateral margins to facilitate swimming.
extreme anterior end of the body is the head. It
is poorly developed but may be set off from the rest of the body by a slight
constriction (Fig 1). The
lateral margins of the head are deeply split by two ciliated chemosensory lateral
cephalic slits, one on each side (Fig 1, 11-9A). The
ciliary current in the slits runs from anterior to posterior. It
enters at the anterior tip of the head and exits at the posterior end of the
Figure 1. Ventral view of the anterior end of
the heteronemertean Cerebratulus from
Coos Bay, Oregon. Nemertea7La.gif
magnification place a drop of isotonic seawater/milk suspension to the water in
front of the head and watch for its slow entry into the cephalic slit. Watch
the posterior end of the slit for its reappearance.
at the ventral surface of the head. The
tiny proboscis pore is
located at the anterior end on the ventral surface (Fig 1, 11-2A,B). It
is a midventral, longitudinal slit that is usually closed. Open
it with minuten nadel and
fine forceps to reveal the proboscis lumen. The proboscis is everted through the
proboscis pore and mouth open independently of each other in Cerebratulus and
other Anopla. The mouth,
also ventral, is much larger than the proboscis port (Fig 1). It
is farther posterior, located at about the level of the posterior end of the
lateral cephalic grooves. It
is also a median, longitudinal slit. It
opens into the foregut. In
Anopla the mouth is posterior to the brain.
remainder of the body is undifferentiated and nearly featureless although the
anterior end is rounder in cross section and the posterior end flatter. The anus and
short, threadlikecaudal cirrus are
at the posterior end. You
will not see either if the posterior end of the worm is missing, as is often the
the worm dorsal side down in a long, narrow, wax-bottom dissecting pan. Use
two #1 insect pins placed through the edges of the body beside the mouth to hold
the worm in place. The head should
be at one end of the pan.
first region of the gut is the ectodermal pharynx (Fig
is ridged longitudinally. Open
the mouth with fine forceps and nadel and
look inside to see the cavity and its ridges.
the worm on the stage of the dissecting microscope, insert one tip of a fine
scissors in the mouth and cut posteriorly along the ventral midline, through the
body wall, thus opening the gut lumen. Pin
the walls of the gut (and body wall) aside as you go. Use
# 1 stainless steel insect pins and insert them into the wax at 45° angles. Find
the pharynx again (Fig 11-5A). Note
that the diameter of this region is enormous, filling most of the interior of
the worm. It
is so large that you may get the impression you are opening a coelomic cavity
but you are not. Notice
the conspicuous large longitudinal ridge running like a typhlosole along the
dorsal midline of the gut. This
is the proboscis and
proboscis sheath bulging into the gut lumen (Fig 11-2B).
posteriorly the pharynx soon becomes the stomach but
the two are similar and difficult to differentiate. Both
are derived from ectoderm and are part of the foregut. The
pharynx is more heavily ridged than the stomach, the walls of the stomach being
cutting posteriorly until the spacious stomach ends abruptly with a transverse
bulkhead, or sphincter, partially occluding its lumen. Posterior
to the stomach is the endodermalintestine, or midgut, whose
apparent diameter is less than that of the stomach. Its
walls are folded to form a series of pouchlike diverticula extending
to the body wall. The
intestine is characterized by the presence of these diverticula. In
living specimens the epithelium of the intestine may be white, whereas that of
the stomach is often pinkish.
cutting posteriorly, noting that the dorsal ridge representing the proboscis is
still present. The
intestine continues for most of the remainder of the length of the body. The
dorsal bulge becomes very large in the middle region of the intestine. As
you approach the posterior end of the body you should notice changes in the
character of the ridge (proboscis). It
becomes smaller and flatter and its color changes.
the posterior end of the worm, the intestine opens into the short, ectodermal rectum. The
rectum has no diverticula. It
opens to the exterior via the anus at
the posterior end of the body.
like most nemerteans, is gonochoric. Its numerous paired gonads are
located in pouches in the body wall between successive intestinal diverticula
(Fig 11-2A). During
periods of reproductive activity, a temporary gonoduct and gonopore develop for
each gonad. Through
them gametes are shed to the sea and fertilization is external. Are
gonads present over the entire length of the worm or are they restricted to
compare the simplicity of the nemertean reproductive system with the amazing
complexity (and diversity) of that of platyhelminth worms (Fig 10-24, 10-37).
gametes are present in the gonads, make a wet mount with seawater and examine
them with the dissecting microscope. Look
for gametes and determine if your specimen has eggs or sperm. Eggs
are large spheres with visible nuclei. Sperms
are tiny flagellated cells. Is your specimen a male or female?
Rhynchocoel and Proboscis
to the anterior end of the worm. Find
the anterior end of the typhlosole, or ridge, in the roof of the pharynx and
open it with a longitudinal cut made with your fine scissors. This
ridge, remember, contains the rhynchocoel and proboscis. Its
wall, through which you just cut, is the proboscis
sheath (Fig 11-2A, B). The
cavity exposed by the cut is the rhynchocoel. Inside
it you can see the large, very long proboscis. (If
the rhynchocoel is empty, it means the worm has everted and then lost the
sometimes happens during collection and handling of specimens.) The
rhynchocoel is a coelomic space.
cutting posteriorly, opening the rhynchocoel and exposing the proboscis for the
entire length of the gut. Note
that in the areas where the typhlosole is very thick the proboscis is coiled or
the posterior end, the proboscis diminishes in diameter and ultimately attaches
to the walls of the rhynchocoel. There
is no separate proboscis retractor muscle inCerebratulus, instead the
muscular posterior end of the proboscis itself acts as the retractor.
at the anterior end of the worm, extend the longitudinal incision to the
proboscis pore. In
doing this you will cut through nerve ring and expose the lateral lobes of the brain. In
living specimens the brain is red with neuroglobin but
in preserved material it is white. Neuroglobin
is a type of non-circulating hemoglobin that stores oxygen for use by the brain.
opened the rhynchocoel anteriorly to the proboscis pore you can now see that the
proboscis pore opens into a cavity, the rhynchodeum,
occupying most of the interior of the head (Fig 11-2A, B). The
rhynchodeum narrows posteriorly and is continuous with the lumen of the
it narrows, the walls of the proboscis join with the body wall. This
is the anterior point of attachment of the proboscis to the body. The
proboscis is, in fact, a deep invagination of the body wall and its walls are
composed of the same muscle and epithelia layers as the body wall. The
retractor muscle or, in the case of Cerebratulus the
posterior end of the proboscis, attaches posteriorly to the proboscis sheath.
remove the proboscis from the rhynchocoel, extend it and compare its total
length with that of the worm.
nervous system consists of a large dorsolateral brain and two lateral
longitudinal nerve cords (Fig 11-2A). You may have seen the brain already. In
life it is red with neuroglobin but in preserved material is white. In Cerebratulus the
brain is located immediately anterior to the mouth and almost exactly level with
the anterior point of attachment of the proboscis with the proboscis sheath. The
two large lateral ganglia are joined to each other by a large dorsal and small
ventral commissures to form a nerve ring around the proboscis (Fig 11-2B). Note
that the nerve ring is around the proboscis, rather than the gut.
large lateral nerve cord exits each of the lateral ganglia and extends
posteriorly for the length of the worm. The
nerve cords are easily accessible by dissection from the gut lumen. You
can find them by cutting or tearing through the lateral wall of the buccal
Fluid Transport System
is a well-developed fluid transport system with narrow, well-defined coelomic
vessels connecting large sinuses, or lacunae (Fig 11-7C). There
is a large cephalic lacuna in the head. Two
lateral vessels run posteriorly from the cephalic lacuna. There
is also a median, dorsal, longitudinal vessel. It
has been shown that the vessels are coelomic channels rather than blood vessels. The
channels are lined by a ciliated mesothelium. The vessels will not be apparent
in your dissection.
excretory system consists of protonephridia with flame bulbs associated with the
lateral coelomic vessels (Fig 11-8). They
will not be seen in this dissection.
CG. 1950. Amphiporus
ochraceous, pp 209-214 in Brown FA (ed). Selected
Invertebrate Types. Wiley,
New York. 597p.
Coe WR. 1895. Anatomy
of a species of nemertean (Cerebratulus lacteus Verrill)
with remarks on certain other species. Trans.
Connecticut Acad. Sci. 9:479-514, pls. 10-15.
WR. 1905. Nemerteans
of the west and northwest coasts of America. Bull.
Mus. Comp. Zool, Harvard 47:1-320, pls. 1-25.
WR. 1937. Methods
for the laboratory culture of Nemertea. pp.162-165 in Needham
JG. et al. Culture Methods for Invertebrate Animals. Comstock,
WR. 1943. Biology
of the nemerteans of the west coast of North America. Trans.
Conn. Acad. Arts Sci. 35:129-328, pls. 1-4.
RW . 1989. Fresh-water
Invertebrates of the United States, 3 rd ed. Wiley,
New York. 628p.
Ruppert EE, Fox
RS. 1988. Seashore
animals of the southeast. Univ.
South Carolina Press, Columbia, 429.
Ruppert EE, Fox RS,
Barnes RB. 2004.
Invertebrate Zoology, A functional evolutionary approach, 7 th ed.
Brooks Cole Thomson, Belmont CA. 963 pp.
JM. 1991. Nemertinea,
pp. 285-328 in
Harrison FW, Bogitsh BJ (eds.). Microscopic Anatomy of Invertebrates vol. 3
Platyhelminthes and Nemertinea . Wiley-Liss,
JM, Ruppert EE . 1985. Comparative
ultrastructure and the evolution of nemertines. American
Large living or preserved Cerebratulus
Seawater for living specimens
Magnesium chloride crystals if using living specimens
Wax-bottom dissecting pan made from kippered herring tins or
aluminum ice trays
Milk made isotonic to seawater by adding salts
Plastic Pasteur pipet
Dissecting set with microdissecting tools
Stainless steel # 1 insect pins