Bootlace worm
Nemertines - Ribbon Worms
Ribbon Worms: Nemertina (Nemertines, Rhynchocoela)

External Features

The nemertines are acoelomate worms with an anus, circulatory system and an eversible proboscis
enclosed in a tubular cavity (
rhynchocoel) dorsal to the gut. Ribbon worms are elongated, often extremely
so. The head is not very definitely delimited and there is no external body segmentation (though they may
be internally metameric).

The vermiform (worm-like) body is usually slender, elongated and cylindrical or dorsoventrally flattened.
These worms are soft, slimy and very elastic and extensible. Some are string or cord-like, but most are band
or ribbon-like with an elliptical cross-section or flattened ventrally and convex dorsally. Pelagic forms (and
the parasitic
Malacobdella) are shorter, broader and more flattened.

The anterior end is blunt and rounded or pointed. The posterior end tapers and may be pointed and some
species have a small tail or caudal cirrus (5-10 mm long, found in some
Heteronemertini). There is no
definite head, but there may be a lancet, spatula or heart-shaped cephalic lobe. This cephalic lobe is not
the head since it often does not include the brain.

The mouth is ventral and anterior or else there is only a proboscis pore and no mouth. The
proboscis
pore
, near the anterior tip, leads into the rhynchodaeum cavity. The anus is posterior and either terminal
or dorsal to the posterior tip (at the base of the caudal cirrus if this is present).

In swimming bathypelagic nemertines the posterior end is flattened and broadened into a simple or bilobed
caudal fin and lateral fin-like extensions may also be present.
Malacobdella is short and flattened and
inhabits the mantle cavity of clams and snails and is equipped with a posterior adhesive disc.

Shallow transverse grooves, called cephalic grooves, occur on the anterior end, or they may be lateral
cephalic slits instead. Eyes are present (especially in armed nemertines) numbering from 2 or 4 to many
hundreds. These eyes are bilaterally arranged.

The pelagic genera
Balaenonemertes (both the males and the females) and Nectonemertes (males only)
have laterally projecting tentacles or cirri.

The small proboscis pore is situated just below the anterior tip or ventral surface of the cephalic lobe. The
mouth is rounded or slit-like and situated anterior and ventral behind the brain in unarmed nemertines. In
the armed nemertines the mouth is in front of the brain just behind the proboscis pore, or else the proboscis
pore forms a common opening for the digestive tract and the rhynchodaeum and a separate mouth is
absent.

Nemertines vary from white or yellowish, or are brightly coloured, especially on the dorsal surface, with
orange, red, and brown and may be plain or patterned. Stripes and sometimes crossbars of contrasting
colour occur in some species. The cephalic lobe is often patterned differently. Some pelagic forms are more
or less transparent.
Classification

Subclass I: Anopla. In the Anopla the mouth is posterior to the brain. The CNS lies beneath the epidermis or
among the body-wall musculature. The proboscis is unarmed. This subclass is divided in to the orders
Palaeonemertini and Heteronemertini. In the Palaeonemertini there are 2-3 body-wall muscle layers
(innermost circular) and a gelatinous dermis. In the Heteronemertini there are three body-wall muscle layers
(inner longitudinal) and a fibrous dermis.

Subclass II: Enopla. In the Enopla the mouth is anterior to the brain, the CNS lies internal to the body-wall
musculature and the proboscis may be armed. The Enopla consists of the orders Hoplonemertini and
Bdellomorpha or Bdellonemertini. The Hoplonemertini have one or more stylets arming the proboscis and a
straight intestine with paired lateral diverticula. The Hoplonemertini is subdivided into two suborders: the
Monostylifera, with one stylet arming the proboscis and the Polystylifera with numerous stylets. The
Bdellomorpha have an unarmed proboscis, a sinuous intestine without diverticula. The bdellomorphs are
parasitic and have a posterior adhesive disc.

Thus there are two subclasses of nemertine, Anopla and Enopla and four nemertine orders: the
palaeonemertines, the heteronemertines, the hoplonemertines and the bdellomorphs.

Proboscis

The proboscis apparatus consists of the proboscis, the rhynchodaeum and the proboscis sheath. The
external proboscis pore leads into a tubular cavity, the rhynchodaeum. This cavity is very short in pelagic
forms and lacking in the Bdellomorpha. The rhynchodaeum has a ciliated cuboidal to columnar epithelial
lining.

The proboscis lies free in the rhynchocoel, fastened to the posterior wall of this chamber by a retractor
muscle composed of longitudinal muscle fibres from the proboscis wall. A sphincter of circular muscle
usually separates the rhynchodaeum / proboscis boundary. The rhynchocoel is blind at both ends, being
closed anteriorly by the rhynchodaeum to which the anterior of the proboscis is fastened. The rhynchocoel
appears to be a true coelom (a schizocoel).

The proboscis is an elongated muscular tube, blind posteriorly. The proboscis varies from very short to two
or more times the body length (in which case it lies coiled within its sheath). The proboscis may be armed or
unarmed. The unarmed proboscis is a simple tube, mostly unciliated, glandular containing rhabdites
(sometimes with glandular papillae) and may contain nematocysts of unknown origin at its interior. The
unarmed proboscis has the same muscle layers as the body wall, although these layers may be reduced
anteriorly.

The armed proboscis can be divided into anterior, middle and posterior sections. The anterior section is a
thick-walled glandular tube (may contain glandular papillae). The middle is bulbous and armed with stylets
on its anterior face. The posterior is a blind tube with reduced musculature. A muscular diaphragm (pierced
by a narrow canal) separates the bulbous part from the anterior section. The stylets are organic secretions.

The Monostylifera are armed with a central seizing
stylet – a straight or curved thorn – in a pocket on a
conical granular base. There may be two or more lateral pockets containing developing reserve stylets. In
the Polystylifera, e.g. Drepanophous, there are numerous minute central stylets and numerous reserve
stylet pockets.

The bulb has a thick muscular wall, made up mostly of longitudinal fibres and diagonal fibres, and has a
sphincter at each end. In the Bdellomorpha the proboscis is a simple elongated tube.

Gorgonorhynchus (a heteronemertine) has a proboscis that branches dichotomously to give up to 32
proboscides. This proboscis resembles a bunch of writhing worms when everted. A fold forms a valve at
each bifurcation.

The proboscis is used in prey capture and defense. It can be everted with explosive force by muscular
contraction exerting pressure on the rhynchocoel fluid. The anterior end of the proboscis is attached to the
inner end of the rhynchodaeum, so that the proboscis turns inside-out when protruded, moving the
glandular inner lining (which may possess glandular papillae) to the outside. A sticky secretion holds the
prey and the proboscis wraps around the prey. Stylets, if present, pierce and hold the prey (and are
thought to be poisonous). The proboscis is withdrawn by a retractor muscle (or other muscles if a specific
retractor muscle is absent).
Locomotion

Locomotion is by ciliary creeping in most species. The cilia act against secreted slime. In larger species
muscular waves assist the ciliary creeping. The proboscis may also be used to pull the animal along.
Malacobdella grossa is very sluggish and moves in a leech-like fashion with the use of its adhesive disc.
Burrowing forms, like
Cerebratulus, may use the proboscis for burrowing. Some species swim by undulating
their flattened bodies.

Coordination

The nervous system consists of the brain, the main ganglionated nerve cords, which are plexus thickenings
and the main plexus. Subsidiary plexi may also be present. The brain and nerve cords are sited in the
epidermis, dermis, body-wall musculature or in the mesenchyme internal to the body-wall muscles. The
brain is usually 4-lobed, consisting of paired dorsal and ventral ganglia more or less fused.

A dorsal commissure above the rhynchodaeum connects the dorsal ganglia. The ventral ganglia are
connected by a ventral commissure, the whole forming a central nerve ring. The ventral ganglia give rise to
2 large lateral or ventrolateral nerve cords connected near the anus by an anal commissure, either dorsal
or ventral to the intestine. There is also a number of minor ganglionated nerve cords.

The anterior faces of the cerebral ganglia send out cephalic nerves to innervate the anterior tip, eyes and
other anterior sense organs. In hoplonemertines there is also a pair of dorsolateral nerves from the dorsal
ganglia. Most ribbon worms also possess a middorsal or dorsomedian nerve. This usually projects from the
dorsal commissure (but is not directly connected to the brain in pelagic nemertines). The middorsal nerve
may give off a lower dorsal nerve that innervates the proboscis sheath.

A pair of foregut or oesophageal nerves spring from the ventral ganglia or the ventral brain commissure.
These nerves may be interconnected by transverse connections and innervate the foregut. Some
palaeonemertines also have a midventral nerve.

Proboscis nerves from the cerebral ganglia or ventral commissure innervate the proboscis. There is one
pair of these nerves in unarmed spp., but there are 7-50 proboscis nerves in species with an armed
proboscis. These nerves form a circle in the proboscis wall. There is also one or more plexi in the proboscis
wall.

In palaeonemertines and heteronemertines the lateral cords are connected to each other and with the
middorsal nerve by commissures.

In pelagic hoplonemertines each lateral cord gives rise to 3 main nerves and some smaller nerves – dorsal,
lateral and ventral peripheral nerves interconnected by the nerve plexus. The nerve plexus is intermuscular
in hoplonemertines, located between the outer circular and longitudinal muscle layers.

The brain and lateral cords have a peripheral layer of ganglion cells surrounding an inner fibrous mass of
nerve fibres and connective tissue fibrils continuous with the neurilemma sheath. The neurilemma sheath
encloses the brain.

When the anterior end of gliding animals is touched it draws back. The posterior end often flattens and
muscular waves pass from behind forward, beginning more and more posteriorly, until the posterior end is
reached and the animal has moved backward.

When the posterior end is touched it is drawn into a spiral, muscular waves run from the front backwards,
beginning more and more anteriorly and the animal moves forward. The middle of the body is very
insensitive to touch. Strong stimulation of the middle body will cause the body, or part of it, to coil.
Decapitated worms respond normally to touch.

In most species the lateral cords are extensively connected by a peripheral plexus, except in
Oerstedia in
which the cords only connect with each other via the brain (very strong stimuli will travel through the plexus
in
Oerstedia).

In
Malacobdella grossa the adhesive disc is controlled by ganglia in the disc itself, which will stay attached
even after severance from the trunk. The disc detaches only after the ventral surface of the anterior end
has secured a firm hold. The isolated disc can not reattach once detached. In this worm the cilia are not
under nervous control and beat forward dorsally and backward ventrally. There is also little reaction to
external stimuli in
Malacobdella grossa, except touch. Touching the anterior end elicits contraction, while
touching the posterior end elicits extension of the worm.

Nemertines exhibit righting reactions and even headless pieces right themselves.

Sense organs

Tactile epidermal cells are present, especially in the anterior and posterior ends. Sensory pits are strewn
over the body in pelagic forms and on the anterior end of
Carinoma (a palaeonemertine).

Some palaeonemertines, e.g.
Tubulanus have lateral organs, which are protrusible ciliary pits. There is
one pair of these organs, one on each side near the excretory pore.
Cephalic grooves are especially
frequent in the hoplonemertines. These consist of a groove or row of sensory pits. The cephalic slits found
in heteronemertines are deep grooves.

Frontal organs are flask-shaped protrusible pits on the anterior tip and are found in hoplonemertines.  
Frontal glands may open in the frontal organs via ducts.

Some nemertines possess
eyes, numbering 2, 4 or 6 to 250 in median or paired clusters or rows. The
number of eyes varies with species and may vary with the individual and with age. The eyes are usually
subepidermal, but may be sited in the dermis, in the muscle-layers, in the mesenchyme or attached directly
to the brain. They are inverted pigment-cup ocelli. In the terrestrial
Geonemertes the eyes are closed ovoid
bodies.

Statocysts are found in Ototyphlonemertes (a hoplonemertine). These are 1-2 pairs of vesicles in the
dorsal ganglion layer of the ventral brain ganglia.

Many ribbon worms possess cerebral organs. These are blind invaginated epidermal canals that open to
the exterior via the cephalic grooves or slits or via pits if cephalic grooves / slits are absent. A water current
is maintained in the cerebral organ canals. This water current increases in the presence of food,
suggesting that these organs have a chemoreceptive function. However, these organs are also supplied by
glands that are often bathed in blood and so may have an endocrine function.
Respiration

Gas exchange is by diffusion through the body surface assisted by the narrowness or flatness of these
worms. Some pump water in and out of their foregut, in which case the foregut is permeated with blood
lacunae (blood-filled spaces) for gas exchange.

Circulatory System

Nemertines have a closed circulatory system with blood vessels and lacunae. The simplest type
consists of one pair of
lateral vessels in the mesenchyme, alongside the digestive tract with no major
branches. These vessels are connected above the rhynchodaeum by the cephalic lacuna and posteriorly
below the anus by the anal lacuna. This is the type of blood system seen in
Cephalothrix.

A second type of circulatory system, as seen for example in
Carinoma, may possess oesophageal lacuna,
one pair of rhynchocoel vessels and one pair of lateral rhynchocoel vessels alongside the proboscis
sheath and transverse posterior connections between the lateral vessels.

Some palaeonemertines and heteronemertines have a ventral connective connecting the lateral vessels
below the rhynchodaeum, as seen for example in
Tubulanus. From this connective springs a middorsal
vessel that connects to the lateral vessels in the intestinal region by transverse vessels and also by the
anal lacuna. In the region of the foregut the middorsal vessel gives off lateral vessels that form a lacunae
network around the foregut wall. Hoplonemertines have a similar circulatory system, e.g.
Amphiporus.

The larger vessels are contractile and their lumens are lined by endothelium. The smaller vessels are non-
contractile. Usually the blood travels forward in the middorsal vessel and backward in the lateral vessels,
but the flow may intermittently reverse or flow in the same direction in all three vessels at the same time.

The blood is usually colourless, but may be yellow, green, orange or red. The blood carries blood
corpuscles, including amoebocytes and flattened nucleated discs that may contain a respiratory pigment
that may be Haemoglobin (Hb). Hb may also be present around the brain and main nerves. In
Lineus there
are four types of blood amoebocyte.

Excretory System

Most nemertines possess one pair of protonephridia in the foregut region. These open to the outside by
one pair of nephridiopores, one on each side. One nephridium runs in contact with each
lateral blood
vessel
. The terminal flame bulbs push against the walls of these blood vessels. In some this area of the
blood vessel walls is absent and the flame bulbs are bathed directly in blood. Nephridia are absent in
bathypelagic nemertines.

Hetero- and hoplonemertines often have richly branched nephridia with many capillaries and flame bulbs
closely applied to the lateral vessels or foregut lacunae and the nephridia may extend into the intestinal
region or the posterior end. In some the flame bulbs are not closely applied to the blood vessels, but occur
throughout the tissues. Some have several ducts and nephridiopores, for example
Amphiporus and some
Prostoma spp. Some possess several nephridia, each with its own duct and pore, as in most Prostoma
spp. and
Geonemertes.

Geonemertes palensis, a terrestrial nemertine of the South Pacific Islands has many thousands of
nephridia and dense masses of nephridia in the head and the flame bulbs are in contact with the cephalic
lacunae.

Some
Baseodiscus spp. Have one pair of nephridia with numerous ducts, some of which open to the
outside, while others open into the foregut.

The tubule walls are usually ciliated. In
Malacobdella the tubules are surrounded by large mesenchyme
cells or athrocytes that discharge waste into the tubule lumen.

Nutrition

In some nemertines the digestive tract is an unspecialized tube, as in Tubulanus. In most nemertines,
however, it can be divided into foregut and midgut / intestine. The intestine may have several to many
pairs of lateral diverticula. The foregut can sometimes be further subdivided into the buccal cavity,
oesophagous and stomach.

The gut usually has no muscles of its own, except the posterior part of the foregut (the stomach) which has
longitudinal fibres derived from the body-wall musculature. Body-wall muscles may surround the gut when
the innermost layer of this musculature is circular, as it does in some palaeonemertines. Sometimes the
innermost longitudinal layer may also ensheath the gut. Alternatively, the gut is surrounded by
mesenchyme. The intestine may also have a very thin layer of circular muscle.

Nemertines are carnivorous and feed mostly at night on live or dead animals, like annelids, molluscs,
crustaceans and fish. The proboscis is rapidly everted and spirals around the prey. The prey is swallowed
whole or sucked out. Digestion is very rapid and the swallowed part may disintegrate before the rest is
swallowed!

Bioluminescence

Emplectonema kandar (a Japanese species) can luminesce along its entire body surface except the
anterior tip. Touch elicits a local flash. If the worm is stretched then the whole body glows. Epidermal gland
cells are the producers of this light, which is not produced by a luciferin-luciferase reaction.

Reproduction

Most nemertines are dioecious, but some hoplonemertines are hermaphroditic, especially freshwater and
terrestrial forms. Hermaphroditic forms may be more or less protandric. The gonads are sacs that are
usually limited to the intestinal region, forming a row on each side (between the intestinal diverticula if the
latter are present).

In the semiparasitic genera (
Malacobdella, Carcinonemertes, Gononemertes) and the commensal
Nemertopsis actinophila the gonads are very numerous and strewn throughout the mesenchyme.

Hermaphroditic forms have mixed gonads or separate male and female gonads. In
Dichonemertes the
anterior gonads are male, the posterior gonads female. Sexual dimorphism occurs in
Nectonemertes
mirabilis with the males possessing a pair of cirri. Ripe sex cells may also be visible through the body wall
of many nemertines, producing sexual colour differences in mature individuals.

Gonads form from the mesenchyme. At maturity a short duct grows from the gonad to the adjacent exterior
to form an external pore. This produces two dorsolateral or lateral rows of gonopores.

A copulatory organ occurs only in the pelagic genus
Phallonemertes, in which each male pore is at the end
of a finger-like projection.

In male
Carcinonemertes the sperm ductules enter a median longitudinal sperm duct that discharges into
the rear of the intestine and the sperm are emitted through the anus.

Mature hoplonemertines contain one egg at a time per ovary. Most palaeo- and heteronemertines contain
up to 50 eggs at once per ovary. Body contractions squeeze the eggs through the gonopores or else the
eggs escape by body-wall rupture. Spawning may occur without contact, or the male may crawl over the
female, discharging his sperm, or two or more worms may enclose themselves in a common mucous
sheath. The sperm may enter the ovaries where fertilisation takes place; or else the eggs are fertilised on
discharge. Self-fertilisation may occur in hermaphrodites. Some nemertines, like
Lineus, are viviparous
(give birth to live young) with the eggs developing within the ovaries.

The eggs are laid in gelatinous strings or masses and are separate or grouped into capsules. Annual
cycles vary with species.

Development

Cleavage is spiral and determinate and gives rise to a ciliated coeloblastula, which gives rise to the
gastrula. Development of the gastrula may proceed by three different routes, depending upon the
species. These are direct development, indirect development with a pilidium larva and indirect
development with a Desor’s larva.

Direct development occurs in the hoplonemertines. The gastrula is bilateral and may have an anterior
sensory ciliated plate. Development occurs within the egg until a small ciliated worm leaves the egg
membranes.

Heteronemertines show either type of indirect development. The pilidium larva has an apical sensory
organ and several ciliated oral lobes and swims about feeding on minute organisms.  The pilidium
blastocoel is filled with gelatinous fluid and amoeboid branched mesenchyme cells, some of which
differentiate into muscle bands. The apical sensory plate is the only component of the larval nervous
system.

A Desor’s larva is more common in forms living in shallow, variable waters. The Desor larva occurs for
example in Lineus. It is an oval ciliated postgastrula and remains inside the egg membranes and lacks an
apical plate and tuft and lacks oral lobes and lacks ciliated oral bands.

Metamorphosis is similar for both larval types. A series of ectodermal invaginations form discs or plates.
There is one pair of anterior cephalic discs, one pair of lateral cerebral discs (form the cerebral canals),
one pair of posteroventral trunk discs, an unpaired posterior dorsal disc and there may also be an
unpaired proboscis disc. The paired discs invaginate and separate from the ectoderm as flattened sacs
and form an outer amnion membrane and an inner columnar epithelium. The unpaired discs form by
delamination and do not form an amnion.

Inside the larva the discs grow, spread, flatten and fuse together, enclosing the larval gut. The amnion
forms the embryonic membrane, while the epithelium becomes the future epidermis, enclosing the gut and
mesenchyme. When metamorphosis is complete, the larva sheds its larval ectoderm (including the apical
plate) and the amnion and emerges as a young worm.

In
Lineus ruber green-coloured individuals lay eggs in a string that undergo the Desor mode of
development. Red-coloured individuals lay egg strings containing many small eggs of which many do not
develop. Those that do develop eat the remaining eggs (with their large mouths and larval foreguts).

Asexual Reproduction and Regeneration

When ribbon worms are handles, irritated or stressed they break-off their posterior pieces and their
proboscis. The anterior pieces regenerate their posterior regions. Only in some
Lineus spp.
(heteronemertines), however, can the posterior pieces regenerate a new head.
Lineus spp. Reproduce
asexually by fragmentation and regeneration. For example, in
Lineus socialis and Lineus vegetus, the
posterior half to two thirds form 2o or more fragments by strong muscular contraction. These fragments
may encyst themselves in mucus while they regenerate. Since this fragmentation is an active process
requiring strong muscular contractions, it is inhibited in the cold and so occurs mainly in summer.

Nemertines can survive starvation for a year or more.  Pigment, much of the digestive tract, muscles and
gonads are phagocytosed. The epidermis and eyes dedifferentiate. The nervous system persists for a
long time. Starved anterior tips of
Lineus reduce in size and come to resemble Paramecium ciliates. Upon
further dedifferentiation they resemble pilidium larvae. After two or more years of starvation an ovoid body
remains with an inner and an outer epithelium with loose cells in between forming masses of large round
cells.

Ecology

Ribbon worms are mostly marine bottom dwellers found along predominantly temperate coasts under
stones, among plants and in mud, sand, and gravel, etc. Some dwell in mucus-lined tubes or parchment-
like tubes. They are also found in the Arctic and Antarctic but are less common around tropical and
subtropical shores.

Some ribbon worms occur down to 15 00 m or more in depth. There are some freshwater species of
Prostoma, and terrestrial species of Geonemertes. An eyeless variant of Prostoma is found in European
caves. Terrestrial forms are well equipped with mucus and frontal glands to reduce desiccation.
Geonemertes arboricola in the Seychelles inhabits leaf bases of the screw pine (Pandanus) up to 40-50
feet above ground.

Some nemertines are commensals living in the pharyngeal cavity or atrium or under the pedal disc of
tunicates, in the bivalve mantle cavity and in the gills and egg masses of crabs. Probably none are true
parasites. These commensals rely on the host ciliary currents. They have enhanced reproductive capacity,
but the proboscis, eyes and may be other organ systems are reduced.
Above: a computer model of a bootlace worm
(
Lineus longissimus). These remarkable worms,
which can be most easily found strewn and coiled in
tangled masses on estuarine mudflats at low tide
regularly reach 5 m in length, though 20 to 30 m is
not uncommon and lengths of 60 m have been
reported (though one has to be careful not to
stretch the worm when measuring it). Care must of
course be taken on mudflats, as quicksand is never
far away, but if one is found it is worth untangling it
and measuring it. The longer specimens are the
longest known animals on Earth, but are only a
few mm in diameter!

Left: a computer model showing the head of a
(generic) nemertine worm, which characteristically
has several eyes (the black spots seen here) in
patterns characteristic of the species.

Ribbon worms are often very long and the body may
be cylindrical or dorsoventrally flattened in
cross-section. They are often beautifully coloured
and patterned and very elegant when in the water
and a real treat to observe.
Nemertine cross section
Nemertine body wall
Pov-Ray computer model of a nemertine ribbon worm
Body wall

The external covering is a columnar glandular ciliated epidermis. The epidermal cells have broad distal
ends and taper to slender filamentous bases. One or two types of gland cell are also present, as well as
interstitial cells and sensory cells. Packet glands are present in some. These are clusters of cells opening to
the surface by a common duct. In palaeonemertines these packet gland cells occur within the epidermis, in
the heteronemertines they are subepidermal.

The interstitial cells are small and form a branched anastomosing network or syncytium in the base of the
epidermis. Hard rod- or sickle-shaped bodies occur in the epidermis of a few nemertines. These are
glandular products and may be equivalent to the rhabdites of platyhelminthes.

Pigment granules providing the animal its colour may occur in ciliated, gland or interstitial epidermal cells.

Beneath the epidermis is the
dermis (matrix or cutis) a connective tissue layer. This is a homogeneous
gelatinous stratum with nuclei and a few connective fibres, except in the heteronemertines in which the
dermis is a thick fibrous layer that often contains glands and muscle fibre strata and may be penetrated by
longitudinal muscle fibres from the body-wall musculature.

Beneath the dermis is a thick, powerful muscle stratum of body-wall musculature. Palaeonemertines and
hoplonemertines usually have two body-wall muscle layers, an outer circular and an inner longitudinal.
Sometimes there are two thin layers of diagonal fibres between the circular and longitudinal layers.
Palaeonemertines may have an additional circular layer inside the longitudinal layer, especially towards the
anterior of the worm and there may also be muscle fibres crossing from the longitudinal layer to the inner
circular layer, connecting these layers together. Heteronemertines have 3 body-wall muscle layers, an outer
longitudinal layer, a middle circular and an inner longitudinal.

There may be strengthening of the anterior longitudinal muscle layer to form tip retractor muscles. Radial
fibres permeate the body-wall musculature and extend to or into the epidermis.

The mouth is ventral and anterior or else there is only a proboscis pore and no mouth. The proboscis pore,
near the anterior tip, leads into the rhynchodaeum cavity. The anus is posterior and either terminal or dorsal
to the posterior tip (at the base of the caudal cirrus if this is present).

Dorsoventral muscle bundles may occur in the mesenchyme (and may connect intestinal diverticula if the
latter are present). These muscle bands are especially strong in flattened swimming forms and in the fin-like
posterior end of pelagic forms.

Floating pelagic nemertines have reduced body-wall musculature. The circular layer is especially reduced
while the longitudinal layer may form dorsal and ventral plates. Nemertine muscle fibres are all of the smooth
muscle type.

The anterior end is equipped with clusters of glands that usually open via a common pore on the anterior tip
above the proboscis pore, forming the cephalic or frontal glands. These glands may also open into a pit or
flask-shaped depression with sensory cells, forming a frontal organ.

Submuscular glands may be present in the mesenchyme of some hoplonemertines. These are
mucus-secreting and open on the body surface (usually ventrally).

Connective Tissue

Connective tissue occurs in the dermis, between the muscle layers, around blood vessels, nerves and
nephridia and fills the space between the digestive tract and the body-wall (this space is very variable in
size). The connective tissue is gelatinous and contains rounded vesicular cells. Fibrous connective tissue
may occur in the dermis and between the muscle layers, consisting of branched cells that possibly form a
syncytium. Lymphocytes are also present in the connective tissue.