Download Segmented worms: Annelida

Segmented worms: Annelids
Class Polychaeta:
There are over 5300 species of known polychaete.
Basic body plan
Polychaetes are metamerically segmented worms, with cylindrical body segments. Each
segment is equipped with a pair of lateral, fleshy, paddle-like appendages (parapodia).
The anterior end bears sense organs and is called the prostomium. The ventral mouth is
located between the prostomium and the postoral peristomium. The posterior end is the
unsegmented pygidium, which bears the anus.
Errant polychaetes are free-moving and may be pelagic, benthic or active burrowers.
They have well-developed prostomial sense organs: eyes, antennae and ventrolateral
palps. Other polychaetes are sedentary and reside in tubes or burrows.
The parapopdia are biramous, consisting of an upper notopodium and a ventral
neuropodium, both of which are supported by internal chitinous rods (acicula). A
tentacle-like cirrus projects from the dorsal base of the notopodium and the ventral base
of the neuropodium. The parapodia may bear chitinous bristles called setae or chaetae.
Each seta is produced by a single cell at the base of a setal sac. New setae replace lost
setae. The fireworms (F. Amphinomidae) possess brittle, tubular and calcareous setae
containing poison.
Body Wall
The outermost layer is the epidermis of cuboidal or columnar epithelial cells. This is
covered by a thin collagen cuticle and contains mucus-secreting glands. Beneath this is a
sheath of circular muscle overlying a longitudinal muscle layer. This longitudinal muscle
may form a sheath, but is often arranged in 4 bundles: 2 dorsolateral and 2 ventrolateral.
Beneath these muscle layers is a thin peritoneum, which lines the coelom.
Coelom
The coelom is spatious. Gut mesenteries divide the coelom into left and right halves,
though these mesenteries may partially or completely disappear. Transverse septa divide
the coeloms of each segment.
Life-styles
Surface-dwelling polychaetes. Surface-dwelling polychaetes are crawlers. These include
the fireworms, the nereids and the scale worms. The scale worms are covered dorsally in
scales or plates, called elytra, mounted on short stalks. These form channels for the
ventilating current when these worms are buried. In the sea mouse the dorsal surface,
including the elytra, is covered with a felt of setae, borne on the notopodia. The parapodia
and setae act like legs. Waves of parapodial activation travel down each side of the body,
alternating out of phase. These parapodia enable the nereids, for example, to crawl and
swim. When moving rapidly, both forms of movement are aided in the nereids by
longitudinal muscle contraction giving rise to body undulations. The power stroke of the
parapodia occurs on the crest of the undulatory wave.
Pelagic polychaetes. Pelagic polychaetes are transparent and exhibit swimming
movements similar to the crawling polychaetes. Examples include the Alciopidae with
their enormous eyes and the Tomopteridae, which lack setae and have numerous
parapodial pinnules instead.
Burrowing polychaetes. Burrowing polychaetes may be active, living in mucus-lined
galleries, or they may be sedentary, living in fixed U-shaped or vertical burrows.
Burrowers have reduced parapodia, a small pointed prostomium and usually lack eyes,
palps and antennae. Many move by peristalsis, for which they have well-developed
circular muscle and septa. These adaptations are similar to those found in the
earthworms.
Sedentary burrowers include the arenicolidae and the terebellidae. The parapodia are
reduced to transverse ridges bearing hook-like setae called uncini. The head may possess
specialised feeding structures. Some burrowing nereids and nephtyds have retained
surface-dwelling features, with well-developed parapodia and prostomial sense organs.
Tubicolous polychaetes. Tubicolous polychaetes live in a protective retreat or tube. This
may also serve as a lair for catching prey, in which case the worms may resemble
surface-dwelling polychaetes. The tubes allow the worms to gain access to clean water
above a muddy bottom, or may inhabit hard bare surfaces, such as rocks, shells, coral,
etc.
Tubes may be made from a secreted material (often produced by ventral glands) or from
sand grains cemented together, or from a combination of these two methods. The
parapodia are used to crawl through the tube. The quill-worms, Hyalinoecia (an onuphid)
secrete translucent quill-like tubes that lie horizontal and can be moved about by the
worm. The carnivorous Diopatra and Onuphis live in heavy membranous tubes with
funnel-like openings, which are camouflaged.
Many tubiculous polychaetes resemble sedentary burrowers with reduced prostomial
sensors. They have specialised feeding structures and move about in their tube by
peristalsis, using their uncinate setae to grip the tube wall. Bamboo worms (Maldanidae)
are intertidal worms living upside-down with their anterior end downward in their tube.
They have truncate heads and parapodia reduced to ridges.
The Sabellariidae and Pectinariidae live in tubes composed of sand-grains. Their heads
carry heavy setae and they have an operculum (2 fused segments grown forward and
dorsally) that can plug the tube entrance when the worm is retracted inside. Sabellaria
form reeflike colonies of aggregated tubes in turbulent water. The Pectinariids also live in
tubes of sand-grains. The head of the worm is equipped with large setae for digging in
sand and mud. Owenia tubes consist of inner secreted, membranous linings with an outer
covering of sand grains. Flat, overlapping sand grains are attached at one edge of the tube
to give it some flexibility. A ventral pouch beneath the mouth stores sand grains suitable
for tube construction collected whilst feeding. Paired glands in the first seven trunk
segments (14 glands in total) produce the tube secretion, which is then applied by the
parapodial setae as the worm revolves.
Fan worms, feather dusters and Christmas tree worms (F. Sabellidae, Serpulidae) have a
funnel-shaped or spiral crown of prostomial palps bearing pinnate processes called
radioles. These radioles are rolled-up or closed together when the worm withdraws into
its tube.
In serpulids the most dorsal radiole has a long stalked knob that functions as an
operculum. The peristomium is folded back to form a collar, which molds additions to the
tube. Serpulids have two calcium carbonate glands beneath the collar folds and ventral
surface glands on the anterior segments secrete an organic cement.
The Sabellids live in membranous or sand tubes, whilst the serpulids live in calcareous
tubes attached to rocks, shells, algae, etc. Sabella tubes consist of sand grains embedded
in mucus. The worm sorts detritus collected by the ciliated radioles. A pair of mucusproducing ventral sacs below the mouth store sand grains suitable for tube construction.
The collar folds mold rope-like strands of mucus and embedded sand grains and attaches
them to the ends of the tube.
Boring Polychaetes. These burrow in dead or living calcareous shells and coral.
Christmas tree worms (the sabellid genus Spirobranchus) lives on the surface of living
coral. Polydora excavates U-shaped burrows in oyster shells, which may give rise to socalled mud blisters when the oyster reacts defensively to the shell damage. These blisters
reduce the marketable value of the oysters.
Interstital Polychaetes. These are minute and some are very aberrant.
Commensalism. Tube dwelling and burrowing polychaetes may act as hosts to scale
worms, crustaceans, including hermit crabs. Scale worms may also live in the ambulacral
grooves of sea stars, sea urchins and sea cucumbers and in corals, and in the burrows of
echiuroid worms, polychaetes and crustaceans. Those living in other polychaete burrows
may possess clinging setae and often are a similar colour to their host.
Parasitism. The parasitic lifestyle is not common among polychaetes. However,
Labrorostratus and other arabellids live in the coelom of other polychaetes and may be
almost as big as their host. The bloodsucking Ichthyotomidae are ectoparasites that attach
to the fins of marine eels. The peculiar myzostomes are commensals and parasites of
echinoderms.
Nutrition
Raptorial feeders. These include surface-dwellers, pelagic, tubiculous eunicids and
onuphids and gallery dwellers like the glycerids and nephytids. They eat small
invertebrates, including other polychaetes. They have an eversible pharynx (proboscis)
equipped with two or more horny jaws of tanned protein used to catch their prey.
Protractor muscles or an elevated coelomic pressure, as a result of body wall muscle
contraction, Everest the pharynx, which is withdrawn by retractor muscles (connecting
the body wall to the pharynx). Enlarged anterior parapodia may grip the prey.
Tubuculous raptorial feeders may leave their tube partially or completely to seize prey.
Chemoreceptors may monitor the ventillating water current for signs of nearby prey.
Glyceran live in galleries in muddy bottoms. These galleries have numerous loops
opening to the surface. The worm waits at the bottom of one of these loops and detects
surface movements of prey by changes in the water pressure. They slowly move to the
burrow opening and seize their prey with the proboscis. The proboscis lies in the first 20
segments when retracted and has 4 jaws on its tip. Each jaw is connected to a poison
gland at the jaw base, via a canal.
Herbivores, omnivores, scavengers and browsers. Nereis pelagica, Nereis virens and
Nereis diversicolor eat algae, other invertebrates and detritus. They use their jaws to tear
off pieces of algae. Nereis succinea and Nereis longissima eat detritus. Nereis fucata is
carnivorous.
Nonselective deposit feeders. These worms have a simple non-muscular proboscis, which
is everted due to an increase in coelomic pressure. These include burrowers and tube
dwellers. Arenicola lives in an L-shaped burrow open to the surface. The head of the
worm is in the blind horizontal part and continually ingests sand. Arenicola irrigates its
burrow by peristaltic waves (the water leaves the burrow by percolating through the
sand). At cyclic intervals the worm backs-up to the surface to defecate castings of
mineral material.
Bamboo worms (Maldanidae) ingest the substratum at the bottom of their sand tube. Cilia
within the everted pharynx drive particles into the gut, and select those particles the right
size for consumption. These worms also periodically back-up to defecate out of the top of
their tube.
Selective deposit feeders. These lack a proboscis, but instead they have specialised
feeding structures that collect particles on mucus secretions, which are conveyed to the
mouth along ciliated tracts or grooves. The Terebellids: Amphitrite, Terebella and
Neoamphitrite deploy contractile prostomial tentacles that stretch over the surface by
ciliary creeping. Surface detritus adheres to mucus secreted by the tentacular epithelium.
Entrapped particles are moved down a ciliated gutter formed by the rolled tentacle. Food
accumulates at the base of the tentacles. The tentacles are wiped over the upper lip
bordering the mouth and cilia on the lip drive the food into the mouth.
Owenia has a similar prostomial crown of flattened, branched filaments. Pectinaria, the
cone worm, lives buried head down in the sand. These worms select organic material: an
organic content of 32% in the sediment is concentrated to 42% in the gut, of which 30%
is utilised.
Tubiculous spionids, e.g. Spio, are suspension feeders. They have two long, tentacular
palps that lash water or project from the tube and extend over the bottom. Particles are
conveyed to the mouth in a ciliated channel.
Filter feeders. These are sedentary burrowers or tubiculous. Feeding structures collect
particles, which are moved to the mouth along ciliated tracts. Serpulid and sabellid fan
worms have crownlike, bipinnate radioles that form a funnel of 1-2 spirals. Cilia direct
water flow through the radioles upward into the funnel and out. Particles trapped on the
pinnules move along a ciliated groove to the base of the radiole. A sorting process occurs:
the largest particles are rejected, fine particles move along a ciliated tract to the mouth
and medium sized particles may be stored for tube construction.
Chaetopterids, e.g. Chaetopterus, live in U-shaped burrows. Notopodia on the 12th
segment are long and aliform (winglike) and covered by a ciliated epithelium, richly
supplied with mucus glands. Notopodia on segments 14, 15 and 16 are modified and
fused into semicircular fans that project against the cylindrical wall of the tube. These
fans beat, generating a current of water, which enters the tube near the head end and
flows out at the tail end. The paired aliform notopodia are stretched out around the walls
of the tube with a sheet of mucus secreted between them, forming a bag of mucus, which
is continually secreted. A ciliated cupule (food cup) on the middorsal side of the worm
(behind the aliform notopodia) grasps the posterior end of the mucous bag and strains out
detritus and plankton. If the peristomial cilia detect large objects, then the aliform
notopodia are pulled back to let them pass by. The food-laden mucous-bag is continually
being rolled up into a ball by the dorsal cupule. When a certain size, the bag is cut loose
from the notopodia and rolled up. The cupule projects forward, depositing the mucous
food ball onto the ciliated middorsal groove, which conveys it to the mouth.
Spiochaetopterus may process up to 13 mucous bags at once. Some genera use cilia
rather than parapodial pumps to generate the water current.
Alimentary canal. This is a straight tube from the anterior mouth to the posterior anus.
Often this tube is regionated into a pharynx or buccal cavity, a short oesophagus, a
stomach (in sedentary spp.) or an anterior intestine, which secretes enzymes, an
absorptive intestine and a rectum. However, the gut often appears as a uniform tube
(differentiated histologically). The intestine may have folded walls, to increase its
working surface area. Nereis has two large glandular caeca that open into the anterior
intestine into which they release their enzymatic secretions. Neoamphitrite has a coiled
intestine.
Waste may be ejected into the tube water currents, or the pygidium thrust out of the
opening of a sand or mud burrow. In fan worms, ciliated grooves carry faecal pellets
from the anus, anteriorly out of the tube. Compacted faecal pellets and strings reduce the
risk of fouling.
Respiration
Gills are common in polychaetes. Theses may be modified dorsal cirri of the notopodia
of the parapodia. In Amphitrite the gills are arborescent anterior outgrowths. Gills may be
absent from very small or threadlike polychaetes. In scale worms the dorsal body surface
is covered by cilia that generate a respiratory water current that flows posteriorly beneath
the elytra. Sea mice (Aphrodita) have no cilia; instead the elytra are tilted up then down
in sequence, to generate a water current. Gill cilia or gill contractions may provide
ventilation. Tubes and burrows are irrigated by water currents, which may be generated
by body undulations or peristalsis. Such a ventilating rhythm may increase oxygen
consumption 15 times, but there is still a 50% net gain.
Internal transport
A blood vascular system is present in some polychaetes. Blood flows to the anterior in a
dorsal vessel. An anterior network of connecting vessels around the gut carries blood
from the dorsal vessel to the ventral vessel. The ventral vessel conveys blood to the
posterior, and gives off intestinal vessels and parapodial vessels (1 pair per segment) and
vessels supplying the nephridia and the body wall. Either afferent and efferent loops
supply the gills; or else the gills are supplied by coelomic irrigation. In fan worms each
radiole contains a single vessel.
The blood circulatory system is closed, comprising vessels. These vessels are sinuses
with basement membrane lining the lumen and contain myoendothelium (or lack
endothelium). The blood is pumped by peristaltic contractions of the vessels, especially
the dorsal vessel. The circulatory system is often supplied by accessory, heartlike pumps.
Plasma respiratory pigments may be present, especially in large worms and burrowers in
soft sediments. Haemoglobin (Hb) or green chlorocruorin (esp. in fan worms) may be
present. Both are iron porphyrins. Magelona has anucleated corpuscles containing
haemerythrin. This is an iron-containing non-porphyrin pigment. Arenicola has plasma
haemoglobin, consisting of 96 haeme units, with a molecular weight of about 3 x 106.
(Mammalian haemoglobin contains 4 haemes and has a molecular weight of 6 x 104).
Coelomic fluid may contain low molecular-mass haemoglobin in the coelomocytes. In
some terebellids, e.g. Glycera (bloodworm) and Capitella, the blood-vascular system is
reduced or absent and the coelomic system dominates the circulatory function.
Plasma pigments seem to be present in animals lacking capillaries, and results in less
viscous blood than those with pigment-carrying corpuscles.
Serpula contains haemoglobin and chlorocruorin. Most terebellids and ophelids have
coelomic red cells and a blood-vascular system with a different haemoglobin. For
example, Amphitrite coelomic Hb has a greater oxygen affinity at low oxygen tensions
(the dissociation curve is shifted to the left). This facilitates the passage of oxygen from
the blood-vascular system to the coelomic fluid and thence to the internal tissues.
Annelids are oxyconformers, altering their oxygen consumption according to supply. For
example, at low tides Euzonus mucronatus has an oxygen store that lasts 2-4 hours,
accompanied by a reduction in oxygen consumption. However, this worm may switch to
anaerobic respiration and hence endure 20 days without replenishing its oxygen supply.
Excretion
Polychaetes may possess blind-ended protonephridia or metanephridia. Generally
there is one pair per segment, but some have only one pair per worm. The anterior end of
the nephridial tubule sits in the coelom of the segment anterior to its nephridiopore
opening. The tubule, which may be coiled, penetrates the posterior transverse septum and
opens anteriorly to a nephridiopore (neuropodium).
In protonephridia, solenocytes form the preseptal end. These are tubules containing a
single flagellum. The walls of these tubules consist of parallel rods connected by thin
lamellae. These fenestrations allow the fluid transit.
In metanephridia the preseptal end is an open ciliated funnel, or nephrostome. The
postseptal canal is often greatly coiled (for secretion or absorption?). The tubule lining is
ciliated. Nereids, for example, have metanephridia.
Fan worms have one pair of functional nephridia that join in the midline via a single
median canal and a nephridiopore on the head.
Some polychaetes live in estuaries, Nereis succinea, for example, has ion absorbing cells
in its gills. A few polychaetes are freshwater species, for example Manayunkia speciosa
lives in the Great Lakes. A few species of polychaete are terrestrial.
The chlorogogen tissue, coelomocytes and the intestinal wall are also possible excretory
routes. The chloragogen tissue is composed of brown-greenish cells on the wall of the
intestine or the walls of some blood vessels. This tissue is involved in intermediary
metabolism and Hb synthesis (a functional liver?).
Nervous System
The brain is usually bilobed and situated dorsally in the prostomium. It sends nerves to
the palps, eyes, antennae and nuchal organs and circumpharyngeal / circumoesophageal
connectives that surround the anterior gut. These connectives join to a double ventral
nerve cord. The two tracts of this cord are more or less fused. This ventral cord is
ganglionated, with 1 ganglion or 1 ganglion pair per segment. In fan worms there are
transverse connectives between separate ganglia in each ganglion pair. Each ganglion or
ganglion pair gives out 3-4 pairs of nerves per segment that innervate the body wall.
Giant axons innervate the longitudinal muscles and are involved in a defensive
contraction reflex. These large fibres conduct signals rapidly at 12 m/s, compared to 0.5
m/s for more typically sized polychaete axons.
Sense Organs
Eyes are best developed in errant polychaetes, in which there are 2,3 or 4 pairs on the
prostomium. Some polychaete eyes are capable of image formation. The radioles of some
sabellids are equipped with eyespots. Serpulid radioles possess dispersed photoreceptors.
Characteristically polychaetes exhibit a shadow escape reflex.
The nuchal organs are a pair of ciliated sensory pits or slits, often eversible. They are
involved in food detection and best developed in predators.
Statocysts are found in many sedentary burrowers and tube dwellers and allow
coordination of burrowing directionality. In Arenicola statocysts are found in the body
wall of the head. Each opens, via a canal, to the outer lateral body surface. Each of these
statocysts contains spicules, diatom shells and quartz grains covered in chitinoid material.
Regeneration
Tentacles, palps and heads can be regenerated, especially in burrower and tubiculous
forms. A new head will regenerate where the nerve cord is severed.
Reproduction
Asexual reproduction occurs by budding or fission in some forms, but most polychaetes
only reproduce sexually. Most are dioecious. The gonads are masses of developing
gametes and not distinct organs. They develop from specific regions of the peritoneum.
They occur in most segments, or only the abdominal segments.
Some fan worms are hermaphroditic. The anterior abdomen contains eggs while the
posterior abdominal segments contain sperm. The gametes are shed into the coelom
where they mature. In pelagic forms they are released by rupture of the body wall and the
worm dies during spawning. If present, 1 pair of coelomoducts / gonoducts per segment
develop at sexual maturity. These open via a ciliated funnel. The coelomoducts may join
the nephridia, and open via external nephridiopores. In many male nereids the sperm exit
through special anal apertures.
Epitoky. Production of pelagic reproductives is called epitoky and such a pelagic
reproductive individual is known as an epitoke. Epitokes have greatly different posterior
body segments, packed with gametes and equipped with swimming setae, which are long
and spatulate. Atokous forms may give rise to epitokes from caudal buds. In nereids
atokous forms transform into epitokes. In yet other polychaetes the posterior part of the
body detaches as an epitoke. Epitokes may swarm, luminescing as they do so in
Odontosyllis enopla. Swarming is induced by pheromones and is often synchronised with
lunar periods and occurs at dawn and dusk (in response to light levels).
Courtship behaviour may be apparent. In Autolytus (a syllid), males swim in circles
around the female, touching her with antennae and releasing sperm.
Egg Deposition. Eggs may be shed into sea water and become planktonic. Alternatively
eggs may be retained in tubes or burrows or attached to objects in mucous masses.
Brooding may occur within tubes, within the coelom, or within the cavity of the
operculum. In Autolytus the eggs are brooded in a secreted ventral sac attached to the
body.
Development
Cleavage is spiral. Gastrulation produces a trochophore larva. This larva may be
planktotrophic (feeding on plankton) or lecithotrophic (yolky) as in nereids and eunicids,
and non-feeding, existing near the bottom. Alternatively, the trochophore may be retained
in the egg prior to hatching, giving rise to an adult or free-swimming larva. The
trochophore undergoes metamorphosis into a young adult. Segments develop from the
anterior to the posterior, and so the oldest segments are near the head of the adult. The
developing larva may sink or remain planktonic during metamorphosis.
Annual species live for 1-2 years and spawn once. Their larvae are planktonic for about
one week or more. These forms produce many small eggs. Perennial species breed more
than once a year, and produce fewer, large, yolky eggs that give rise to non-feeding
benthic larvae. Finally, multiannual species have short lifespans and undergo several
generations per year. They produce few, large, yolky eggs that also give rise to nonfeeding benthic larvae.
Ecology
Polychaetes are extremely abundant. For example, in Tampa Bay, Florida, estimates
revealed an average of 13,425 polychaetes per m2, representing some 37 different
species. Polychaetes make-up 40-80% of infauna on the upper continental slope and the
deep ocean floor. Predation and other factors limit polychaete numbers and not the
resources available.
Pogonophora (Beard Worms)
External characteristics
Very long, slender worms that inhabit a close-fitting tube of their own secretion. Length
varies from under 10 cm to the 3m long Riftia pachyptila. Most are very thin, ranging in
diameter from <0.5 mm to 2.5 mm, but Riftia pachyptila is 4 cm in diameter.
The tube has a smooth contour, or is made-up of rings or funnel-like pieces, and may
show alternating dark and light bands. The oral end of the tube is thinner, less opaque and
may bear a funnel expansion. Zenkevitchiana longissima is 35 cm long, including
tentacles, and lives in a 1.5m tube that sits erect in the bottom ooze.
The body is divided into a protosome, mesosome and a metasome (trunk). The protosme
– mesosome division is often not externally visible, giving rise to a protomesosome. A
visible constriction separates the mesosome from the metasome, which marks the
position of the muscular mesosome-metasome septum.
The protosome is made-up of the anterior cephalic lobe, which contains the central
nervous ganglion, and 1-200 or more tentacles. The tentacles bear a fringe of lateral
pinnules on one or both sides and spring from the base of the cephalic lobe in a single
spiral. There is no mouth, anus or digestive tract.
On the mesosome is a region called the girdle or bridle, which possess a pair of ridges of
thickened cuticle, which may fuse ventrally. These are thought to hold the worm in its
tube when it protrudes. Belts are formed from 2 adjacent girdles, and may bear hardened
platelets bearing denticles. This belt divides the worm into preannular and postannular
regions.
A midventral groove begins behind the mesosome-metasome septum partway along the
preannular region. The edges of this groove bear conspicuous glandular papillae, which
may be covered in hardened platelets. It is thought that these glands may produce the tube
secretion or an adhesive secretion. There is also a preannular dorsal longitudinal
ciliated strip. Postannular papillae may be present, grouped either irregularly or in
ventral rows, giving the appearance of segmentation (pseudosegmentation).
Body Wall
The glandular epidermis is more or less columnar and covered with a cuticle. Beneath
this there is a thin layer of circular muscle, followed by a thicker layer of longitudinal
muscle, which is especially thick in the mesosome. There may be 2 longitudinal bundles
in the anterior protosome that contribute to the muscles of the tentacles.
The hollow tentacles contain an inner peritoneum lining, enclosing the coelom that
houses (usually two) blood sinuses. A tentacular nerve occurs outside the coelom, and
there are 2 muscle bands in each tentacle. The pinnules are long, slender extensions of
epidermal cells. The two adjacent epidermal cells are ciliated, giving rise to two
longitudinal ciliated tracts.
Nervous System
There is an intraepidermal nervous system. The brain (ring-shaped?) in the cephalic lobe
thought to project a posterior middorsal nerve underneath the middorsal ciliated band.
The brain also gives off tentacular nerves. There is a diffuse nerve ring in the mesosomemetasome septum.
Coelom
The eneterocoelous coelom lacks a definite peritoneal lining. The protosome contains a
single protocoel, which is connected to the exterior by one pair of coelomoducts. These
coelomoducts may represent nephridia, although they lack a nephrostome. The protocoel
is divided by dorsolateral muscle bands and gives rise to the tentacle coeloms. The paired
mesosomal coeloms lack coelomoducts. The paired metasome coeloms connect to the
exterior via gonoducts.
Circulatory System
The circulatory system is closed and well-developed. A middorsal blood vessel and a
midventral blood vessel run the length of the mesosome and trunk. There are also 2 pairs
of lateral vessels in the trunk. The ventral vessel is enlarged into a heart in the protosome.
The heart gives off anterior branches into the tentacles. An afferent and an efferent vessel
supply each tentacle. Both of these vessels give rise to a loop into each pinnule. The
efferent tentacular vessels connect to the dorsal vessel in which the blood runs backward.
Blood runs forward in the ventral vessel. No blood cells have been observed.
Nutrition
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No alimentary canal is present at any stage
No known active feeding processes
No extracorporeal enzymes known
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Assumed to feed by:
(i) absorbing dissolved organic compounds
(ii) pinocytosis?
(iii) symbiotic bacteria
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Amino acids, glucose, fatty acids - taken up from water and concentrated in blood.
Well-vascularised tentacles may enhance absorption
Protein and ferritin also absorbed, pinocytosis? - body cuticle?
Tube walls contain chitin (= diatom -chitin) and are known to be permeable at least
to water, NaCl, sucrose and phenylalanine.
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Trophosome tissue, in the trunk, has a good blood supply and contains symbiotic
chemoautotrophic Gram-negative bacteria – these oxidise hydrogen sulphide (with O2
or nitrate) and use the energy to produce organic carbon compounds.
ATP + reducing power
reduce + fix CO2
(H2S) Sulphur oxidation
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Hydrothermal vents are sulphide rich. Sulphide and oxygen bind to haemoglobin in
the tentacle plumes and are carried to the trophosome.
The bacteria are acquired through the mouth and gut of the young, which disappear in
the adult.
Reproduction
Pogonophores are dioecious. The position of the gonopores is the only sexually
distinguishing character. The gonads are a pair of elongated bodies in the metacoel (one
in each half of the metacoel). The cellular walls of the gonads are all that separates them
from the coelomic fluid. The testes open via a pair of ventral gonopores behind the mesometasome septum. The sperm ducts are filled with spermatophores. The ovaries open via
a pair of posterior oviducts in the middle of the trunk. The eggs are large and yolky, and
many species brood their eggs in the tube. Each batch consists of 10-30 eggs.
Development
The young are found in the parental tube and resemble young stages of Cephalodiscus
buds. The pole of the egg facing the tube outlet becomes the anterior of the new worm.
Cleavage is holoblastic and unequal, forming larger vegetal blastomeres. No blastopore is
formed, gastrulation occurring by delamination. The embryos show bilateral symmetry
and contain a spongy digestive tract rudiment, which is yolky and consumed for nutrition.
Adult worms have no trace at all of a digestive tract.
Ecology
Riftia pachyptila was recently discovered around deep-sea hydrothermal vents, over 1
mile deep on the floor of the Pacific Ocean. These belong to the order vestimentifera,
which is sometimes given its own phylum status. Here they belong to ecosystems
dependent on the activity of chemoautotrophic bacteria, and independent of a direct solar
energy input.
Classification
Class I Frenulata
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Frenulum or bridle - a ridge of tissue running obliquely round the forepart
Usually setae on trunk
One to 200+ tentacles
Tubes found in soft sediments
O. Thecanephria
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Anterior 'coelom' of cephalic lobe horseshoe - or corkscrew-shaped, opening to
exterior by 2 median coelomoducts
Flat spermatophores
1-200+ tentacles
e.g. Polybrachia, Lamellisabella, Spirobrachia.
O. Athecanephria
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Body cavity of cephalic lobe sac-shaped, communicating with exterior by 2 lateral
coelomoducts
Spermatophores cylindrical
1-20 tentacles
e.g. Oligobrachia, Siboglinum.
Class II Afrenulata
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No frenulum, but a pair of lateral tissue folds meeting in dorsal mid-line
(overlapping), extend anteriorly
No setae on trunk
Obturaculum - plug of hardened tissue among tentacles, used to plug tube
1000+ tentacles
Tubes attached to hard substrates
O. Vestimentifera
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e.g. Lamellibrachia, Riftia.
Summary of Pogonophora
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About 100 known species
Most up to several cm in length
Riftia pachyptila, living around hydrothermal vents in the eastern Pacific Ocean, may
reach 3m.
Marine, free-living, benthic
Sedentary, living in secreted tubes
No pelagic larva
Development: total, unequal and bilateral
Adult polymeric (with a number of segments): divided into 4 regions:
 tentacular region
 vestimentum (forepart)
 trunk
 opisthosoma
Tentaculate
No digestive system or alimentary canal
Simple nervous system (subepithelial nerve plexus) with a median (ventral) cord
Closed blood-vascular system, each tentacle has two vessels
Coelomate - a coelomic compartment in each body division and the coelom extends
into the tentacles
Dioecious
Two cylindrical gonads - one on each side in the trunk coelom
Two male gonopores at anterior part of trunk
Two female gonopores farther back on the trunk
Many species brood the eggs within the tube
1-1000+ tentacles, may be arranged in complex patterns
Lamellibrachia barhami has 2000 tentacles arranged in 25 concentric lamellae around
and attached to a double lophophore.
Haemoglobin extracellular - in both vascular blood and fluid of the body cavities in
Riftia pachyptila - similar to annelid haemoglobin.
Echiura (Spoon-worms)
There are about 135 species of known echiuroid.
External Characteristics
Echiuroids are coelomate burrowing worms, and are unsegmented in the adult. The body
may be up to 40 cm in length, excluding the proboscis. The body is rounded or a
cylindrical sausage-shape and is smooth or papillate. The proboscis may be very long and
extensible; for example, Ikeda has a 40-cm trunk and a 1.5-m trunk. Bonellia has an 8cm
trunk, but can extend its proboscis up to 2 m. In some species, however, the proboscis is
much shorter than the trunk, for example in Echiurus. The proboscis is actually a cephalic
lobe (cf. The prostomium in annelids) containing the brain and is non-retractable. It is
able to extend by ciliary creeping and is very mobile. The proboscis is flattened and may
bifurcate and usually has a ciliated ventral gutter. The mouth opens at the proboscis base.
Just posterior to the proboscis is one pair of ventral chaetae / setae, which are curved or
hooked. In addition Urechis and Echiurus also possess 1 or 2 circlets of setae around the
posterior of the trunk. There is also one pair of anal vesicles on the cloacal region of the
intestine.
Echiuroids are usually drab gray or brown in colour. Some (e.g. Bonellia) are green,
while some are red or rose.
Nutrition
Most echiuroids burrow in mud, sand or else reside in small natural openings in rocks and
among shells. Most echiuroids are detritus feeders and obtain food from surrounding
surfaces with their proboscis. The proboscis is projected from the burrow and stretches
over the substrate by ciliary creeping, due to the cilia on its ventral surface. The extended
proboscis sweeps the substrate and detritus adheres to secreted mucus covering the
proboscis. And is driven back into the median ventral ciliated groove, which conveys the
particles to the mouth. The anus is on the posterior terminus. The gut, especially the
intestine, is highly coiled.
The innkeeper worm, Urechis caupo lives in a U-shaped burrow. The proboscis is very
short. A circlet of mucous glands girdles the anterior part of the trunk just behind the
setae. The glands are brought into contact with the burrow wall and their secreted mucus
is spun-out as the worm backs-up, forming a funnel of mucus. Peristaltic action pumps
water through the burrow, all the water passes through the funnel. The mucous funnel
traps particles, and when it is laden with food this mucous net is detached from the body.
The worm backs-up, seizes the net with its proboscis and swallows it. About 30 litres of
water pass through a Urechis burrow each day.
Circulation & Coelom
The blood-vascular system is closed (except in Urechis) and similar to that of annelids.
The blood is colourless, but some coelomocytes contain haemoglobin. The schizocoelous
coelom is a large, fluid-filled cavity in which the fluid and amoebocytes circulate. The
circumintestinal vessels are contractile and body wall movements also aid the circulation.
Excretion
Spoon-worms possess 1-100’s of pairs of large sac-like metanephridia. (Bonellia has one
pair, Echiurus two pairs and Ikeda 100’s of pairs). In Bonellia and Echiurus the anterior
nephridiopores open to the outside just behind the anterior setae. In Thalassema the males
have more nephridia than the females. Echiuroids also possess one pair of anal sacs,
which are simple or branched diverticula arising from each side of the rectum. These sacs
posses numerous ciliated funnels over their surface. These funnels open into the coelom,
and their collected waste is eliminated through the anus.
Respiration
In Urechis the hindgut pumps water in and out and is a major site for gas exchange. Gas
exchange also occurs over the body wall, especially the proboscis, which has a large
surface area. Peristaltic waves ventilate the burrow. The frequency of these waves
increases in oxygenated water, suggesting that echiuroids are oxyconformers.
Movement
Spoon-worms move within their burrows by peristaltic contractions. The setae provide
traction for gripping the burrow walls. Some echiuroids can squeeze through very
confined spaces. The body is very well muscularised and a spatious coelom acts as a
hydrostatic skeleton. However, these worms show no locomotory movements outside
their permanent residence. Longitudinal, circular and oblique muscles are present in the
body wall.
Urechis controls its internal hydrostatic pressures by adjusting volume of water in the
hindgut. When the internal pressure increases due to a peristaltic pulse, the anus opens
expelling water resulting in a drop in internal; pressure. If internal pressure falls, then
water is inhaled into the hindgut. Forced exhalent currents relieve pressure in the gut and
clear the burrow of its contents.
Nervous System
There is a single, solid ventral nerve cord, which bifurcates anteriorly into 2
circumpharyngeal connectives that unite in the proboscis in the central nervous mass or
brain. The ventral nerve cord may be sinuous, contains no ganglia and gives off 100’s of
nerves. A multicellular giant fibre in the ventral nerve cord conducts signals at 1.5 m/s in
both directions. Pacemakers in the nerve cord regulate anal pumping and peristalsis.
Peristalsis is initiated in the proboscis, which contains the brain.
Reproduction & Development
The sexes are separate. The gonads are in the peritoneum of the ventral mesentery in the
trunk. Gametes are released into the coelom where they mature and subsequently escape
through the nephridia. Fertilisation is usually external, except in Bonellia. In this species
the dwarf males are ‘parasitic’ on the females. The males are minute and ciliated and
have no proboscis, no blood system and a vestigial gut. The males are found in the female
nephridia, in the coelom, or in the oesophagus / pharynx or, as in Pseudobonellia, in
specialised male tubes. Developing males may occur on the female proboscis. The eggs
are fertilised in the nephridia.
In Bonellia viridis the female trunk is up to 8 cm long, while the male is only 1-3 mm
long and lives in the female oesophagus or nephridia. Sex is determined at the larval
stage. Any larva contacting and entering a female develops into a male, under the
influence of a female hormone. Initially the larva contacts the female proboscis, and
attaches to it via an adhesive secretion. The proboscis secretes the hormone that causes
the larva to initiate male development. After a few days on the proboscis, the male passes
into the female oesophagus and into a nephridium or a special male tube, where it
matures in 1-2 weeks. One female may house about 20 males. Larvae that do not
encounter a female become females and require over one year to reach maturity.
Spiral cleavage gives rise to free-swimming trochophore larvae. These trochophores pass
through a metamerically segmented stage with 10 pairs of rudimentary coelomic pouches.
Spoon-worms are thus thought to be related to annelids and sipunculoids.
Ecology
Most species live in sand or mud burrows or in natural crevices or spaces among rocks
and shells. Thalassema inhabits dead sand dollar shells and becomes too large to leave its
permanent abode. All are marine, most sub-littoral and a few live in deeper waters.
Spoon-worms are cosmopolitan and some species live in brackish water.
Hemichordata: Enteropneusta (Acorn Worms)
External Features
The acorn worms are solitary cylindroid worms with numerous gill slits, but no
tentaculated arms. Length varies from several cm to over 2 m, for example, Saccoglossus
pygmaeus is 2-3 cm, while Balanoglossus gigas is 1.8 – 2.5 m. Enteropneusts are soft and
covered with slime. They have no external appendages and no exoskeleton. The body is
divided into three regions: an anterior proboscis (protosome), a middle collar (mesosome)
and a trunk (metasome).
The proboscis is short, rounded or conical, or elongated in Saccoglossus, and is circular
in cross-section. The proboscis may have a deep middorsal groove and may possess a
midventral depression in its base. The proboscis stalk is more or less concealed by the
collar, and is continuous with the inner dorsal surface of the collar.
The collar is a short cylinder, usually shorter than the proboscis. The funnellike anterior
part forms a collarette that embraces the proboscis stalk (and sometimes the posterior
proboscis). A circular indentation marks the collar off from the trunk. The mouth is
ventral and situated inside the collarette.
The trunk has a middorsal and a midventral ridge. These accommodate median
longitudinal nerves and blood vessels. The trunk musculature consists of a longitudinal
layer beneath the epidermis, which is usually thicker ventrally and diminishes posteriorly.
This muscle layer is very thin or absent on the hepatic sacculations. Saccoglossus has two
thick ventral longitudinal muscle bands, which cause a spiral twisting of the trunk. There
is also a circular muscle layer inside or outside of the longitudinal muscle layer.
The posterior trunk may possess visible sacculations where the hepatic part of the
intestine shows externally, as in Balanoglossus and Ptychodera. The anus is terminal.
Visible trunk annulations are formed by annular grooves with low poorly glandular
epidermis and high glandular epidermis between the grooves.
Enteropneusts are usually drab buff coloured, but the proboscis, collar and/or body may
be orange or reddish. The hepatic region is brown. The colours of the ripe gonads may
show through the body wall.
Body wall
The external surface consists of a tall, slender, glandular epidermis. The gland cells are
classifiable into mulberry cells containing coarse granules, reticulated cells and goblet
cells. The bases of each epidermal cell gives rise to a fine fibril that crosses the epidermal
nervous layer situated at the base of the epidermis. These epidermal fibres are
continuations of a strong elastic fibre traversing the cell. The epidermis and the epidermal
nervous layer rest upon a basement membrane, comprised of two lamellae pressed
together. An additional reticulated membrane may be present between the main
epidermal layer and the basal nervous layer.
The basement membrane is thickened to form the proboscis skeleton in the proboscis
stalk. This consists of a median plate with 2 posterior horns and sometimes a midventral
keel. This skeletal plate is a combination of epidermal and coelomic tissue secretion.
A definite peritoneum is only present in Protoglossus. Instead the coelomic cavities are
mostly filled with connective tissue and muscle fibres.
Nervous System
An intraepidermal nervous layer occurs in the base of the epidermis (cf. Asteroid nerve
cords). This nervous layer is thickened to form cords in the middorsal and midventral
grooves of the proboscis. There is an anterior nerve ring at the proboscis base.
The trunk contains middorsal and midventral cords, which terminate at the anterior of
the trunk. A circumenteric (prebranchial) nerve ring connects the dorsal and ventral
cords. The dorsal cord extends to the collar, where it leaves the epidermis and runs
through the coelom above the buccal tube as the collar cord (‘neurocord’). This collar
cord is the nervous centre and develops from an epidermal invagination. The collar cord
may have a continuous open lumen, or else contains numerous small cavities. The collar
cord is connected to the exterior at both ends via anterior and posterior neuropores, in
epidermal depressions. The collar cord is a conduction path only, contains no cell body
concentrations and gives off no nerves and is covered externally by a basement
membrane derived from the epidermis. Ciliated epidermis lines the internal cavities of the
cord. The collar cord contains 10-160 giant cells.
There is an additional nervous layer in the base of the digestive tract epithelium, with
midventral and middorsal thickenings connected by a ring-like thickening at the entrance
to the buccal tube. Nerve fibres from ventral mesentery plexi supply the blood vessels
and muscles.
Sense Organs
Epidermal neurosensory cells are distributed over the epidermis. These are more
numerous on the proboscis, especially at its base. There may be a middorsal organ –
presumably sensory - is connected via a strong nervous strand to the middorsal cord of
the proboscis.
A preoral ciliary organ is present in some species on the ventral side of the proboscis at
the junction with the stalk, or on the stalk itself. This is a U-shaped epidermal depression
bounded by an epidermal ridge.
Photoreceptors are present as modified neurosensory cells and all parts of the surface are
photosensitive, especially the proboscis and collar. The proboscis is essential for response
to light and an isolated proboscis will exhibit negative phototaxis.
If agitated acorn worms retreat by reversed posteroanterior peristalsis. If the proboscis is
prodded there is a shortening reflex, accompanied by coiling of the trunk. It has been
hypothesised that giant cells mediate this retreat reflex.
Coelom
The coelomic wall gives rise to muscular and connective tissue (an ontogenetic
peculiarity of the group). The adult coelom is reduced and lacks a definite peritoneal
lining. The proboscis coelom is an unpaired cavity partially divided into 2 ventrolateral
chambers and 2 dorsolateral chambers. The left dorsolateral chamber is generally larger
than the right. The right is usually blind and the left opens to the dorsal surface via a
proboscis canal and proboscis pore in an epidermal invagination. This pore may be
medial. In other species the right chamber opens in a pore whilst the left is blind, or both
may open via two pores. There is a sphincter muscle at the junction of the proboscis canal
and coelomic sac.
In Protoglossus there is a pair of collar coelomic sacs, but in other genera these are
reduced or absent. These continue anteriorly into the proboscis stalk. The proboscis stalk
and the ventrolateral proboscis coelomic chambers secrete chondroid tissue around the
proboscis skeleton. The collar coeloms may open via a pair of collar canals and collar
pores. A transverse collar-trunk septum separates the collar and trunk coeloms.
The trunk coelom is paired, giving rise to dorsal and ventral mesenteries above and below
the digestive tract. The dorsal mesentery is usually perforated, allowing the two trunk
coeloms to communicate directly. The trunk coeloms do not open via external pores, but
are closed. In the trunk, coelomic fluid derived from the peritoneum contains amoeboid
coelomocytes.
Musculature
The muscles are of coelomic origin and course through the coelomic spaces and the inner
surface of the epidermis. The fibres are of the smooth muscle type. Circular fibres occur
beneath the epidermal membrane in the proboscis. Longitudinal fibres almost fill the
proboscis interior (coelom), but connective tissue fills the proboscis centre. Dorsoventral
fibres attach to the median sagittal plate in the posterior proboscis. The other proboscis
fibres attach to the epidermal basement membrane. The collar musculature is highly
variable and the subepidermal musculature is generally poorly developed here.
Nutrition
The digestive tract consists of a straight epithelial tube with little or no intrinsic
musculature. There are no longitudinal muscle fibres in the gut wall. Circular muscle
fibres may be present around the branchial and oesophageal regions. Radial fibres cross
the trunk coelom between the digestive tube and the trunk wall.
The large mouth is situated in the ventral collar, between the collarette and the proboscis
stalk. This opens into the buccal tube (pharynx) occupying the centre of the collar
interior. This has a glandular, ciliated luminal epithelium, with a basal nervous layer. The
buccal tube leads into the oesophagus, which leads into the anterior intestine in the
hepatic region. The intestine luminal surface is ciliated. Oesophageal canals may be
present, opening to the exterior by pores. These are dorsal paired structures. The canal
may be supported by skeletal elements. One to 15 pairs are present. There may also be as
many as 60 unpaired or paired irregular pores (aborted gill slits?). The anterior intestine
leads into the hind intestine, which may form a distinct rectum. The rectum or hind
intestine opens via the terminal anus, which may be equipped with a sphincter muscle.
The entire luminal epithelium of the digestive tract is ciliated. These cilia move the food
cord backwards. Burrowers may eat sand or diatoms and protozoans. Some are mucousciliary feeders. Particles touch the proboscis and become entangles in mucus and the
mucous strands are passed back along the proboscis to the collarette where they are
rejected if the animal is not feeding or else are directed to the mouth by ciliary currents.
The preoral ciliary organ directs some particles into its grooves and then conveys them to
the mouth. These grooves are possibly chemoreceptive. The gill apparatus maintains a
ciliary current entering the mouth and exiting via the gill pores.
Branchial Apparatus
The dorsal pharynx is equipped with gill slits, while the ventral pharynx is digestive (and
reduced to a hypobranchial ridge, a midventral strip, in Schizocardium). A diverticulum
sprouts from the roof of the buccal cavity, into which it opens, and runs forward to the
posterior proboscis. This has been considered to be a notochord by some zoologists,
though this argument seems unconvincing. This diverticulum bears pairs of ventrolateral
pockets or sacculations and may continue forward into the proboscis as an anterior
appendix.
There are two longitudinal dorsal rows of gill slits either side of the middorsal strip /
epibranchial ridge. The gill slits open into branchial sacs, which in turn open to the
outside via gill pores. Each branchial sac is partially divided by a U-shaped tongue bar,
which contains a hollow coelomic cavity. The bars of the U point towards the middorsal
line. The gill or branchial region is behind the collar and there is a longitudinal row of gill
pores either side of the middorsal ridge, which may be mounted in depressions, or borne
on ridges.
In some species the first 2-4 sacs open via a common pore. All the gill pouches on each
side of Stereobalanus canadensis are fused and open by a single longitudinal slit. The gill
pores may be equipped with sphincter muscles. The pore rows may be sunken into
branchiogenital grooves (which also receive the gonopores). There are no gills (i.e. no
thin-walled projections from the gill septa) associated with these ‘gill’ slits and pores.
There may be from a few pairs of gill pores to hundreds of pairs. The number of gill
pores may increase with age as new ones are added posteriorly.
Branchial support. Septa form solid partitions between successive gill slits. The tongue
bars never reach the ventral end of a gill slit. Skeletal rods (pharyngeal epithelium
basement membrane thickenings) support the tongue bars and the septa. The rods
supporting the tongue bars are trifid (3-pronged), with the median prong in the septum
and the 2 lateral prongs in the adjacent tongue bars. The tongue bars hang freely in the
gill slit or are joined to the septa by immovable cross-connections (synapticules).
Haemal System
This is located between the two lamellae of the basement membrane of the body
epithelium or between the two leaves of the mesenteries. There are two main vessels, the
dorsal and ventral longitudinal vessels. The dorsal vessel is located in the dorsal
mesentery below the dorsal nerve and runs from the anus to the collar where it passes into
the venous sinus that passes into the central sinus. The heart vesicle contains no blood,
but is muscular and its contractions move blood in the central sinus into the glomerulus.
The central sinus is non-contractile and its endothelial lining is often incomplete. All
other parts of the haemal system are lacunae or spaces rather than capillaries. The blood
is colourless and contains a few cells, which may be detached endothelial cells. It is
thought that the branchial apparatus may aerate the blood.
The proboscis complex consists of the buccal diverticulum, heart vesicle, central sinus
and the glomerulus, and projects into the coelom of the proboscis base and is coated with
peritoneum. The glomerulus comprises fingerlike out-pushings clothed with peritoneum
(see excretion below).
The dorsal and ventral longitudinal vessels are lined by endothelium encircled by a layer
of muscle and are contractile.
Excretion
It is thought that the glomerulus may function as an excretory organ. It consists of a mass
of peritoneal evaginations (simple or branched) with blood-filled interior cavities. The
blood they contain is continuous with that in the central sinus.
Metamerism?
Enteropneusts are not metameric since there is no repetition of the body parts in the three
regions. However, the branchial apparatus and the gonads may show metameric
tendencies.
Locomotion
Coordinated beating of the strong trunk cilia, borne on the trunk ridges between the
annulations, and peristaltic movements propel the animal in a crawling/creeping
movement. Mucus coats the burrow and hardens to a smooth surface. Mucus is also used
in feeding, to cover the animal with sand and may have a pungent iodoform odour. The
animal is constantly covered by mucous, which is moved towards the posterior by ciliary
action.
Burrowing is achieved by proboscis action. The proboscis extends and contracts in cycles
of about 12 per minute. Cilia assist in removing particles from the path. When it gains
headway, a peristaltic bulge anchors the proboscis while the body passively follows.
Some times enteropneusts take to the surface and swim in swarms. The proboscis is the
most active part of the animal; the rest of the body is sluggish.
The proboscis may effect crawling on the surface, with the aid of cilia and
anteroposterior peristalsis of the body. Movement in the burrow is possibly by cilia action
alone. The proboscis exhibits constant exploratory movements.
Luminescence
Some species of the family Ptychoderidae luminesce. For example, the whole body of
Ptychodera flava, except the branchial region, luminesces bright green. The
luminescence of this species is inhibited by light. It is thought that the luminescence may
be due to luminescent slime.
Reproduction
The gonads occupy the lateral regions of the anterior trunk, and may give rise to visible
lateral trunk swellings, or genital ridges, or to genital wings, which are flattened
extensions. These wings may be curved dorsally giving the false impression that the
animal has been split dorsally as in Balanoglossus and Ptychodera. There may be 1 to
several longitudinal rows to the sides of the digestive tube in the anterior trunk. These
gonads occupy the trunk coelom and are enclosed in a membrane continuous with the
epidermal basement membrane. Each gonad opens via a canal and external pore in the
branchiogenital grooves.
Stereobalanus has four short fluffy genital regions – 2 ventral and 2 dorsal. The gill
openings are concealed in a groove on each side between the dorsal and ventral genital
swellings. Thus, thus region of the body becomes the branchiogenital region. In many
species the gonads do not show externally, in which case the worm trunk is divisible into
branchial and postbranchial regions. In those genera with hepatic sacculations, the body
may be divisible into branchiogenital, hepatic and posthepatic/caudal regions. The caudal
region may taper to the anus. The gonopores are small and invisible.
Acorn worms are dioecious and the sexes are indistinguishable, except for the colour of
the ripe gonads showing through the body wall. An exception to this is Ptychodera flava,
in which the male has brown flecks on its genital wings. In this species a sex ratio of 1
male to 65 females has been measured.
In species with direct development the eggs are large (up to 1 mm) and yolky (F.
Harrimaniidae). In those with a tornaria larva stage, the eggs are small and poorly
provisioned with yolk. The sperm are flagellated. In Saccoglossus horsti a mucous cord
with 2000-3000 embedded eggs issues from the burrow as a coiled mass. About 20
minutes later the males release sperm from their burrows.
Asexual reproduction is known in Balanoglossus capensis. The juvenile phase (which
lacks hepatic sacculations) cuts off small pieces from its tail end in the summer. These
pieces regenerate into sexual adults, which are found in winter.
Development
Development is either indirect, with a tornaria larva stage or direct. Cleavage is
holoblastic, approximately equal and mostly radial. After 6-15 h a coeloblastula results,
which forms a blastula after 12-24 h.
Indirect development. Depending upon the species, this blastula gives rise either to a
benthonic larva (after 7 days) or to an embryo that elongates, becomes ciliated, and
escapes (after 24-36 h) to a planktonic life lasting days or weeks. This larva is ciliated,
and when ciliated bands differentiate it becomes a tornaria larva (less than 1 mm to 9 mm
long) with an equatorial constriction. Eventually the ciliated bands disappear and the
larva becomes wormlike and benthonic. It may have a prehensile tail (later resorbed) for
anchorage in its burrow. Such a larva may move in its burrow by ciliary gliding or
leechlike crawling, attaching its proboscis, releasing the tail, drawing the body forward,
reattaching its tail and then extending its body forward.
Direct development. This gives rise to a wormlike benthonic larva, usually via a ciliated
larval stage that lasts for about one day.
Regeneration
Acorn worms are fragile and break into pieces when handled. Trunk pieces regenerate.
The proboscis (with or without the collar) can not regenerate, but lives and moves about
for some time. Coelomocytes remove old muscle and connective tissue and new tissue is
derived from the coelom.
Ecology
Some acorn worms live in burrows, e.g. Balanoglossus, Saccoglossus, while others live
under stones or in plant tangles, etc. Most are upper littoral or intertidal, but some occur
down to 180 m, while Gland. Abyssicola occurs at 450 m. Burrows are U-shaped, with an
anterior funnel and posterior casts on the surface.
Numerous types of protozoan have been found as parasites in enteropneusts, as have
trematodes (in the coeloms of the proboscis and collar), nematodes (in the proboscis
muscles) and copepods. These copepods are much reduced and wormlike (males are also
smaller than the females) and form galls or tumours on the genital wings. Polychaetes
have also been found in the dorsal trough between genital wings and crabs and
amphipods in enteropneust burrows. Some enteropneusts only live in the burrows of
other larger enteropneuists.
Hemichordata: Pterobranchs (C: Pterobranchia)
Classification
Pterobranchs are small hemichordates. They may possess gill slits and have 2 or more
tentaculated arms borne by the mesosome. The digestive tract is recurved. They live in
colonies housed in an externally secreted encasement. There are three genera:
Cephalodiscus, Rhabdopleura and Atubaria and about 15 species.
Cephalodiscus
Coenecium
Unconnected individuals arise by budding from a single progenitor in a secreted
encasement or coenecium, which also incorporates adherent foreign objects like spicules,
bits of shells and tubes, etc.
The Orthoecus type of coenecium, found for example in Cephalodiscus densus, consists
of upright tubes, each of which is occupied by an individual zooid. The coenecium is
permanently fixed to the substrate and the tubes are more or less adherent to each other,
especially basally.
The Idiothecia type of coenecium, as found for example in Coenecium nigrescens, is
branching or arborescent. Small projections open at the tips to single tubular cavities
housing the individuals.
The third type of coenecium is the Demiothecia type. This may be adorned by
filamentous projections that give it a seaweed appearance. Spines (spicules) may support
the projections.
Coenecia are often a drab yellowish-brown colour, but some are orange, red or brown.
External Features
The zooids consist of a body, usually < 5 mm long, but up to 14 mm in length and a stalk
up to 4 cm long. The body is plump and bears feathery arms on the dorsal side of the
neck. The long attachment stalk gives off buds near its distal end.
The protosome forms the buccal shield or cephalic shield. The cephalic shield is flat and
discoid or disciform and tilted towards the ventral side concealing the mouth. The shield
is equipped with lateral indentations (notches) connected by a curved red pigment band.
The shield continues dorsally with the collar or mesosome (no stalk connects the two).
The collar is longer dorsally than ventrally and so is curved.
The dorsal collar bears the arms. The collar has bilateral rows of 5-9 arms each. The
arms within each row are fused basally. The arms consist of a stem bearing a ventral
groove and a fringe of 25-50 tentacles on each side. Each tentacle may terminate in a
glandular knob. The number of tentacles is variable and may increase with age. The oral
lamella is a fold of the body wall that partly encircles the collar and extends from the
base of the most posterior arms around to the ventral side below the mouth. This lamella
directs the food caught by the arms to the mouth.
The trunk or metasome consists of an anterior sacciform, plump region that contains the
recurved digestive tract and the gonads and a posterior stalk. The trunk bears the anus and
2 dorsal gonopores near to the collar. There is a single pair of gill slits situated laterally
on the trunk behind the posterior border of the collar. The stalk is hollow and muscular
and extends from the rear end.
Body Wall
The epidermis is ciliated, especially on the ventral surface, on the grooved surface of the
arms and tentacles, on the oral lamella and on the dorsal trunk sac near the anus. The
epidermis is also glandular, especially on the central part of the ventral cephalic shield,
which is possibly responsible for secreting the coenecium. The dorsal arm and tentacle
surfaces and the trunk sac epidermis are also glandular. The terminal knobs are involved
in prey capture and possibly produce a sticky secretion for this purpose. Attachment of
the zooid to the coenecium is possibly by glandular secretion or a vacuum cup.
There is an intraepidermal nerve plexus, like in enteropneusts. Beneath the epidermis is a
strong supporting basement membrane. Beneath this there are subepidermal longitudinal
muscle fibres. The peritoneum is partially transformed into muscle and connective tissue,
but remains intact in places.
Nervous System
There is an intraepidermal nerve plexus in pterobranchs. There is also an
intraepidermal collar ganglion in the dorsal wall of the collar between the bases of the
arm rows. This ganglion is a plexus thickening. It gives out nerves that are really plexus
thickenings. It gives out an arm nerve to each arm, which course along the dorsal side of
each arm, and median dorsal and paired laterodorsal nerves in the cephalic shield and a
short middorsal nerve to the anus region.
The ganglion also gives out a pair of circumenteric connectives that course ventrally
approximately along the collar-trunk boundary behind the gill pores to the ventral side of
the trunk sac where they form a midventral trunk nerve. This midventral nerve gives out
one or two pairs of lateral trunk nerves. The midventral and lateral trunk nerves continue
into the ventral side of the stalk. The midventral nerve of the stalk may turn around at the
end of the stalk and continue to run forward dorsally.
Coelom
There is an unpaired protocoel in the shield that opens to the outside via one pair of
ciliated canals and pores. The mesocoel (collar coelom) and the metacoel (trunk coelom)
are each subdivided by a median dorsoventral mesentery and so both are paired. A
septum separates the protocoel from the mesocoel.
The dorsal mesentery is a site for muscle attachment. The arms and tentacles are hollow
and contain extensions of the collar coelom or mesocoel. There is a ciliated collar canal
and pore for each mesocoel half just in front of the gill pores on the neck. A septum
separates the mesocoel from the metacoel.
The metacoel or trunk coelom is almost filled by the digestive tract and gonads. It
extends into the stalk (where there is no mesentery) where its lumen is almost filled by
muscles and connective tissue.
Musculature
There is a subepidermal thin longitudinal layer, which is possibly of peritoneal origin.
This layer is lacking in the muscular ventral part of the shield. Fibres radiate through the
protocoel from the shield-collar septum to the ventral wall of the shield (and some also
extending to the dorsal wall).
The oral musculature in the collar can narrow the mouth and buccal tube, but the mouth
is closed by bringing the cephalic shield against it. Strong longitudinal fibres can contract
the trunk and stalk. These extend into the trunk sac as one pair of ventral bundles that
extend to the gill region. Strong muscle bundles extend from the collar canal to the
adjacent body wall.
All or most of the muscle is smooth muscle. The muscles attach to the septa, basement
membrane and the mesenteries.
Digestive System
The mouth is transversely elongated and more or less concealed by the cephalic shield.
The upper and lower lips are formed of thickened epidermis.
The buccal tube contains mucous gland cells and a buccal diverticulum hangs from the
buccal roof forming a tubular evagination that extends forward along the shield-collar
septum. The dorsal collar mesentery attaches the diverticulum to the dorsal epidermis.
The buccal tube continues as the pharynx. The pharynx passes through the collar-trunk
septum. The pharynx contains gill passages in its dorsolateral walls. There are no tongue
bars and no branchial skeletal supports.
The pharynx continues into the oesophagous that connects to the stomach. The stomach is
saclike and fills the greater part of the trunk sac. The stomach connects to the tubular
intestine that curves dorsally forward to the anus, with its ventral wall in contact with the
stomach. The intestine leads into a rectum expansion that opens via the anus. The anus is
transversely elongated and situated on the dorsal surface of the anterior trunk sac.
Circulatory System
The structure of the circulatory system is not well known. It is wholly lacunar. A dorsal
sinus originates from lacunae in the stomach wall and runs forward above the
oesophagous and pharynx and connects to sinuses around each gonad, then continues
beneath the collar ganglion and terminates in the central sinus. The central sinus is
associated with a contractile heart vesicle.
The central sinus gives out the ventral shield sinus that runs back beneath the buccal tube
and buccal diverticulum as a pair of peribuccal channels or a lacunae system. From these
peribuccal channels or lacunae originate the main ventral sinus. This sinus extends
posteriorly from the collar-trunk septum along the ventral side of the stalk and then turns
dorsally at the end of the stalk and continues forward along the dorsal side of the stalk
and along the dorsal side of the intestine where it disappears.
In enteropneusts the heart vesicle and central sinus are positioned above the buccal
diverticulum, whilst in Cephalodiscus they are positioned in front of the buccal
diverticulum.
Glomerulus
The glomerulus is the much-folded wall of the ventral shield sinus where it passes
beneath the buccal diverticulum. These folds are surrounded by altered peritoneal cells
that are possible excretory. It is thought that these cells may detach and be emitted
through the canals and pores of the cephalic shield and collar.
Reproduction
Cephalodiscus are dioecious and the sexes are not distinguishable externally, except in
Cephalodiscus hodgsoni in which the female is red and has 12 arms while the male is
brown and has 10-11 arms. The coenecium may be of one sex or a mixture of both sexes.
Hermaphrodite individuals often occur with one male and one female gonad.
In Cephalodiscus sibogae one coenecium was found with neuter and male zooids and no
females. The neuters had 4 pairs of tentaculate arms and lacked gonads. The males had 2
arms and no tentacles and a vestigial digestive tract, with the space filled by the testes
instead.
There is one pair of gonads in the anterodorsal trunk in front of the stomach. These
gonads are oval or sacciform and each opens to the outside by a short gonoduct with the
gonopore on the anterodorsal surface just behind the collar-trunk boundary. The two
gonads are separated by the median dorsal trunk mesentery. Breeding is probably
seasonal. In Cephalodiscus gilchristi the eggs are shed into the coenecium cavities where
they develop.
Development
The eggs are quite large and yolky. Cleavage is holoblastic and more or less equal. A
blastula results (hollow or solid?) and subsequent endoderm formation results in an
embryo that is completely ciliated and escapes from the egg. This undergoes a brief
swimming period and possesses an apical sense organ and may have an apical tuft of long
cilia.
The larva elongates and resembles a young enteropneust. The intestine is initially straight
but later bends dorsally (as the dorsal surface shortens) to acquire the adult U-shape. The
heart vesicle is possibly of coelomic origin.
Asexual Reproduction
Buds form from a zone near the distal end of the stalk. There are usually 1-14 buds per
zooid at any instant. These offspring remain in the parent colony. The coenecium is
asexually produced from a sexually produced progenitor. Each bud contains an extension
of the stalk coelom. Soon the bud develops a stalk, cephalic shield with pigment stripe,
and then the trunk sac develops proximal to the shield as an enlargement. The arms
appear as buds (hollow extensions of the collar) and the collar becomes delineated from
the trunk sac. The heart vesicle develops from part of the protocoel and gills, buccal
diverticulum, and coelomic canals and pores form.
Ecology and Behaviour
The stalk stretches and the zooids reach out of the coenecium openings and wind about
the coenecial projections, using their adhesive cephalic shield as a holdfast. Alternatively
the zooids emerge totally and hold on by the shields of their buds.
The arms are adhesive and presumably capture minute organisms and particles. These
particles are conveyed along the arm grooves, by ciliary action, to the oral lamella and
then to the mouth.
Coenecia grow on muddy, sandy, gravelly or rocky bottoms and sometimes on other
sessile animals. Hydroids and bryozoans may grow on the coenecia. Sporozoan parasites
have been found in the intraepidermal nervous layer and copepod parasites have been
found in the stomach. Cephalodiscus occurs from 50 m to 650 m in depth. Most species
are Subantarctic and Antarctic.
Atubaria
Atubaria is found at 200-300 m and lacks a coenecium. The stalk twines about larger
hydroids. The anatomy is very similar to Cephalodiscus. There are 8 tentaculated arms
borne on the collar. There is a cephalic shield with a red pigment stripe. The trunk sac is
plum and the stalk is long, flexible and muscular. The 2nd pair of arms have long and
rodlike distal parts devoid of tentacles and of a glandular nature. The stalk has no
adhesive tip. There is one pair of gill slits and the anus is more posterior than in
Cephalodiscus. Other internal structures are the same as in Cephalodiscus. Juveniles bear
only two arms and have no tentacles. There is some evidence of budding.
Rhabdopleura
Coenecium
The rhabdopleuran coenecium is a pale brown prostrate branching tube partitioned into
chambers and cemented to the substratum. It gives off short erect tubes at frequent
intervals, each of which houses a zooid. The coenecium usually grows on hard surfaces,
such as rocks, shells, bryozoans, and tunicates, etc, but can grow on sandy bottoms. The
maximum colony diameter is 10 cm. The coenecium is formed by budding from a single
progenitor and the branch ends continue new growth.
The prostrate tube is flattened on the side in contact with the substrate. They may
incorporate foreign particles. The erect tubes are cylindroid, clean, translucent and 6-7
mm tall. The coenecium walls are secreted by the glandular centre of the cephalic shield
and is of an unknown non-chitinous chemical nature.
The black stolon is a black cord embedded in the attached wall of the creeping tube. It
gives off a branch to the base of each zooid at each transverse partition. It consists of a
hard black hull enclosing living vacuolated tissue with a central denser syncytial core that
connects the zooids and terminates as a knob in the zooid stalk coelom. The periphery of
the living stolon is separated from the zooid by up to 10 partitions in each side branch.
The progenitor secretes a hemisphere encircled by a ring tube enclosing the ring-shaped
Black stolon. A creeping prostrate tube grows from this hemisphere. The stolon gives off
buds into each transversely partitioned chamber.
External Features of Zooids
The zooids resemble Cephalodiscus and are small, being less than about 1 mm long
excluding the stalk. They are dark brown in colour. They have a cephalic shield, a collar
bearing one pair of tentaculated arms or plumes. The cephalic shield has a pigment stripe
but no lateral notches.
When the zooid is extended the arms are held curved backwards, but when in its tube the
arms are held straight forward.
There is a marked asymmetry in favour of the right side. The right fold of the oral lamella
is larger than the left. The anus is dorsal and just behind the arm bases on the right side.
There is a single gonopore on the right dorsal surface just in front of the anus. The stalk
attaches to a branch of the black stolon.
Internal Structure
The internal structure of Rhabdopleura is similar to that of Cephalodiscus. The epidermis
is ciliated, especially on the tentacles, ventral sides of the arms and the groove of the oral
lamella. The cephalic shield is similar to that of Cephalodiscus. The protocoel has one
pair of canals with external pores just anterior to the arm bases. The collar has a pair of
coelomic cavities continuous with the arm coeloms and oral lamella. There are the usual
collar canals and pores, though the pores are very minute.
The circulatory, nervous and digestive systems are very similar to those of
Cephalodiscus. There is one pair of gill pouches or dorsolateral pharyngeal grooves with
no external pores. There is a subepidermal longitudinal muscle layer, but this is very
weakly developed. The stalk is well muscled and possesses two ventral longitudinal
muscle bands that connect to the dorsal arm musculature and the mouth and buccal
musculature. There is glomerular tissue around the central sinus and the ventral shield
sinus.
Reproduction
Most of the Rhabdopleura colonies found have been sterile. Some, however, have been
found with male or female zooids. In those with male zooids, about ⅓ of the zooids are
male, the rest neuter. Females are very scarce.
A single gonad in the right metacoel opens via a short duct and gonopore to the right of
and behind the anus. The testis is elongated with the proximal portion producing sperm
and the distal portion forming a seminal vesicle for storage of ripe sperm. The ovary is
rounded and contains only one very large yolky egg at any one time.
The eggs are discharged to the exterior and have been found adhered to the arms.
Embryonic development is not known. Only the actively growing stolon ends bud. These
young stolons consist of epidermis enclosing a pair of coelomic cavities lined by
peritoneum and separated by a median mesentery. Older stolons secrete a wall around
themselves, which turns black and the coelomic cavities are lost.
Black stolons can not bud but are possibly able to regenerate. Some buds are sterile and
do not develop, but remain encapsulated in the hull. These are possibly dormant buds,
since they are common in autumn and may represent an overwintering stage.
Zooids may degenerate from the arms basally and regenerate again from the stalk, from a
new regeneration bud.
Ecology and Behaviour
Stalk contraction withdraws the zooid into its tube, as a protective response. Extension
from the tube is very slow and achieved by application of the cephalic shield to the tube
wall.
At least some rhabdopleurans prefer temperatures in the range of 5-10oC and survival at
12oC is reduced to days. There is no apparent response to light. Diatoms, radiolarians and
crustacean larvae have been found in the stomach. Rhabdopleura is found from 2 to 550
m depth.
Nematoda (Nemata, Round Worms)
External Features
Most nematodes are < 2.5 mm long, and are often microscopic, however, some marine
species reach 5 cm. Dioctophyme renale, the kidney worm, and Dracunculus medinensis,
the guinea worm, may reach 1 m in length. Nematodes are of circular cross-section and
fusiform or filiform worms. The posterior end is usually curved in the male in which it
may bear papillae and wing-like alae. Nematodes are transparent, whitish or yellowish
(due to cuticle). There are no definite regions and no distinct head. In the family
Draconematidae, a constriction demarcates the anterior end, including the entire
pharyngeal region.
There is a midventral anterior excretory pore, a midventral and posterior female gonopore
(or vulva) and a posterior midventral anus. The postanal region forms the tail.
The mouth is in the centre of the anterior tip and may be surrounded by 6 liplike lobes in
primitive marine forms, three on each side, but there are often only three lips in terrestrial
and parasitic species, as a result of fusion. Primitively the lips bear 16 sensory papillae or
setae. Small interlabial lobes may also be present. A circular groove separates the labial
region from the anterior tip. The anterior end has a hexaradial-biradial structure.
Probolae, or rigid lip protuberances, occur in certain terrestrial nematodes, e.g. Acrobeles
and Wilsonema. There are 3 or 6 (2 circles of 3) of these probolae. These probolae are
forward projecting, rounded, conical, forked or antler-like projections, movable only by
lip motions, and of unknown function.
In many Strongylidae the lips form an upstanding collar, with up to 40 or more lobes or
teeth on the inner surface, forming the leaf crown (corona radiata) and there may be a
similar additional inner leaf crown.
In the order Spiruroidea (the spirurine nematodes) the lips are encircles by a cuticular
collar, which may be extended dorsally and ventrally, to form head shields. Some
members of the spirurine family Physalopteridae have an upstanding collar or collarette
into which the anterior end can withdraw. In the spirurine family Acuariidae have 2, 4 or
6 anterior longitudinal cuticular chord-like thickenings or grooves, called cordons or
epaulets, which may be straight, recurved or looped, or horseshoe-shaped. In Seuratia
and some oxyuroids the free posterior edge of the cordons forms spines and there are 4
cordon scallops. Heterocheilus has a cuticular thickening behind the lips. Typhlophorus
has a similar thickening with longitudinal ribs.
In several spiruroid genera, appendages project from the head. There are 4 feathery
projections in Ancyracanthus; 4 pointed wings in Schistorophus; 8 simple lobes in
Ancyracanthopsis; 2 split lobes, giving rise to secondary lobes in Histiocephalus and a
circle of variously shaped processes in Serticeps.
In the spiruroid family Gnathostomidae, 4 cuticular inflations or ballonets form a
swollen band or head bulb behind the lips. This may be armed with circlets of spines or
transverse striations.
Spines, warts and longitudinal ridges may be present. These ridges may form 1,2 or 4
wings or alae. These alae may be cervical or caudal (latter involved in copulation). Some
nematodes have bristles, for example the stilt bristles or ambulatory setae, used in
locomotion in Epsilonematidae and Draconematidae. The body sense organs are
concentrated in 4 or 8 longitudinal rows.
In many free-living nematodes adhesive caudal glands open via a pore at the posterior tip.
This pore may be mounted on an adhesive tube (similar to that structure seen in rotifers
and gastrotrichs).
Body Wall
The outer cortical layer is bounded externally by a thin epicuticle, which may be
quinone-tanned and is typically ringed / annulated. The median layer consists of struts,
skeletal rods, fibrils, canals or is uniform and granular. The basal layer is striated,
laminated, or may contain spiral fibres.
The outermost layer is the epicuticle and is 6-45 nm thick. This consists of a 3-layered
osmiophilic membrane, which may have an outer glycocalyx. Inside this membrane there
may be an inner osmiophilic layer. There is a more or less clear layer between these two
osmiophilic membranes. Annulations are transverse grooves in the epi- and exocuticles
and in Enoplus they reach the mesocuticle cavity which may open to the outside at
places. Pores or striations may also cross the epicuticle. Scales or longitudinal ribs may
be present.
Beneath the epicuticle is the exocuticle, which varies in thickness from 0.2 m in small,
free-living forms to ~ 10 m in large, parasitic forms. The exocuticle is, however, absent
in some small Rhabditia, Monoposthia, Ascaris and Oxyuris. The exocuticle consists of
striations perpendicular to the surface, composed of crystalline protein. The periodicity of
these striations is 10-17 nm in longitudinal section and 20-27 nm in transverse section.
The exocuticle either closely adjoins the epicuticle or is separated from it by a thin layer
of homogeneous material or, in Rhabditia, by a thick homogeneous layer or longitudinal
fibrous material.
Beneath the exocuticle is the mesocuticle. This is usually the thickest layer of the cuticle,
and is the most varied component, but is sometimes reduced. The mesocuticle may
contain an intracellular cavity, which is fluid-filled or filled with loose or porous material
and is intersected by columns of dense osmiophilic material supporting the epi- and
exocuticles. In Paracanthonchus the cavity accounts for 75-80% of the cuticle thickness.
In Sphaerolaimus the cavity is filled with a coarsely alveolar osmiophilic mesh. The
mesocuticle usually also contains 2-3 layers of oblique protein fibres at 60-75o to the
longitudinal axis. These form spirals in opposite intertwining directions around the worm.
These fibre layers range from 1 m to 25 m in thickness.
The innermost layer of the cuticle is the endocuticle. This layer is homogeneous and
consists of alternating dense, less-dense layers. The internal surface is often wavy. The
Enoplida, e.g. Enoplus and Deontostoma have the most complicated cuticle structure.
At the head end the mesocuticle thickens greatly to form a shock absorber. The
sclerotised outer layers form the head skeleton or endocupola.
The lumens of the stoma, pharynx and posterior intestine are also lined by cuticle, though
this internal cuticle is simpler in structure and consists of epi- and endocuticle only.
Nematodes moult their cuticle 4 times during growth. Both the external and the internal
cuticles are shed.
The nematode cuticle is selectively permeable and may be important in the uptake of
certain materials, especially in Enoplida where pores give the cuticle a high permeability.
This semiporous property is thought to be particularly important in parasitic nematodes.
The cuticle is also an effective barrier to many materials. The cuticle also functions in
protection. Those layers of the cuticle that appear clear and homogeneous under the
electron microscope are especially hard and contain scleroprotein. The lacunar layers are
effective shock absorbers, especially at the anterior end of the worm. This protective
function is important to nematodes, since these worms characteristically occupy ‘tight’
interstices in soil, plant tissues and animal guts, where they are otherwise prone to
mechanical damage.
Most nematodes move by undulations of the body or by serpentine crawling. They are
not capable of significant alterations in body shape. Rather their bodies are stiff and
turgid. The turgor of the body tissues keeps the cuticle stretched tight and when the worm
flexes its body by muscular contraction the elasticity of the cuticle straightens the body
back out. Thus, essentially we have the muscular-flexing of an elastic cylinder. The
compression of the striations is particularly important in providing a restoring force. The
layers of obliquely oriented fibres, present in many nematodes, presumably function in a
similar manner to the spiral fibres in the basement membranes of Turbellaria and
Nemertinea. In nematodes, the spiral cuticular fibres are at 60-75o to the long axis.
Beneath the cuticle lies the epidermis. The epidermis (hypodermis) is usually cellular, but
may be syncitial. The epidermal cytoplasm extends into the pseudocoel to form
middorsal, midventral and midlateral longitudinal cords, containing the epidermal nuclei
in rows. Hemidesmosomes connect the hypodermis to the cuticle. Tonofibrils from these
hemidesmosomes penetrate the cuticle. Interchordal hypodermis is attached to the
underlying musculature by a layer of amorphous intercellular materiel. The total
thickness of the interchordal hypodermis is 0.1-0.8 m in free-living forms, but in
parasitic forms the subcuticular layer may reach 30 m in thickness.
The hypodermal cells are arranged in 5-12 regular longitudinal rows, and thus there are
also 5-12 hypodermal chords, grouped into lateral, and often also ventral and dorsal rows.
The dorso-ventral flexing of the nematode during locomotion, means that the cell-cell
junctions of the lateral hypodermal cells must be particularly strong and these cells have
interdigitating margins that serve to increase the contact area between adjacent cells. The
hypodermis is also an important storage organ, and stores glycogen and fat.
Musculature
Beneath the epidermis is a single muscle layer composed entirely of longitudinal
obliquely striated fibres, arranged in bands. The contractile filaments are limited to the
base of broad, flat fibres or to the bases and sides of tall, narrow fibres. Each muscle fibre
has a slender arm extending to the dorsal or ventral nerve cord.
The muscle cells are divided into basal contractile zones. The muscle cells are obliquely
striated, with the contractile filaments at an angle of 14-17o to the z-bands. There are 1012 actin filaments around each myosin. The apical cytoplasmic zone contains the cell
nucleus, mitochondria and glycogen granules. The innervation process extends to a nerve
trunk or to the nerve ring. Each cell may give out several branched processes. These form
a spatial network in the pseudocoel. Neighbouring muscle cells are connected by
electrical synapses.
In the subclass Chromadoria and some of the Rhabditia, the muscle cells are flat
platymyarian muscle cells: the z-bands are perpendicular to the base of the cell and at
an acute angle to the cell axis. In free-living forms of the order Enoplida, the muscle cells
are primary coelomyarian muscle cells: the z-bands are parallel to the cell base in
transverse section, and diagonal in longitudinal section. Large parasitic nematodes have
secondary coelomyarian muscle cells. These form ontogenetically from platymyarian
muscle cells. In some Rhabditia (e.g. Filariina) the contractile zone forms a ring along the
whole length of the cell and these muscle cells are called circomyarian muscle cells.
These form from 2o coelomyarian cells. The coelomyarian type of muscle cell is found in
large nematodes and serves to increase the cross-sectional area of the contractile zone.
Polymyarian nematodes have ~10 to ~100 muscle cells, whilst meromyarian nematodes
have only 8 muscle cells in transverse section. Small forms and juveniles tend to be
meromyarian. Polymyarianism evolved secondarily in enlarged parasitic forms, such as
Ascaris, which has 600 muscle cells per transverse section.
Locomotion
Many nematodes can swim intermittently for short distances. A few can crawl,
undulatory waves of muscular contraction act against the substrate, aided by the grip
provided by the sculptured cuticle. The hydrostatic skeleton and elastic cuticle are
antagonistic to muscle contraction. Nematodes move through soil pores 15-45 m in
diameter, and the pore size for optimum movement is about 1.5 times the worm’s
diameter.
In one species, with a ringed cuticle, crawling is earthworm-like. Others may crawl like
caterpillars, and others move like inchworms.
In swimming the dorsal and ventral muscles contract alternately, causing the body to
undulate. The dorsal and ventral muscles are well developed and equally well developed,
with equal numbers of muscle cells.
Nutrition
The nematode digestive tract can be divided into an ectodermal foregut, comprising the
pharynx and, if present, the buccal cavity, an endodermal middle intestine and an
ectodermal posterior intestine. Many nematodes are carnivorous, but some are
phytophagous. Many terrestrial nematodes pierce plant root cells and suck-out the
contents. Many are deposit feeders, ingesting the substrate particles and associated
microbes. Similarly, some feed on dead organic matter, such as dung, corpses, etc. and
associated microbes.
The mouth of the nematode is usually terminal and either opens into a buccal cavity, or
stoma, or directly into the pharynx. The stoma, or buccal capsule, is tubular and lined
with cuticle, which is often strengthened by rhabdions, which are ridges, rods or plates,
and may bear teeth. For example, Mononchus papillatus is carnivorous and toothed. It
has a large dorsal tooth opposed by a buccal ridge. One such individual may consume up
to 1000 other nematodes during its 18 week lifespan. It attaches its lips to its prey and
makes an incision and pumps out the prey’s contents with its muscular pharynx. In
Enoplida the stoma contains 3 cuticular mandibles (jaws or gnathi). In some nematodes
the buccal cavity is divided into 3 regions, an anterior vestibule enclosed by the lips, a
middle protostom, which is the longest region, and a posterior telostom (or glottid
apparatus). The mouth is often surrounded by 3 or 6 labia (see description under Sensory
Systems).
Some carnivorous roundworms, and many species that feed on plant cells, have a long
hollow spear or stylet or a solid odontostyle, housed in the buccal capsule. This stylet
can be protruded through the mouth. Enzymes are released, and in the case of hollow
stylets, the digested food is pumped into the worm via the stylet.
The buccal cavity leads into a tubular pharynx or oesophagus. The buccal cavity is itself
sometimes embedded in and surrounded by pharyngeal tissue. The pharyngeal lumen is
triradiate in cross-section and lined with cuticle. The wall is composed of myoepithelium
and gland cells. Frequently, there is more than one muscular swelling or bulb, acting as
pumps. Valves are frequently present.
The triradiate symmetry allows the pharynx to be easily and completely closed. The
pharynx consists of two concentric tubes, the cuticular inner lining and the outer basal
lamina, with radial muscle fibres in between. Contraction of these fibres opens the
pharynx, whilst upon relaxation of these muscles, hydrostatic pressure closes the lumen.
Pharyngeal contractions travel from anterior to posterior. The muscular cardia valve
separates the pharynx from the middle intestine.
The second index of de Man gives the ration, b, of the body length to the pharynx length.
In small nematodes b is smaller (the pharynx is relatively longer). In juveniles b ≈ 2. In
small nematodes the undulatory movements of the worm body reduce the intracavity
pressure. This adversely affects pharynx function in small nematodes. To overcome this,
the pharynx is relatively longer (i.e. b is smaller) or a different means of locomotion is
employed.
In cross-section the pharynx has three muscular cells (one dorsal and two subventral) and
three marginal cells (one ventral and two subdorsal) opposite to the lumenal radii. Three
pharyngeal digestive glands open via ducts into the pharyngeal lumen. One of these
glands is unicellular and dorsal; the other two comprise two cells each and are subventral.
The subventral glands may open via a common duct. There are three pharyngeal nerves,
one dorsal and two subventral. The muscle cells have radial myofilaments and are crossstriated and extend between the pharynx cuticle and the basal lamina. These muscle cells
are only one sarcomere long. The marginal cells are non-contractile and contain fibrils
that connect to the inner cuticle and to the outer basal lamina by hemidesmosomes.
The nematode has a constant cell number. In Caenorhabditis elegans there are 34 muscle
cells, 9 marginal cells, 5 glandular cells and 20 nerve cells. Large parasitic forms have
larger cells, but the same cell number. In large nematodes the gland cells contain many
nuclei, for example Ascaris has ~ 104 nuclei per gland cell.
The pharynx may be simple and cylindrical or may taper anteriorly or it may possess a
basal and/or median muscular swelling with valves, called a bulb, or without valves,
called a pseudobulb. In some nematodes, the pharynx is long, tubular and non-muscular,
as in mermithoids. In most Plectida, the pharynx consists of a wider anterior part, or
corpus (divisible into anterior procorpus and posterior metacorpus), a narrow middle part
or isthmus and a posterior bulb.
The pharynx leads into the intestine. A single-celled epithelial layer lines the intestine
lumen. A valve at each end prevents food from being forced out by the hydrostatic
pressure of the pseudocoel. The middle intestine has a fixed cell number in some
nematodes; for example it comprises 18 cells in Turbatrix aceti. In this case the cell size
increases during growth, and the cells may become giant (up to 4 mm by 0.5 mm) and
multinucleate. In some nematodes the middle intestine cell number is not fixed and may
number ~ 102 to 106 cells. When not fixed in number, these cells can be regenerated and
replaced. In this latter case, whole sections of the intestine can regenerate if a section is
removed.
The intestine leads into a short, cuticle-lined rectum (cloaca in males) which connects the
intestine to the anus. The anus is on the midventral line, just in front of the posterior tip of
the body.
The pharyngeal glands and intestinal epithelium produce digestive enzymes. Digestion
begins extracellularly, but is completed intracellularly.
Digestion begins in oral cavity and is completed in the middle intestine, where it is both
extracellular and intracellular. The intestine lumen contains a microvilli brushlike border,
and may also possess synchronous cilia. Extracellular digestion occurs both in the lumen
and on the surface of the microvilli.
Excretion and Osmoregulation
Protonephridia are absent and some nematodes have no specialised excretory system, but
many do possess gland cells, with or without tubules, with some excretory function.
In the class Adenophorea, which includes most aquatic species, there is usually one large
gland cell or renette gland located ventrally in the pseudocoel near the pharynx. It has a
necklike duct that opens ventrally on the midline as an excretory pore.
In the class Secernentea, which includes many terrestrial species, three long canals are
arranged to form an H-shape. Two of the canals are lateral, located in the lateral
longitudinal cords and the third transverse canal connects these. From this transverse
canal a short excretory canal leads to the excretory pore. In some species, the anterior
lateral canals may have disappeared, forming a horseshoe shape. In others the system is
asymmetric.
Nervous System
The nervous centre is a circumenteric (circumpharyngeal) nerve ring with ganglia
attached dorsally, laterally and ventrally. These ‘ganglia’ are not true ganglia, since they
lack neuropil and have no external sheath covering (only in Siphonolaimus do they have
sheaths) but are simply swellings composed of cell bodies. The neuropil is in the ring
itself, and the whole is functionally equivalent to a ganglion. Hence the swellings will be
referred to here as pseudoganglia.
In the Rhabdita the lateral pseudoganglia comprise 30-45 nerve cell bodies, the ventral
pseudoganglia 16-33 cells, and the dorsal ganglia contain 1-7 cells only, or are absent.
The nerve ring gives off sensory nerves to the head. These nerves are deep and are
adjacent to the body wall. A subdorsal nerve innervates one labial papilla and one pair of
cephalic setae. A subventral nerve also innervates one labial papilla and one pair of
cephalic setae. Lateral nerves each innervate one labial papilla, the corresponding lateral
cephalic seta and amphid.
Nematodes possess 8-12 longitudinal nerve cords. Free-living nematodes have the most
complex nervous systems, such as those of marine nematodes in the order Enoplida. In
the main body of the worm there are 10 longitudinal nerve cords: ventral, lateral, paired
subdorsal, subventral, ventrolateral and dorsolateral nerves. These are linked by semicircular commissures and by individual nerve fibres. The ventral nerve cord (VNC) is the
most developed and connects to the circumpharyngeal nerve ring, as may the other cords.
The VNC contains hypodermal and subcuticular nerve fibres. The other longitudinal
nerves contain subcuticular fibres only.
Lateral sensory plexi innervates the somatic sensory setae and the female vulvar region.
In the posterior of the worm the ventral cord bifurcates to form the circumanal
commissure and branches of the ventral cord connect to the lateral cords.
In Ascaris, the ventral nerve cord contains 55 fibres, the dorsal cord 3 fibres, the
submedian nerve cords 4 fibres each and the lateral nerve cords also contain 4 fibres
each. The fibres may be up to 40 m in diameter.
In all nematodes the VNC is the most developed and contains sensory, motor and
interneurons. The VNC connects to the circumpharyngeal nerve ring by a pair of rootlets
(subventral nerves). The VNC innervates the dorsal musculature with motor neurons. It
also sends out motor fibres to the dorsal nerve cord (DNC) which go on to innervate the
dorsal muscles. These muscles are responsible for the swimming undulations. They are
equally developed dorsally and ventrally and hence equally well supplied by motor
fibres. THE DNC gains fibres as it proceeds posteriorly. In its anterior portion it contains
just 3 fibres in Ascaris, whilst posteriorly it contains 13 fibres.
The DNC and submedian nerves are usually devoid of nerve cell bodies. The submedian
nerve cords (SNCs) are motor, whilst the lateral nerve cords (LNCs) are sensory. The
motor nerves (VNC, DNC, SNC) are apparently cholinergic, as are the amphids,
phasmids, genital sensilla and head sensilla. The sensory system, however, is generally
catecholaminergic. In saprobiotic and parasitic nematodes, the sensory systems are
reduced and only 4 catecholamine neurons remain. These innervate the cephalic papillae
(outer circle), the male genital papillae and 1-2 pairs of lateral sensory neurons.
The pharynx is innervated by its own pair of subventral longitudinal nerves and a dorsal
longitudinal nerve, connected by circular commissures at the base and anterior end of the
pharynx (and also by sector commissures). In Caenorhabditis the pharynx consists of 34
muscle cells, 9 marginal cells, 9 epithelial cells, 5 glandular cells and 20 neurons
(including sensory, motor and interneurons). Longitudinal nerves and a sensory nerve
plexus innervate the middle intestine of parasitic nematodes. Parascaris has nerve plexi
on the walls of the gonoducts. Bipolar neurons may also be associated with excretory
canals.
In the order Enoplida, each worm contains thousands of nerve cells, and the number is
variable. In Rhabdita each worm contains between 150 and 300 neurons, and the number
is constant within a species. In small nematodes, the nervous system may account for
40% of the cells in the worm’s body. It is interesting to note that the cephalic nerves have
hexaradial symmetry, whilst the motor systems have biradial symmetry.
Sensory Systems
The thick nematode cuticle prevents a potential barrier to sensory information. To
overcome this, nematodes have receptors with specialised cuticular accessories that allow
specific sensory information through the cuticle at specific sites on the external body
surface. These accessory structures serve to filter sensory modalities and in some cases
amplify the sensory signals. The labial papillae and cephalic papillae are cuticular
projections, each containing a nerve fibre that branches from the papillary nerves.
Mechanoreceptive setae are present. Amphids are blind, pouchlike or tubelike
invaginations of the cuticle. These are chemoreceptive and possibly have other sensory
functions. Cervical papillae are situated just behind the anterior region of the worm. The
papillae are conical and often contain a terminal pore or are reduced to mere pores or
extended into bristle-like setae. Mechanosensory setae amplify tactile information
detected by a sensory neuron at the base of the seta. Cephalic slits occur in Enoploidea
as a pair of pouch-like cephalic sense organs adjacent to the amphids.
Somatic sense organs are well-developed in free-living forms, as in the subclasses
Enoplida and Chromadoria. These take the form of pores, which are usually
chemoreceptive, and of bristle-like setae. In Paracanthoncus, pores open at the bottom of
cuticular depressions and are arranged in rows over the whole body. Each pore opens into
a canal that opens into a receptor cavity. Each is innervated by 1-2 sensory dendrites.
These dendrites are modified non-motile cilia. Also associated with each sense-organ or
sensillum (pl. sensilla) are two modified hypodermal cells, a glandular sheath cell and a
socket cell.
Setae are hollow cuticular processes innervated by 5-6 dendritic branches. The lumen of
the sensillum opens to the outside via a pore in the tip of each seta. In Deontostoma
californicum the somatic setae are cones, each with a pore opening in its tip.
The order Trichocephalida is equipped with bacillary bands: rows of pores and glandular
cells of the lateral hypodermal nerve cords and 4-6 dendritic branches innervate each
pore. These branches are embedded in the glandular sheath cells. These embedded
dendrites possibly function as hygroreceptors. Caenorhabditis possesses aporous
cuticular papillae each innervated by one electron-dense dendrite.
Sensory dendrites in contact with the outside through a pore are probably
chemoreceptive. These are also associated with glands whose secretions bathe the
dendrites. Dendrites with electron-dense tips are thought to be mechanoreceptive and
hygroreceptive. Some sensilla are possibly dual-function chemo-mechanoreceptors, since
they possess dendrites extending to pores and electron-dense dendrites that often
terminate in the sensillum base.
In free-living enoplians (O. Enoplida) the labial papillae and cephalic papillae are conical
and each has an internal canal that opens at the tip via a pore. Shorter sensilla are papillae
whilst longer sensilla are setae. Setae also cover the general body surface, and are
irregularly distributed, but with a tendency to concentrate into two paired sublateral rows.
In contrast, the receptors of the anterior end are more precisely arranged. Some examples
of the arrangement of these anterior sensilla follow.
1. Pontonema vulgare (O. Enoplida) has 6 labial papillae, 10 cephalic setae and one pair
of pocket-shaped amphids. The labial papillae and cephalic setae each contain 9-12
dendritic branches, one wide with 80-100 microtubules per cross-section embedded in
electron-dense material, whilst the other dendrites are threadlike with ~1 microtubule per
cross-section. The amphidial nerve has 17 dendritic branches. One of these is widened
and vacuolated and contains electron-dense material. The other 16 are threadlike. All end
at a pore opening at the base of the amphidial cavity. The amphidial cavity in turn opens
to the outside by a pore or slit. The amphidial cavity is filled with a glandular secretion
originating from a gland in the nerve ring, far from the head-end.
2. Xiphinema (O. Dorylaimida) possesses an inner circle of 6 labial papillae and 6
cephalic papillae and an outer circle of 4 cephalic papillae. There is also one pair of
pocket-shaped amphids. The inner circle papillae have terminal pores and 4 dendritic
branches, with one thicker dendrite. The outer circle papillae each have 3 dendritic
branches, with one thicker dendrite. The amphids contain 14 neurons each (5 of the
neurons have 2 dendritic branches, giving a total of 19 dendritic branches. One dendrite is
short and does not reach the amphidial cavity, whilst the rest enter the cavity.
3. The amphids of Oncholaimus vesicarius (O. Enoplidia) contain 38 dendritic branches,
from 3 dendrites. A 4th dendrite gives rise to 10 branches that enter a depression in the
sheath cell and do not reach the amphid cavity or the pore in the base of the cavity.
4. Tobrilus aberrans (O. Enoplidia) have pocket-shaped amphids with 16 dendrites, each
with a single branch.
5. The parasitic Capillaria hepatica (O. Enoplidia) has an inner circle of 6 labial papillae
and 6 cephalic papillae, an outer circle of 4 cephalic papillae, and one pair of amphids.
All these sensilla have pores. Each dorsolateral and ventrolateral papilla has two thin
dendritic branches that extend to the terminal pore. Each lateral labial papilla has 3
dendritic branches, 2 of which are threadlike and do not extend to the pore, while the
third widens at its distal end and contains electron-dense material. Each lateral cephalic
papilla also has three dendritic branches. Each amphid has 10 dendritic branches, each
ending at a different level inside the amphidial cavity or canal.
6. Sphaerolaimus balticus (SC. Chromodoria) has 6 labial papillae, 10 cephalic setae and
8 pairs of cervical setae. Each of these sensilla has one dendritic branch 25 m long,
which extends to the pore and has its cell body deep in the cephalic tissues. These
dendritic branches have electron-dense tips. This species also has one pair of circular
amphids. Threadlike spiral branches fill the amphidial cavity. One of these dendritic
branches is thicker and forms a descending spiral.
7. Paracanthonchus has 6 labial papillae and 10 cephalic setae, similar in morphology to
somatic setae, and one pair of spiral amphids. Each labial papilla has 5 dendritic
branches. Each cephalic seta has 6 short and 4 long dendrites. The amphids form a spiral
trough, filled with secretion, on the cuticle surface. Each amphid has 18 dendritic
branches, one of which is non-emergent and resides in the sheath cell recess.
8. Caenorhabditis elegans (a saprobiont) has 6 labial papillae, 6 cephalic papillae, 4 outer
cephalic papillae and one pair of porelike amphids. Each labial papilla has a small
terminal pore and 2 dendritic branches, one of which extends to the pore, the other, which
is electron-dense, ends in the base of the papilla. The subdorsal and subventral papillae of
the inner and outer circles pair to form double submedial papillae. In each of these, two
dendritic branches with electron-dense tips extend into the cuticle, connected by a strand
of dark material to the outer cuticle layer. Each amphid contains 10 dendritic branches (8
dendrites, 2 with 2 branches) and three more dendritic branches enclosed within the
sheath cell and another dendrite, equipped with about 50 microvilli, is enclosed in the
sheath cell, which is situated in lateral pseudoganglion at the level of the nerve ring. Thus
each amphid contains a total of 12 dendrites.
9. Nippostrongylus brasiliensis (an animal parasite, SC. Rhabdita) has 6 labial papillae, 4
double submedian cephalic papillae (the lateral cephalic papillae are reduced) and one
pair of porelike amphids. Each labial papilla contains one dendritic branch that terminates
in an electron-dense tip in the cuticle, and is connected to the surface layers of the cuticle
by a dark strand. The submedian cephalic papillae each contain 2 dendritic branches with
electron-dense termini. One of these is bent parallel to the outer cuticular surface. Each
amphid contains 13 dendritic branches extending to the pore, and 2 unbranched dendrites,
equipped with microvilli, that terminate in the sheath cell.
10. Heterakis gallinarum possesses three secondary labia around the mouth, which bear 6
labial papillae. It also has 4 double submedian cephalic papillae, formed from the fusion
of the inner and outer circle cephalic papillae, and there is one pair of amphids. The two
pairs of lateral papillae are reduced and in their place are cervical papillae that have
moved anteriorly and are just posterior to the secondary labia. The labial papillae are
pointed and each has a single dendritic branch in the cuticle at its base. Each submedian
papilla has one pair of dendritic branches that terminate in its cuticle. These have
widened distal ends and are arranged at right angles to each other. The lateral papillae are
cuticular processes each with a pore in its tip and containing two dendritic branches, one
extending to the pore, the other is electron-dense and ends at the base of the papilla. Each
amphid contains 13 dendritic branches extending to the amphidial pore and two
additional branches end inside a hypodermal cell.
11. In the order Tylenchida, the inner cephalic papillae are generally not connected to the
external environment. Each contains a single papilla terminating in the cuticle. The outer
circle papillae usually have no openings. The amphids contains 7 dendritic branches
extending to the amphidial pore, 5 dendritic branches terminating in the sheath cell and
another dendritic branch, equipped with about 200 microvilli, also terminates in the
sheath cell.
Labial papillae may have pores that connect to the inside of the oral cavity, as in
Radopholus, Rotylenchus and Macroposthonia.
Thus, the receptors of the anterior end are precisely arranged in various patterns. All
these patterns, however, are possibly variations on the same probable ancestral plan. This
consists of rings of 6 setae. Thus, one would have an inner circle of 6 labial setae, and
two outer circles of 6 cephalic setae (6 + 6 + 6 arrangement). The lateral setae of the third
circle are though to have developed into the amphids, leaving a 6 + 6 + 4 arrangement.
The 4 setae remaining in the third ring often united with the second ring to give the 6 +
10 arrangement of two circles commonly seen, as in the Enoplida. In some nematodes,
these have been supplemented by cervical setae uniting with the cephalic setae. Thus, we
have a concentration of receptors at the anterior end. Furthermore, the most anterior setae
often shorten into papillae. This is possibly a mechanical adaptation, since the anterior
setae are likely to be involved in forceful collisions with obstacles. These changes were
also accompanied by changes in the mouth, from a triradiate mouth following the contour
of the triangular pharynx and lacking labia, to mouths with three and then sometimes six
labia. The anterior sense organs may be reduced in parasitic forms.
Although the oral aperture has tri- or hexaradiate symmetry, the labial papillae have
hexaradiate symmetry, the amphids are biradially arranged and cephalic setae are
biradially arranged in the Enoplida (2 pairs subdorsal, 2 pairs subventral and 1 pair
lateral), the overall body symmetry is bilateral about the sagittal plane.
Ocelli are found either side of the pharynx in some aquatic nematodes. Stretch receptors
in the epidermal cords are involved in regulation of locomotor movements.
Metanemes are proprioceptors situated in the lateral hypodermal chords. Each metaneme
consists of a primary sensory cell whose ciliary dendrite is inserted into the receptor
cavity of a secondary sensory neuronal dendrite. The wall of the receptor cavity contains
granules and is in synaptic contact with the ciliary dendrite of the primary cell. These
receptors apparently detect the ventrodorsal bending of the nematode body during
locomotion.
The sensilla all have glandular cells associated with them. These produce secretions that
bathe the dendrites and are necessary for sensory function. This arrangement has lead to
the evolution of organs with dual sensory and glandular functions. Adhesive setae occur
in O. Desmoscolecida and the F. Draconematidae. The adhesive secretion is emitted
through the terminal pore of the seta and is employed in the caterpillar-like crawling
mode of locomotion of these nematodes.
In male nematodes there is a midventral preanal row of 10-20 supplementary organs
that have dual sensory and adhesive functions. They serve to attach the male to the
female during copulation. Male nematodes of the SC. Rhabditia also have paired pre- and
postanal papillae. In Aphelenchoides each papilla has a terminal pore with a single
chemoreceptive dendrite terminating just beneath this pore and a basal mechanoreceptive
dendrite with an electron-dense tip. In Dipetalonema, each papilla contains a single
dendrite with an expanded mushroom-shaped electron-dense tip that is connected to the
outside via a narrow canal.
In nematodes of SC. Rhabditia, there may be a copulatory bursa supported by ribs. In
Pelodera there are dendritic branches that terminate beneath pores at the end of each rib,
and are presumably chemoreceptors.
In male nematodes the spicules are hollow protuberances with distal pores, and contain
sensory dendrites. These are presumably tangoreceptors, though in Tylenchulus
semipenetrans they also have mechanoreceptive dendrites and are dual function
receptors. In Pelodera, the spicules are protruded only after the final attachment of the
male to the female. The male gubernaculum is also innervated by sensory dendrites, that
are enclosed and not in contact with the external medium.
A pair of unicellular glands, called phasmids, open separately on either side of the tail in
some nematodes (Secernentea). These are most well developed in parasitic nematodes
and may function as glandulosensory chemoreceptors. They are also often better
developed in female nematodes.
Glandular Systems
The cervical gland or renette opens via a cuticularised canal to the ventral anterior
excretory pore, anterior or posterior to the nerve ring. This gland has a neurosecretory
function, in some nematodes, releasing an enzyme that dissolves the old cuticle during
molting. In general, though, the cervical gland is thought to have an excretory function.
Free-living Enoplia and Chromadoria have caudal glands that form a complex organ: the
spinneret. This secretes a sticky substance that allows the nematode to adhere to the
substrate. It may be used to stick the nematode to a particle of detritus that is
subsequently ingested.
Crytobiosis (anabiosis)
Cryptobiosis is a state of dormancy, in which many species can survive dry conditions
for several years. Inactivity is accompanied by water loss and a very low metabolic rate.
These dormant states can also tolerate low temperatures.
Reproduction
Most nematodes are dioecious. The males are often smaller, and have a hooked tail. The
gonads are tubular. The germ cells usually arise from a single large terminal cell located
at the distal end. The germ cells gradually pass through the gonad, maturing as they do
so.
The male system comprises 1-2 testes, which pass into the long sperm duct, which
widens to form a long seminal vesicle. A muscular ejaculatory duct connects the
seminal vesicle to the rectum / cloaca. The ejaculatory duct is lined with prostatic
glands, which secrete an adhesive material to aid copulation. The wall of the cloaca is
evaginated to form two pouches, which join before they enter the cloacal chamber. Each
pouch contains a pointed curved spicule. Special muscles can cause the spicules to
protrude from the anus or vent. In many nematodes the dorsal walls (and sometimes
ventral and lateral walls) of the pouches bear special, cuticular pieces forming the
gubernaculum that guides the spicules through the cloacal chamber.
The female system consists of 1-2 ovaries, usually oriented in opposite directions. The
ovary gradually extends into a tubular oviduct and into a widened, elongated uterus. Each
of the two uteri opens into the vagina, which opens to the outside via the gonopore
located on the midventral line in the middle of the body. The upper end of the uterus
usually functions as a seminal receptacle.
The females secrete a pheromone that attracts males. The curved posterior of the male
nematode is usually coiled around the body of the female in the region of the genital
pores. The male extrudes its copulatory spicules, which are used to hold open the female
gonopore during sperm transmission. The amoeboid sperm migrate to the upper end of
the uterus, where fertilisation occurs.
Some terrestrial nematodes, e.g. rhabditoids, are hermaphroditic and eggs develop after
sperm develop in the same gonad (an ovatestis). Self-fertilisation then occurs.
Parthenogenesis also occurs in some nematodes.
Marine species produce about 50 eggs, laid in clusters. Terrestrial forms produce several
hundred eggs. Viviparity occurs in many parasitic nematodes and also in some freeliving
species, as in the vinegar eel.
Development
After fertilization the oocyte cytoplasmic membrane exfoliates and becomes the vitelline
membrane. A new cytoplasmic membrane is formed beneath this vitelline membrane. In
the uterus, the egg envelope is formed with a chitinous shell. Free-living nematodes
deposit 1-50 ova at a time, stuck together in chains or clusters. These eggs may be stuck
to the cuticle of the adult. Parasitic forms produce thousands of ova.
Aberrant cleavage gives rise to a blastula and then a gastrula. The blastula is determinate,
in that cell fate is determined at this stage. The blastopore forms the mouth and anus. The
worms hatch as juveniles. Subsequent development is direct and maturity is generally
reached after the fourth molt.
Ecology
The nematode body-plan is an extremely successful one and there are some 25 000
described species. Nematodes occur in all habitats, from arid deserts to the bottoms of
lakes, rivers and at a range of depths in the oceans and a range of temperatures, from hot
springs to polar seas. Free-living forms are particularly suited to intersticial meiofaunal
habitats, between sediment particles, and their narrow form and tough cuticle enable to
penetrate small spaces without being easily crushed or abraded. This life-style preadapted nematodes to a parasitic mode of life, in the tight tissue spaces of plants and
animals.
Free-living marine herbivorous nematodes eat mostly diatoms, whilst fresh-water
herbivorous forms eat green algae and cyanobacteria. Saprophagous forms eat detritus,
dead and decaying matter and the bacteria and fungi found therein. Carnivorous
nematodes eat mostly rotifers, tardigrades, annelids and other nematodes. Many
nematodes are zooparasites and many are phytoparasites.
Classification
O. Enoploidea: Free-living, mostly marine.
O. Dorylaimoidea: common in soil and fresh-water.
O. Mermithoidea: juvenile stages parasitic in terrestrial or fresh-water invertebrates,
usually insects, whilst the adults are free-living in soil or water. Up to 50 cm long and
filiform.
O. Chromadoroidea: mostly marine with spiral amphids and usually a ringed or punctated
cuticle.
O. Araeolaimoidea: fresh-water or terrestrial with 4 conspicuous cephalic bristles.
O. Monhysteroidea: aquatic or terrestrial, but mostly marine.
O. Desmoscolecoidea: short, plump, marine nematodes with heavily ringed and more or
less bristled bodies and hemispherical amphids and a demarcated armoured head with 4
bristles.
O. Rhabditoidea or Anguilloidea: papillate cephalic sensilla and 1-2 pharyngeal bulbs.
Semiparasitic or epizoic, or parasites / facultative parasites in vertebrates and
invertebrates, or phytoparasites – often gall-forming, or terrestrial and free-living, inc.
vinegar eels and nematodes found in felt beer mats.
O. Rhabdiasoidea: vertebrate parasites with complicated life-cycles.
O. Oxyuroidea: zooparasites, esp. of vertebrates, with a one-host life-cycle.
O. Ascaroidea: vertebrate intestinal parasites.
O. Strongyloidea: bursate nematodes. The male has a conspicuous, expanded bursa
supported by muscular rays. Intestinal parasites of herbivorous mammals and human
hookworms, which penetrate the skin.
O. Spiruroidea: parasites of mammals, inc. eye worms (Thelazia, Oxyspirura), birds and
fish. May have invertebrate intermediate hosts.
O. Dracunculoidea: filiform, parasitic in connective tissue or coelom of vertebrates. Have
an intermediate host (typically a copepod).
O. Filarioidea: filiform, males much smaller than females. Endoparasites of vertebrates,
transmitted by blood-sucking insects.
O. Trichuroidea (Trichinelloidea): inc. the whipworms, mammalian intestinal parasites
and parasites of the urinary bladder and air passages of mammals and endoparasites of
mammals.
O.Dioctophymoidea: endoparasites of mammals, e.g. the kidney worm, which infects the
kidneys (usually the right kidney) and abdominal cavity of dogs and is up to 1 m long.
(In some classification systems, ‘oi’ is replaced by ‘I’ and ‘ae’ by ‘a’).
Platyhelminthes (Flatworms)
Class Turbellaria
External Characteristics
The turbellarian flatworms are mostly free-living, with some commensal or parasitic
species. They are unsegmented worms, mostly < 5 mm long, but land planarians reach up
to 60 cm in length and polyclads (O. Polycladida) and triclads (O. Tricladida) are also
usually large.
Land planarians (O. Tricladida) are long and slender, whilst polyclads are often leaf-like
ovals or circular in margin. Flatworms are mostly dorsoventrally flattened (some are
cylindroid), with a flat ventral surface and a convex dorsal surface.
There are usually no projections, but Amphiscolops and Polychoerus have 2 caudal lobes
and Polychoerus and some sand-dwellers have a small tail filament. Some forms bear
tentacles. Vorticeros has one pair of anterior tentacles, while many polyclads have one
pair of nuchal tentacles on the dorsal surface near the brain or as marginal tentacles – a
pair of folds on the anterior margin. The temnocephalids (SO. Temnocephalida of O.
Rhabdocoela) have 2-12 tentacles at the anterior end or along the body sides. The dorsal
surface of some polyclads is covered in tubercles or papillae. In Thysanozoon an
intestinal branch occurs inside each papilla. Adhesive organs are also common.
Some fresh-water planarians have sensory lateral projections of the head called auricles.
A neck-like constriction may separate the head from the trunk, especially in fresh-water
planarians like Dugesia and land planarians like Bipalium.
Colours vary and include: translucent, white, brown, gray, black, bright colours, stripes,
bars and flecks. Land planarians and polyclads are especially colourful. Land planarians
often have white, yellow or orange stripes on a dark background, or vice versa.
Body Wall
The body covering is a one-layered epidermis. The epidermis may be cellular, especially
in larger forms, consisting of flat or columnar cells, or it may be syncytial or partly
syncytial, especially in acoels, rhabdocoels and alloeocoels. In the acoels the syncytium
may be ‘insunk’ – that is the nuclei and some cytoplasm may descend into the
mesenchyme beneath the subepidermal musculature. Fluid-filled spaces may divide the
syncytial cytoplasm into vertical strands or columns, especially in the Acoela.
This epidermis may be completely ciliated (especially in smaller forms) or only the
ventral surface may be ciliated (as in many marine and fresh-water forms). Ciliation is
reduced in commensal and sand-dwelling forms. Cilia are absent on glandular and
adhesive areas of the epidermis and well-developed on sensory areas where they bring
water-currents to the sensors. The ventral cilia are often longer and are used in
locomotion. Many land planarians, especially cylindroid forms, have a ventral median
strip or creeping sole bearing especially powerful and densely packed cilia for
locomotion. The polyclad Errantia has secreted spines along its body margin.
There is no cuticle or protective layer, except in nonciliated temnocephalids where it
forms a clear layer over the epidermis.
Large oval epidermal gland cells may open to the exterior between the cilia bases. These
glands secrete a mucous substance. The body wall contains secretory inclusions called
rhabdoids of which there are three types: rhabdites, rhammites and chondrocysts. They
are used to grip the substrate, but may have other unknown functions.
Rhabdites are straight or slightly curved rods, shorter than the height of the epidermis,
with their long axis at right angles to the surface. They occur singly or in bundles and are
secreted by gland cells in the epidermis (epidermal rhabdites) or mesenchyme (adenal
rhabdites). They are more numerous dorsally, along the body margins, and at the body
ends.
Rhammites are long, slender, and often sinuous and are longer than the height of the
epidermis and are adenal (secreted by mesenchyme gland cells). Chondrocysts occur in
land planarians and are especially large, granular rhabdites.
Each rhabdoid comprises a hull enclosing fluid and have an unknown function. They are
absent in many Acoela, some Rhabdocoela, some Alloeocoela, sand-dwelling
Rhabdocoela, parasitic and commensal species, and are absent on glandular and sensory
areas and the creeping sole.
Pseudorhabdites are masses of slimy secretion inside epidermal cells, found especially in
polyclads, alloeocoels and some rhabdocoels. Sagittocysts are pointed vesicles enclosing
a central protrusible rod or needle, and are found in some Acoela.
Nematocysts occur in the epidermis of a few species. These originate from ingested
hydroids and are carried to the epidermis by mesenchyme cells, after passing through the
gastrodermis. They are used in prey capture. These flatworms may eat hydroids only
when they need to replenish their nematocysts. Microstomum may even regurgitate food
to make room for a hydra if in need of nematocysts.
A basement membrane may be present as a homogeneous or lamellated membrane. It
may be very thick, but is lacking in Acoela and some others.
Subepidermal Glands
These are usually unicellular and are derived from epidermal, or insunk epidermal cells.
Cyanophilous glands stain blue with connective tissue stains and take haematoxylin.
They are especially abundant in the anterior ventral mesenchyme and secrete slime. This
slime is used in prey entanglement and also forms the slime trail in locomotion, and is
also protective. Some turbellarians may use the slime to encyst themselves. The slime
protects the worm against secretions from its own eosinophilous glands. Cyanophilous
glands open on the sole of land planarians with a creeping sole. A cyanophilous frontal
gland may be present near the brain. This gland opens on the anterior tip via one or more
pores on the sensory area. The secretion is involved in food capture.
Eosinophilous glands contain coarse granules that stain red with eosin. These include
the rhabdoid-forming cells and the marginal adhesive glands. The latter are found in
fresh-water and marine triclads and open via a ring of pores encircling the ventral surface
near the margin, forming a marginal adhesive zone devoid of rhabdoids and cilia. The
pores open in protrusible adhesive papillae. In land planarians lacking the ventral
creeping sole (and creep on their entire ventral surface instead) there is a white strip, the
glandular margin, near the body margin bounding the creeping surface. Eosinophilous
caudal glands open at the posterior tip or along the posterior margin. These are
protrusible or else mounted on permanently protruded finger-like adhesive papillae.
Multicellular glands occur in the acoel genus Convoluta.
Muscular System
The subepidermal musculature consists of a stratum beneath the epidermis (beneath the
basement membrane when this is present). It comprises an outer circular layer and an
inner longitudinal layer and there may also be a diagonal layer between the circular and
longitudinal layers. The subepidermal muscle layer is thin in small forms (e.g. Acoela)
but very thick in larger forms (e.g. planarians and polyclads) and is often thicker
ventrally.
In polyclads the ventral side of the subepidermal musculature usually consists of 5-6
layers. These usually are the outer circular, outer longitudinal, outer diagonal, inner
circular, inner diagonal and inner longitudinal layers. In large forms the longitudinal
fibres may group into definite bundles. In some acoels, rhabdocoels and all alloeocoels
the musculature is epidermal as in coelenterates.
The parenchymal musculature is situated deeper in the mesenchyme. It consists of
dorsoventral, transverse and longitudinal fibres and may be very complex in large forms.
The dorsoventral and sometimes the longitudinal muscles are the best developed. The
fibres may be striated and are ensheathed in a protoplasmic envelope containing the
nucleus and cell processes. It is hypothesised that these processes may have a nutritive
function. The parenchymal muscle fibres commonly attach to the epidermal basement
membrane by one or more fine filaments that may extend into the epidermis.
Mesenchyme
This is parenchymatous connective tissue that fills much of the body and occupies the
space between the body wall and internal organs. It is composed either of rounded cells
or a fixed net-like syncytium through which free amoebocytes wander. The syncytium is
nucleated and contains numerous interstitial fluid-filled spaces. These spaces possibly
constitute a pseudocoel.
Pigment
Pigment occurs in the epidermis and/or mesenchyme as granules, rods or dissolved
pigment and may be contained in pigment cells. The dorsal side is especially pigmented.
Zoochlorellae or zooxanthellae in the mesenchyme often cause the greens and browns,
found in the Acoela and Rhabdocoela.
Adhesive Organs
Turbellarians are richly supplied with adhesive organs. Glandulo-epidermal adhesive
organs occur on altered areas of epidermis that usually lacks cilia and rhabdites.
Subepidermal eosinophilous glands open here via neck-canals and pores. An example is
the marginal adhesive zone, which may form an adhesive plate or disc of adhesive
papillae.
Glandulo-muscular adhesive organs are common among fresh-water and terrestrial
triclads as cushions, swellings, pockets or pits and may be protrusible. These form
crescentic to circular protrusions and are found in fresh-water planarians and triclads.
Fresh-water planarians usually have one glandulo-muscular adhesive organ in the centre
of the anterior margin or on the ventral side of the head. There may be 2 or 4 such organs
on the ventral side of the head. In Polycotylus (from Lake Baikal) there are about 200 in a
row along the lateral body margins.
Kenkia rhynchida (from an Oregon cave) has a peculiar muscular, cylindrical snout with
a central lumen and equipped with distal eosinophilous glands and proximal
eosinophilous glands. There is a strong retractor muscle on the eosinophilous region.
These various adhesive organs are used in leech-like locomotion and prey capture.
Planarians seize crustaceans with their adhesive organ (when present) and rhabdites along
the body margin grip the substrate while the animal struggles with its prey.
In rhabdocoels the anterior proboscis is actually a glandulo-muscular adhesive organ,
which can be protruded through an anterior pore and is used in prey capture. Polyclads
(SO. Cotylea) have a ventral adhesive ‘sucker’. The genital ‘suckers’ between the male
and female gonopores in Leptoplana assist copulation. These are not true suckers, but
rely on secretion for their adhesion.
A true sucker or acetabulum is found in some species. This is separated from the body
parenchyma by a muscular septum. They have a circular raised rim with a central
depression, and may be stalked. They adhere by vacuum suction.
Nervous System
The simplest type found in turbellarians is of the coelenterate type and occurs in some
Acoela and Alloeocoela. This comprises an epidermal nerve layer outside the
subepidermal musculature. Anteriorly this exists as a more or less continuous sheet and
posteriorly as a net-like plexus with about three pairs of longitudinal strands. The brain is
thought to be either a pair of lateral thickenings in the main stratum or the nervous layer
over the statocyst.
In all other Turbellaria the nervous system is sunk into the mesenchyme to form a
submuscular plexus. There are a number of longitudinal cords, of which the ventral pair
is the most prominent. Transverse connections or rings join the cords together. The
ventral cords extend to the brain or cerebral ganglia, which may be bilobed. The brain
gives off nerves to the body margins and sense organs of the head and is connected
directly or indirectly to the other nerve cords. A subepidermal plexus may also be present
beneath the basement membrane.
Some acoels have 3-6 pairs of longitudinal cords (usually 5 pairs): dorsal, dorsolateral,
ventrolateral, ventral and marginal (the marginal is often absent in forms with 5 pairs).
The acoel brain forms a mass, usually of 4 lobes, around the statocyst. There may be an
additional ganglion on each side (in addition to the brain) from which the dorsal cords
proceed, as in Polychaerus. In polyclads there is bilobed brain enclosed in a capsule and
2 frontal accessory ganglia outside the brain capsule.
The nervous systems of other turbellaria are more bilateral and usually possess 3-4 pairs
of longitudinal cords: dorsal, lateral or marginal, ventral and sometimes ventrolateral;
connected by a plexus and ring commissures. There may be vertical commissures
between the dorsal and ventral cords.
Most fresh-water planarians have only the ventral pair of nerve cords. These cords are
well developed with transverse connectives between them and they give rise to lateral
branches and often continue to the brain. The brain may be merely the thickened cord
ends or rounded ganglia.
Land planarians have particularly complex nervous systems consisting of subepidermal
and submuscular plexi. There are no cords, but the submuscular plexus is thickened to
form a ventral nerve plate below the gut. This ventral plate may continue anteriorly as a
submuscular head plate. The ventral plate may have cord-like thickenings. The most
complicated brains are found in the polyclads.
Sense Organs
There is a rich supply sensory bristles on the epidermis that represent subepithelial
sensory cell projections. Up to 3-5 such bristles may pierce each epidermal cell! These
sensory bristles are denser on the tentacles, auricles and body margins where they form a
sensory strip in land planarians.
Chemoreceptors are concentrated at the head end in ciliated pits or grooves. These pits
are epidermal depressions, which may be glandular or non-glandular, but are devoid of
rhabdites. The sides of the pit are ciliated, while the bottom of the pit possesses stiff cilia
(sensory processes?). These organs are connected to the brain directly or via a nerve.
Some land planarians have one pit on the ventral side of the head. Additionally, a pair of
ciliated grooves border the anterior end of the creeping sole or else there is one groove at
the anterior end of the sole. Some rhabdocoels have one pair of pits on the sides of the
head. Some alloeocoels have a transverse ciliated furrow that partly or entirely encircles
the rear of the head.
Marine and fresh-water triclads have an auricular groove (auricular sense organ) on each
side of the head. In dark coloured planarians each organ is visible as a white line along
the head margin or across the base of the auricles (when auricles are present). This line
may be straight, curved or dashed. A water current that flows along the auricular groove
from the front backwards irrigates the organs. The grooves are depressed epidermis
lacking rhabdoids and gland cell pores. There are short cilia (sensory?) and a rich nervous
supply.
The frontal gland is probably also chemoreceptive and tactile (forming a frontal organ).
Rheoreceptors detect water currents. In Mesostoma and Bothromesostoma there are four
pairs of rheoreceptive cells along the body sides. These are large branching cells with
sensory bristles projecting through the epidermis.
A statocyst is present in some turbellarians. This is a nucleated vesicle with 1-2 lithocytes
enclosing a statolith. This statocyst is embedded in the brain or else positioned very close
to it.
Photoreceptors occur over the general body surface and in eyes or ocelli. The eyes / ocelli
are either epidermal pigment spots or pigment-cup ocelli sunken into the mesenchyme.
There is usually one pair of eyes, but 2-many pairs may be present. Many pairs may be
strewn over the head. There may be 2 clusters of eyes as in Polycladodes. In Polycelis
there is a band of eyes around the anterior margin. Geoplanids have a row of eyes along
the whole body margin. The eyes may be borne on tentacles.
Eyes are absent in cave-dwelling planarians and some endoparasitic rhabdocoels, many
acoels, rhabdocoels and alloeocoels, especially sand and mud-dwellers.
Pigment cup ocelli have one or more pigment cells in a bowl or cup. One to many
photoreceptors (retinal cells) project into the cup through an opening. These are bipolar
cells with their expanded distal ends projecting into the cup. This is the inverse type of
pigment-cup ocellus.
The ocelli of some turbellarians lack pigment cells and retinal cells are free in the
mesenchyme near the brain. Land planarians have the most complex turbellarian eyes.
These are of the converse type with a cornea over the mouth of the cup. The cornea may
act as a lens.
Respiration
Gas exchange is by simple diffusion through the body surface, aided by either the
narrowness of the body (cylindroid forms) or the flatness of the body. Respiration is
aerobic.
Nutrition
Most turbellarians are carnivores and eat live or dead animals. Some turbellarians eat
algae. Prey animals include worms, molluscs, crustaceans, ascidians, insect larvae and
fish. Many scavenge corpses. Some are generalists while others predate more specific
prey. Stylochus frontalis slowly devours oysters inside their shell (despite shell partitions
erected against it).
The Acoela lack digestive cavities. They ingest small animals whole and digest them
intracellularly. In most turbellarians, however, the gut consists of mouth, pharynx and
intestine. The endoparasitic rhabdocoel family Fecampiidae lack mouths.
Taste receptors occur on the anterior margin and the pharynx tip. The mouth is midventral or anywhere along the mid-ventral line, anterior or posterior. The mouth is a
circular, longitudinal or transverse slit equipped with radial and circular subepidermal
muscle fibres that operate to open and close the mouth, which is very expansable.
The pharynx is absent in some Acoela and lost in the Fecampiidae. When present, the
pharynx is a muscular tube. This tube may be simple, bulbous or plicate. In the simple
type the pharynx is a short ciliated tube. The bulbous variety is more muscular and
unciliated and may be preceded by a more or less muscular buccal tube. The bulbous
pharynx is equipped with both circular and longitudinal muscle layers and has a sphincter
of radial muscle fibres at one or both ends. It is innervated by a nerve plexus and, in some
species, also by a nerve ring in the pharynx tip. Extrinsic muscle bands connect the
bulbous pharynx with the body wall. These operate to evert and retract the pharynx.
The plicate type pharynx has a cylindrical fold projecting into the large pharyngeal
cavity. This fold may point forwards, backwards or hang straight down. The pharynx is
protruded through the mouth by muscle action when feeding and food is ingested through
the pharynx orifice not the mouth.
Some fresh-water and land planarians have many mouths and many pharynxes
(polypharyngy).
The pharynx passes into the intestine, either directly or via a short oesophagous. The
intestine is absent in Acoela. The intestine is a rounded or elongated sac or else a highly
branched tubular system. It is lined by a gastrodermis monoepithelium, which may be
syncytial. This lining contains phagocytic and granular cells. It is thought that the
granular cells could be storage cells or glands. The gastrodermis may be ciliated and may
have a subepidermal muscle layer beneath it.
In Acoela the mouth opens into the mesenchyme, either directly or via a simple pharynx
or via a protrusible pharynx. Phagocytic cells in the mesenchyme ingest the food for
intracellular digestion. Phagocytic mesenchyme may form a central phagocytic syncytial
region.
Some polyclads have one dorsal anus or several lateral or dorsal anal pores. Most
turbellarians lack an anus.
Excretory System
Most turbellarains have protonephridia equipped with terminal flame bulbs, but these
are absent in Acoela which lack specialised excretory organs. Protonephridia are better
developed in fresh-water than in marine forms. Protonephridial tubules consist of a
single-layered cuboidal epidermis (or a syncytium) that is usually unciliated. There are
numerous branches, called capillaries, each ending in a flame bulb. The flame bulb plus
its capillary comprise a single tubular cell, called a flame cell. In some species, one flame
cell may give rise to several capillaries and flame bulbs). The ciliary flame may be flat,
ribbon-like or a hollow cone or cylinder. The free end of the flame bulb may give out
cytoplasmic processes into the mesenchyme. There may be additional lateral flames of
cilia along the capillary in some rhabdocoels.
The protonephridium opens to the outside via a nephridiopore or sometimes via a
contractile ampulla or bladder in fresh-water forms. There may be one common pore or
several pores or the nephridia may open into the pharynx or into the genital atrium. There
may be one median protonephridium or 1-4 nephridia on each side, which may
anastomose to form a network opening by up to several hundred nephridiopores. There
may be dorsal nephridiopores in addition to the ventral nephridiopores present. In some
species athrocytes can be seen wrapped around the tubules. These athrocytes may be
syncytial.
Locomotion
Small forms up to 2-3 mm long can locomote entirely by ciliary gliding across the
substratum and may also swim occasionally. They may even glide underneath the surface
film. Ciliary waves run from the anterior to the posterior.
Large forms crawl or creep over the substratum by muscular waves (possibly aided by the
action of the cilia) from the anterior to the posterior. These waves may travel anterior to
posterior along the whole width and are then called monotaxic waves, or they may travel
alternately along the two sides, in which case they are called ditaxic waves. Sometimes
these waves are invisible to the naked eye. Swimming is achieved by undulating the body
or body margin. Most turbellarians will also exhibit leech-like movements, especially
when harassed.
Secreted slime aids gliding, especially in terrestrial forms. Slime threads may be used to
drop from one level to another or to bridge gaps in the substratum.
Reproduction
Almost all turbellarian species are hermaphroditic with separate male and female
systems. They may have separate male and female gonopores or the gonads may share a
common antrum and pore. A few species are dioecious and in some only females are
known suggesting permanent parthenogenesis.
At the appropriate season, mesenchyme cells migrate to the gonads and undergo
gametogenesis. There may be definite gonads, or the gametogonia may reside free in the
mesenchyme. The gonads are rounded, oval, elongated, lobed or branched. The follicular
condition pertains when numerous small gonads are present.
Rhabdocoels usually have one pair of testes and 1-2 ovaries. Polyclads have numerous
testes and ovaries. Alloeocoels usually have few to many testes and one pair of ovaries.
The male system consists of one duct when only one testis is present; one pair of sperm
ducts when 2 or more testes are present, and several pairs when multiple copulatory
complexes are present. When there are many testes, several connect with a sperm duct via
their own sperm ductule. Sperm ducts may unite to form a common sperm duct, which
connects to the copulatory complex.
On approaching the copulatory complex each duct or common duct has a tubular or
sacciform expansion called the spermiducal vesicle that stores ripe sperm. The duct or
ducts feed into the ejaculatory duct. There may be one or more copulatory complexes.
There may be a seminal vesicle that stores ripe sperm. Neither, either or both a seminal
vesicle and a spermiciducal vesicle may be present. A prostatic apparatus of prostatic
or granule glands may be present. These glands may open into the penis or ejaculatory
duct or into a prostatic vesicle. There are various arrangements.
The penis is the terminal part of the male canal. It may be a protrusible conical projection
(penis papilla) or a cirrus. The penis may be armed with thorns or spines or a stylet (a
curved or straight hollow tube) or a complex array of barbs and teeth.
The male apparatus opens to the outside via the male antrum and gonopore or via a
common antrum and gonopore or into the pharyngeal cavity. The sperm possess two
flagella or bristles and may be packaged into spermatophores that are discharged into the
recipient.
The female canal or vagina may open to the outside directly via the gonopore or via an
antrum that opens via the gonopore. In some Acoela eggs are discharged through the
mouth or through a rupture in the body wall.
Yolk may not be stored in the egg, but in special vitelline cells (altered ovocytes?)
instead. There may be yolk glands (vitellaria) that open into the oviducts directly, in
which case the oviducts are really ovovitelline ducts, or via yolk ducts. There is usually
one pair of yolk glands and 1-2 ovaries. When one ovary is present there is one oviduct,
when two ovaries are present there are 2 oviducts. When there are numerous ovaries
these connect via oviductules to two oviducts or ovovitelline ducts to the female
copulatory complex. This complex is typically a copulatory bursa (which receives
sperm from the partner copulant and briefly stores this sperm), but may be a seminal
bursa (stores sperm for longer and also digests excess sperm and prostatic secretion) or a
seminal receptacle (stores sperm indefinitely). A bursa and a receptacle may be present.
This bursal complex may open to the outside via a copulation canal and bursal pore,
which is separate from the gonopore.
Uteri may be present as expanded oviducts that hold shelled eggs prior to laying and may
be involved in eggshell formation. The eggs are either embedded in jelly or one or more
are enclosed in a hard shell (capsule or cocoon) that may also encapsulate hundreds of
yolk cells. This shell hardens and darkens after laying and is coated with an adhesive
secretion of the cement gland. This secretion may be drawn out into an adhesive stalk.
The eggs are fastened to an object and hatch within a few weeks. The hatchlings are
complete tiny worms except in a few polyclad species in which they are free-swimming
larvae.
In some species the eggs develop in the parental body, in the uteri, mesenchyme or
intestinal lumen in which case the young escape through the mouth, gonopore or by
rupture and death of the parent.
Copulation
Fertilization is usually internal. Self-fertilization is rare since the sperm only become
motile once they enter another individual.
There are two types of copulation in turbellarians: hypodermic copulation and the more
normal type of copulation. In hypodermic impregnation the armed penis injects sperm
through the epidermis of the partner. The sperm migrate through the mesenchyme to the
eggs. In ordinary copulation there is a mutual exchange of sperm. The sperm are emitted
into the copulatory or seminal bursa or into the oviducts or the seminal receptacles.
There is a brief courtship with head or body contacts. The copulants usually face away
from each other with their genital regions pressed together and often elevated, while the
rest of the body remains attached to the substratum. Copulation lasts from a few minutes
to an hour or more (aided by adhesive secretion?).
Development
Cleavage is of the spiral determinate type. In most turbellarians the eggs hatch directly
into tiny worms. In some polyclads, however, hatching gives rise to Műller’s larva, a
ciliated free-swimming larva that lasts for a few days. It has apical and sometimes also
caudal sensory tufts and 8 ciliated lappets or lobes, eyes, a mouth and a brain. Graffizoon
lobata exhibits neoteny and the adult is a sexually mature Műller-type larva. Gőtte’s
larva occurs in some Stylochus spp. And differs from Műller’s larva by possessing 4
ciliated lappets instead of 8. After a few days the lobes of both larval types are absorbed,
the sensory tufts disappear, the larva flattens and extra eyes differentiate as the larvae
metamorphose into tiny adults.
Some turbellarians also reproduce asexually and some species are only known to
reproduce asexually. This may be by transverse fission into chains of zooids. The zooids
are well differentiated before they separate from the chain. In some others the rear end
adheres firmly while the anterior advances. This causes the worm to elongate and rupture.
The rear end regenerates into a new worm. Greasing the container inhibits attachment and
the generation of the necessary traction, resulting in very long worms!
Many species undergo asexual fission in summer and sexual reproduction in winter or
spring. Thus, there is often an annual asexual cycle.
Some turbellarians can reproduce asexually by fragmenting into numerous pieces that
encyst. Within the cyst reorganization and regeneration into small adult worms takes
place.
Regeneration
Asexually reproducing forms have necessarily high powers of regeneration. Almost any
piece may regenerate. In turbellarians that lack asexual reproduction, only anterior pieces
or pieces containing the cerebral ganglion can regenerate a new head. Many are capable
of replacing posterior regions, however, including the pharynx and copulatory apparatus
when the cuts are posterior to the cerebral ganglion.
Polarity is usually strongly retained in regenerating pieces. Cuts can induce head or tail
formation in the wrong places and may lead to multiple heads or tails. Regeneration
involves both epimorphosis, or new tissue growth, and morphallaxis, or reorganization of
old tissue.
Longitudinal halves may replace the missing half. In freshwater planarians a moderate
sized piece can regenerate, but smaller pieces have lower powers of regeneration as do
more posterior pieces. Some posterior pieces will fail to develop a head or will develop
reduced heads.
Grafting
Grafting experiments can be carried out on suitable freshwater planarians, e.g. Dugesia.
Regeneration occurs according to the original polarity of the transplanted piece (unless
the piece is very small in which case it may adopt the polarity of the host region).
Cephalic grafts may induce head formation along with one or more pharynges, especially
when implanted posteriorly. Posterior pieces induce tail formation when implanted
anteriorly.
Ecology
Small forms live for weeks to months and may encyst in unfavourable conditions. Large
forms are thought to live as long as 2-3 years or more. Marine turbellarians are typically
bottom dwellers with small forms preferring soft bottoms and large forms hard bottoms.
Marine turbellarians are littoral and non-abyssal. Some are pelagic as adults and some
juveniles and the larval forms are pelagic.
Turbellarians seem to have evolved in the marine habitat, with some adapting to the
freshwater habit and some freshwater forms adapting to humid terrestrial areas.
Terrestrial forms occur especially in tropical and subtropical rainforests and hunt at night
and hide by day under stones, logs, leaves, between the leaf bases of tropical plants or in
earth burrows.
Turbellarians are rarely eaten because their mucus is distasteful or toxic. Dragonfly and
damsel fly nymphs may eat planarians. Turbellarians may also be eaten by other
turbellarians.
Most acoels and some rhabdocoels and some alloeocoels possess symbiotic chlorellae or
xanthellae in their mesenchyme inside the subepidermal muscle layer. The juveniles
acquire these symbionts from their food. Convoluta houses chlamydomonad symbionts
and the adults only feed by digesting some of these chlorellae. These symbionts provide
oxygen and consume host metabolic waste (there is no exctretory system in acoels).
Some marine turbellarians are commensals in echinoderms, molluscs and other
invertebrates. Some polyclads live in snail shells, hermit crab houses. Other polyclads are
ectocommensal on fish or Limulus. These ectocommensals typically show loss of
pigment and cilia (esp. dorsal cilia), rhabdites and sometimes eyes. They have strong
adhesive organs. Endocommensal turbellarians have fewer rhabdites, reduced eyes, but
an enhanced female reproductive system. A few turbellarians are genuine endoparasites.
Turbellarians support their own commensals and parasites, including euglenoid
flagellates and trypanosome-like flagellates in the copulatory bursa or digestive tract.
Ciliates, gregarines and larval trematodes have also been found living within
turbellarians. Nematodes have been found within land triclads.
Classification
Polyclads (Polycladida) occur in littoral zones as bottom dwellers. Some are pelagic
down to 1000 m. Some pelagic forms swim, while others inhabit floating Sargassum.
There is one freshwater species and no parasitic species. They range in size from 2 mm
long to several cm. There are four suborders.
Triclads (Tricladida) are commonly known as planarians and range from 2 mm long to 60
cm. They occur in freshwater, saltwater and humid terrestrial regions. There are three
suborders: Maricola (marine planarians), Paludicola or Probursalia (freshwater
planarians) found especially in temperate regions, and Terricola (terrestrial planarians)
found on the forest floor, especially in tropical or subtropical regions. Most are long and
slender.
Alloeocoels (Alloeocoela) range from 1 mm to 40 mm long and are often cylindroid,
plump to elongate, and some are long and slender. Most are marine littoral, some occur in
brackish water, some in freshwater and some in moist terrestrial regions. There are four
suborders.
Rhabdocoels (Rhabdocoela) range from 0.3 mm to 15 mm long. They range from slender
to plump. The Temnocephalida have 2-12 tentacles. Rhabdocoels are marine littoral, and
are found especially on sandy and muddy bottoms) or freshwater or damp terrestrial.
There are six suborders.
Acoels (Acoela) range from <1 mm to 12 mm long and from elongate to plump. They are
mostly marine littoral, but some are pelagic. There are five families.
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 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.
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
plattyhelminthes.
Pigment granules providing the animal its colour may occur in ciliated, gland or
interstitial epidermal cells.
Beneath the epidermis is the dermis 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, This consists of branched cells that possible form a syncytium.
Lymphocytes are also present in the connective tissue.
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 musclelayers, in the mesenchyme or attached directly to the brain. They are inverted pigmentcup 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 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 respiratory pigment that may be 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. Bodywall 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 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
become to resemble Paramecium ciliates. Upon further dedifferentiation they resemble
pilidium larva. After 2 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.
Priapulida
External Features
By 1979 there were about 9 species of priapulids known. These marine aschelminthes are
cylindroid, warty and have a superficially segmented trunk and an introversible presoma
or proboscis. The mouth and anus are terminal and the gut is straight. The gonads and the
solenocytic protonephridia open via one pair of urogenital ducts and pores near the anus.
Priapulids range from < 1 mm to 8 cm in length and are drab coloured.
In Priapulus the posterior end bears 1-2 conspicuous warty appendages or caudal
appendages. The presoma is about 1/3 of the body length and is plumper than the trunk
from which it is demarcated by a constriction.
In Halicryptus the presoma is less than about 1/6 of the body length and the same width
as the trunk.
An area armed with spines arranged in concentric pentagons surrounds the terminal
mouth. There are 3-4 such pentagons in Halicryptus and 5-7 in Priapulus. The spines
increase in length with distance from the mouth and then decrease again. This spiny
circumoral region is normally invaginated with the spines pointing backwards but everts,
with the spines pointing forward, when in use.
In Priapulus there is a visible ring behind the spine-bearing region that indicates the
position of the nerve ring.
The presoma or proboscis is adorned by 25 longitudinal ridges or ribs of rows of papillae.
The presoma can be invaginated, but is normally evaginated.
The trunk is irregularly strewn with small spines and papillae and annulated with about
30-40 rings in Priapulus or about 100 rings in Halicryptus. Along the midventral line of
the trunk and proboscis is a whitish longitudinal strip caused by the underlying nerve
cord.
In Priapulus the most posterior trunk rings have wart-like protuberances (except on the
midventral line) that are possibly glandular since they are porous. The posterior end of
the trunk bears the anus, 2 urogenital pores and 0-2 caudal appendages. The caudal
appendages have hollow stems covered in hollow round to ovoid vesicles. The function
of these caudal appendages is unknown and they have the same structure as the body
wall.
Body Wall
A cuticle overlies the epidermis. Beneath the epidermis is a circular muscle layer, then a
longitudinal muscle layer and then a thin nucleated membrane. The cuticle has a thin
homogeneous outer layer and a thicker lamellate inner layer. The papillae spines and
warts of the body wall are sensory or glandular. The muscle fibres are obliquely striated.
The epidermis is a single layer of tall slender cells. Only the ends of these cells touch as
they are otherwise separated by fluid-filled cavities. The circular muscle layer forms
separate rings and the longitudinal muscle layer forms either separate strands, as in
Priapulus, or a continuous stratum, as in Halicryptus. There is a longitudinal muscle
under each proboscis rib (here outside of the circular muscle layer).
Nutrition
The mouth opens to the muscular pharynx that contains circular and radial muscle fibres
in its wall and has an external layer of longitudinal muscles. The pharyngeal lumen is
lined by epidermis and cuticle covered with spines or teeth that are continuous with the
spines around the mouth and decrease in size posteriorly. A circle of retractor muscles is
attached to the anterior end of the pharynx. These muscles originate on the body wall and
serve to invaginate the proboscis. Short retractors originate at the junction of the
proboscis and trunk. Long retractors originate on the trunk wall. There are up to 25 short
retractors in Priapulus and up to 8 long retractors. Halicryptus has about 10 of each.
When the pharynx is invaginated the pharynx invaginates into the midgut. The midgut or
intestine is straight or slightly curved. The intestinal wall comprises a luminal epithelium,
bearing circular folds, a circular muscle layer and a longitudinal muscle layer. A
constriction marks the rest of the intestine from the rectum that opens to the anus. The
rectum has a cuticle luminal lining.
Priapulids are predators of slow-moving prey and eat polychaetes and other priapulids.
They swallow their prey whole. Priapulids can survive without eating for months.
Body Cavity
A fluid-filled cavity exists between the body wall and the digestive tube and the cavities
in the caudal appendages and the spaces in the body wall musculature are also filled with
similar fluid. Numerous round cells float in the fluid. These are probably athrocytes as
they have been seen to take up foreign particles and deposit them in the protonephridia.
In some species these cells contain haemerythrin and body-wall movements circulate the
fluid.
The cavity between the body wall and gut is lined by a ‘peritoneum’ that is anuclear and
‘structureless’ and therefore probably not a true peritoneum. This membrane covers the
inner body wall and the outer gut wall and forms webs attaching the gut and urogenital
organs to the body wall. The gonads are attached to the body wall by epithelial
extensions. The true developmental nature of this cavity is unknown.
Nervous System
There is an epidermal circumenteric nerve ring at the level of the beginning of the
pharynx and a midventral epidermal nerve cord. There are no ganglia, but the cord is
slightly broadened in each trunk annulus and has a terminal thickening. Ganglion cells
occur throughout the length of the cord. Each annular thickening gives out one pair of
main nerves and other nerves that go around the body in the epidermis.
The circumenteric nerve ring gives out 13 pairs of nerves to the proboscis epidermis
(these nerves possibly travel underneath the ribs) and 4 pairs of nerves to the pharynx.
Nerve rings connect these pharyngeal nerves. The entire nervous system is epidermal.
Sense Organs
The papillae are presumably tactile and chemoreceptive. No eyes are known in the
priapulids, though they are sensitive to light, which they avoid. A light tactile stimulus
evokes vigorous burrowing. A strong tactile stimulus causes the priapulid to become
shortened and contracted in a defensive posture.
Excretion
There is one pair of urogenital organs in the body cavity, one either side of the intestine.
Each is an elongated warty body supported by a mesentery extension of the membrane
lining the cavity. Each urogenital organ consists of a urogenital duct bearing the gonad on
the side next to the mesentery and clusters of solenocytes on the other side. Each cluster
consists of thousands of solenocytes. Each solenocyte has a flagellum flame. Collecting
canals connect the solenocytes to the urogenital duct. Each urogenital duct opens via an
external urogenital pore at the posterior end of the trunk.
Respiration
Experiments have shown that the caudal appendages are not respiratory. No evident
effects follow their removal and their function remains unknown.
Locomotion
Priapulids live burried in soft bottoms from littoral and intertidal zones to 500 m beneath
the sea. They lie quiescent in a vertical position with the mouth level with the surface,
except when hungry in which case they plough through the mud seeking food.
The proboscis explores the environment and exhibits positive geotaxis. Burrowing is
achieved by alternate invagination and evagination of the proboscis. At its rear end
Priapulus forms a ring-shaped expansion for anchorage. Halicryptus lacks anchorage and
moves by peristaltic waves.
Reproduction
Priapulids are dioecious, though in some species only females are known. Sexual
dimorphism occurs in some and fertilisation may be internal or external. The paired
gonads form a tubular network.
Development
Cleavage is radial, total and equal and gives rise to a rotifer-like larva. The larva
possesses a cuticularised chitinous lorica or armour consisting of a dorsal and a ventral
plate and 3 long narrow lateral plates (and two additional small plates in Priapulus
comprising dorsal and ventral pieces in front of the main plates). The presoma projects
from the lorica and consists of a spiny region and narrowed neck. The presoma can be
withdrawn into the lorica or tunic.
The larva of Priapulus has two pairs of tactile spines (cf. Lateral antennae of rotifers), a
terminal foot and two toe-like caudal projections. This foot gives rise to the caudal
appendages of the adult. The foot is also supplied by one pair of caudal gland complexes
that give rise to the glandular warts of the adult Priapulus and Halicryptus.
The larval internal structure is similar to that of the adult, but the urogenital system is not
fully developed. These larvae live for about two years as benthic detritovores. They molt
and shed their lorica and emerge as young adults that continue to molt as they grow.
Regeneration
A sphincter exists between the caudal appendages and the body cavity and the caudal
appendages can regenerate unless the sphincter is damaged, in which case fluid loss leads
to death.
Ecology
Priapulids are common in temperate waters as carnivores of the muddy sediments in
which they burrow. Priapulus caudatus is bipolar.
Phoronida (Phoronids: Horseshoe Worms)
The phoronids are tubiculous, vermiform coelomates with a terminal horseshoe-shaped
lophophore embracing the mouth. The digestive tract is recurved and the anus is near the
mouth. The circulatory system is closed and contains red blood cells. There is one pair of
metanephridia that also function as gonoducts. These worms are solitary and inhabit a
tube of their own secretion. There is no obvious regionation or segmentation. There are
about 15 species of phoronid and 2 genera: Phoronis and Phoronopsis.
External Features
Phoronids are colourless, transparent or pale or of a greenish colour. These worms range
in length from about 6 mm to 200 mm. From 18 to over 500 (or possibly over 1000?)
tentacles are borne on ridges.
The terminal tentacle crown is borne on the lophophore. The lophophore is a crescentic
or horseshoe-shaped double ridge of the body wall and bears a single row of these
tentacles along both ridges. The mouth is located between the two ridges of the
lophophore. The outer (external) convex ridge is ventral to the mouth, while the inner
(internal) concave ridge is dorsal to the mouth. The limbs of the crescent are directed
dorsally and maybe spirally rolled to increase the space available for the tentacles. The
inner row has a medial break at the generative zone at which new tentacles form to either
side. There is no gap in the genitive zone of the outer row. The number of tentacles
increases with age. The tentacles are all the same length, except for the growing zones,
and comprise hollow ciliated extensions of the body wall. The tentacles are also laterally
flattened and have an oval cross-section. The bases of the tentacles are fused to form a
membrane bordering the mouth and buccal grooves and which is continuous with the
lophophore ridges. A groove or collar-like fold may demarcate the tentacular crown from
the trunk.
The mouth is midventral and positioned between the lophophore ridges and is large,
crescentic and continuous at the angles with the buccal grooves. These grooves run
between the lophophore ridges throughout their length. The upper lip, or epistome,
overhangs the mouth dorsally. The epistome is a fold of the body wall ventral to the inner
ridge.
An anterior concavity, forming a brood chamber, is bounded by the arms of the
lophophore laterally and bounded dorsally by a projection bearing three pores and
bounded medially by the anus. One pair of nephridiopores are situated one either side of
the brood chamber. Lophoral organs may occur in the lophophore concavity. These
organs comprise glandular epidermis and their occurrence depends on species and season.
Alternatively there may be one pair of glandular depressions partly covered by a flap of
body wall. There may also be seasonal white earlike thickenings of the dorsal epidermis
of the bases of the inner tentacles. A ciliated groove connects the lophophoral organ to
each nephridiopore.
The trunk coelom (metacoelom) contains a transverse septum at the level of the
lophophore base. The posterior end of the worm is enlarged into an end bulb. The body
of the worm is usually faintly annulated.
Body wall
The epidermis contains gland cells, especially on the outer surface of the tentacles and
lophophore, the anterior of the trunk and the tip of the end bulb. The basement membrane
is very thick in the tentacles and the base of the lophophore. Beneath this, is a circular
muscle layer and then a stronger longitudinal muscle. Beneath these muscle layers is the
peritoneum. There is a thin cuticle, especially on the outer lophophore. The tentacular
epidermis is heavily ciliated along the inner side. (Anterior trunk epidermis ciliated?). An
intraepidermal nerve plexus is situated at the base of the epidermis. The tentacle
epidermis is columnar (especially the inner surface) and ciliated.
Musculature
The musculature is most ell developed in the trunk. There is an outer thin circular muscle
layer and a stronger inner longitudinal muscle layer. At the two ends of the trunk the
longitudinal musculature forms a simple layer. In the middle of the trunk the longitudinal
musculature forms bundles that project into the coelom as folds. The varying height of
the muscle fibres gives these folds a feathery appearance. The folds serve to increase the
number of muscle fibres and as a result the trunk is extremely muscular and contractile.
There are 18-129 such folds per cross-section, depending on the species. In the rear part
of the end bulb there is an inner circular muscle layer and a weakly developed outer
longitudinal muscle layer.
The muscle layers are syncytial with the nuclei in the fibre bases and not in the folds. The
muscle is of the smooth type. The peritoneum lining is a nucleated syncytial layer. The
tentacles have no circular muscle and have a thick, stiff basement membrane. The
tentacles can only move slightly, by means of a few longitudinal muscle fibres on the
inner side of the basement membrane, at each end of their oval cross-section.
Movements are restricted to extension from the tube, expansion of the tentacular crown,
withdrawal into the tube and folding of the crown. Worms removed from their burrows
will excavate a new burrow. The end-bulb possibly serves to anchor the animal in its tube
during retraction into the tube.
Nervous system
The nervous system consists of an intraepidermal nerve net and an intraepidermal central
nervous ring along the outer edge of the lophophore at the level of the septum. This nerve
ring is thickest middorsally. The nerve ring gives out nerves to each side of the outer
tentacles. The inner tentacles and lophophoral ridge are supplied directly by the nerve
ring with which they are in contact. Spiralled lophophores are accompanied by a nerve
tract for each spiral. The nerve ring also gives out a nerve tract to each lophophoral
organ. Motor nerves from the nerve ring cross the basement membrane and innervate the
anterior ends of the longitudinal muscle bundles.
There is a lateral nerve cord, actually a giant nerve fibre, in the left side only. A right
nerve cord is sometimes present, but may be smaller than the left. P. ovalis has no lateral
nerves. The lateral nerve cord originates in the right side of the nerve ring, crosses over to
the left side of the nerve ring and extends to the beginning of the end bulb. The cord
gives off transverse branches into the trunk musculature through the basement membrane.
The nerve net conducts equally in all directions.
Sensory systems
Epidermal neurosensory cells are arranged in groups on the muscular part of the trunk
and on the tentacles. There is a cluster of these cells associated with the glandular
lophophoral organ on the outer medial side of the covering flap, forming the lophophoral
sense organ. Repeated light touches, or one strong prod, of the tentacles causes cilia
stoppage and tentacle retraction. Jarring causes the same response. A giant nerve fibre
transmits these reflexes. No reactions to light or shadow have been observed.
Coelom
The trunk coelom (metacoel) is divided from the tentacular crown coelom by a slightly
oblique septum (diaphragm) at about the level of the nerve ring, surrounding the
oesophagus. Ventral, dorsal, right and left dorsolateral mesenteries, comprising two
layers of peritoneum, divide the metacoel (usually) into four longitudinal compartments
that may join at the two ends of the trunk. The ventrolateral compartments are larger. The
peritoneal layers of the mesenteries sometimes enclose radial muscle fibres.
The metacoel opens to the outside via the nephridia (coelomoducts). Anterior to the
septum the coelom has no external openings and forms the epistome, lophophore and
tentacle cavities, which are joined together. The lophophore coelom is continuous with
the tentacle coeloms. Muscle fibres cross the epistome cavity and connective tissue
strands and communicates at the sides with the lophophore coelom.
The coelom is filled with a colourless fluid. This fluid contains red corpuscles, spindle
bodies of unknown function and four kinds of coelomocyte: phagocytes, eosinophilous
and basophilous granulocytes, and non-motile eosinophilous cells. Species with a
coloured tentacle crown also have motile pigmented cells in the lophophoral coelom.
Excretion
There is one pair of metanephridia that open via the metacoel coelomoducts. The
coelomoducts open in the anterior end of the metacoel close to the septum and consist of
U-shaped tubes, lined by ciliated epithelium and covered by peritoneum on the coelomic
side. The nephridiopores are located on the sides of the anal papilla. The nephridia are
excretory and also emit the sex cells.
Injected dyes are taken-up by the circulatory endothelium, peritoneum, vasoperitoneal
tissue, nephridial tubes and coelomocytes. The coelomocytes accumulate in the base of
the digestive epithelium and are eliminated. Uric acid derivatives (guanine?) accumulate
as granules on the outside of the vasoperitoneal tissue.
Nutrition
Phoronids are ciliary-mucus feeders. Tentacular ciliary currents deliver particles to the
buccal grooves and hence onto the mouth. The current exits between the tentacle bases
and flows over the anal and nephridial openings, carrying away waste. Unsuitable
particles are rejected at the mouth angles and by the tentacles. Some tentacle cilia beat
towards the tentacle tip, carrying rejected particles. The tentacle tips then bend outward
and the waste particles drop off. All particles become mucus-coated. Individual tentacles
bend suddenly to the mouth, presumably to deposit food.
The digestive tract is a recurved hairpin shape. The crescentic mouth is overhung dorsally
by the epistome and can be closed by bending of the epistome over the opening. The
mouth leads into a ciliated buccal tube, which connects to the oesophagus. The
oesophagus has a thick folded wall, a ciliated glandular luminal epidermis with a nervous
layer underlain by a basement membrane and a circular fibre layer. Radial fibres connect
the oesophagus to the body wall. The oesophagus opens into the prestomach or
proventriculus. The proventriculus possesses a middorsal ciliated strip on its luminal
surface and has no apparent nervous layer and no muscle. Apart from the strip, the rest of
the prestomach lining is weakly ciliated (and may be glandular?). The prestomach leads
to the stomach. The ovoid stomach is situated ventrally in the end bulb. It has a middorsal
ciliated groove on its inner surface, but the rest is weakly ciliated. The stomach is
innervated by a haemal network or haemal plexus, external to its epithelium, which is
invested by a thin layer of connective tissue and muscle fibres. The stomach connects to
the intestine. The first part of the intestine, in the end bulb, is wider and invested in
longitudinal muscle fibres. The remainder of the intestine is narrower and curves forward
and continues dorsally along the length of the trunk. The intestine passes into the rectum,
which opens via the anus on the anal papilla. The gut has an outer covering of peritoneum
and is suspended by mesenteries.
Circulatory system
The circulatory system is mostly closed. There are two longitudinal trunk vessels. A
median, dorsal or afferent blood vessel sends blood to the anterior, while a lateral,
ventral, or efferent blood vessel sends blood to the posterior. The afferent vessel
originates from the haemal plexus and gives off no branches and is situated in the right
dorsolateral coelomic chamber. The afferent vessel is dorsal to the oesophagus and
crosses through the septum into the lophophoral coelom where it undergoes a T-fork to
become a horseshoe-shaped ring vessel in the lophophoral coelom. This ring vessel gives
off one vessel to each tentacle that travels along the inner side of the tentacular coelom.
The blood in these tentacular vessels surges back and forth. Each tentacular vessel gives
off a branch, at the tentacle base, that enters the efferent lophophoral ring vessel, which is
horseshoe-shaped and closely applied to the afferent ring. From the efferent ring a pair of
branches cross through the septum and travel posterior, dorsolateral to the oesophagus,
where they unite at the posterior end of the oesophagus to form the lateral efferent vessel.
The efferent longitudinal vessel travels in the left ventrolateral coelom and terminates in
the haemal plexus of the stomach wall. It also gives off numerous short lateral diverticula
or capillary caecae, which may be simple or branched. The capillary caecae are highly
contractile and contain both circular and longitudinal muscle. The afferent and efferent
longitudinal vessels communicate via the haemal plexus on the stomach wall, just
beneath the peritoneum. The plexus of the stomach wall consists of open sub-peritoneal
sinuses.
The main blood vessels have a flattened epithelial lining and a circular muscle layer and
are clothed with peritoneum. The blood is a colourless fluid containing red corpuscles.
These corpuscles are 10 m discs containing Hb.
The tentacle vessels and capillary caeca are autonomically contractile. The tentacle
vessels fill from the afferent lophophoral ring vessel and empty by contraction from their
tips basally, every 3-10 seconds. The caecae empty by contraction and shortening for 510 seconds, then refill. Contraction waves occur in the two longitudinal vessels,
especially the median vessel, posteroanteriorly, several times a minute. The lophophoral
ring vessel also contracts. Blood descends in the lateral vessel to the sinuses in the
stomach wall.
The blood corpuscles and coelomocytes originate from the endothelial lining of the blood
vessels and peritoneal lining, tentacle linings and vasoperitoneal tissue. The
vasoperitoneal tissue disposes of worn-out corpuscles and coelomocytes.
Tube
The cylindrical tubes usually occur in aggregations. These aggregations result from
asexual propagation in some species. The tube is formed from a sticky, transparent fluid
secretion that sets on contact with water. This material is chitinous and forms a wall of
one or more layers. Shell fragments, spicules and sand, etc. adhere to the tube, though a
clear section often remains at the top. The tubes may form tangles together. The tubes are
found on sandy bottoms, shells and pilings, etc. Two species have their tubes enclosed in
mollusc shell burrows or calcareous rock. The tubes of Phoronis australis occupy the
interstices of tubes of the cnidarian Cerianthus.
Reproduction
The gonads are loose indefinite masses intimately associated with the peritoneum of the
capillary caeca, and sometimes also the peritoneum of the lateral vessel. During spawning
the vessel peritoneum develops into nutritive vasoperitoneal tissue that nourishes the
gonads. The sex cells originate from peritoneal cells on the capillary caeca. As the sex
cells enlarge and mature the vasoperitoneal tissue reduces.
Most phoronids are hermaphroditic, but some are dioecious. When separate sexes occur
they are indistinguishable except for the colour of the ripe gonads, visible through the
body wall. In hermaphroditic forms, the testis is usually situated ventral to the lateral
vessel and the ovary dorsal to the lateral vessel, but this order is reversed in P. pallida.
In the Northern Hemisphere phoronids breed in spring or summer, over 2-3 months from
March to April or June to August. In the Southern Hemisphere they breed from
November to May. Ripe sex cells are shed into the coelom. Fertilisation may occur within
the coelom or outside, after the eggs have been shed into the sea via the nephridia.
Alternatively, the eggs are brooded on the lophophore concavity, being held in place by
the inner tentacles (and possibly also adhesive secretion from the lophophoral organs?).
Or else the eggs are attached to the inside of the tube or to an adjacent rock with the aid
of the lophophoral organ secretion.
The lophophoral organs also occur in some non-brooding species and in some other
species they are only found in ripe males. The functions of these organs are uncertain. A
ciliated furrow extends from the nephridiopore to the lophophoral organ and it is thought
that this may direct the eggs to the lophophoral organ. A flap overhanging the
lophophoral organ may also direct the eggs. It has also been suggested that the
lophophoral organs may act as seminal receptacles by trapping sperm.
Asexual reproduction occurs by transverse fission in P. ovalis. The fission plane is in the
non-muscular part of the trunk near the end-bulb. The posterior end is pinched-off and
develops a new tube continuous with the old tube and at right angles to it. This process
gives rise to tube aggregations. Budding also occurs in P. ovalis, in which the ventral
side of the non-muscular region produces epidermal projections that secrete a tube into
which the descending limb of the digestive tract sends a limb.
Autotomy and Regeneration
Crowns may be shed and regenerated. Only in P.ovalis does the shed crown regenerate
into a worm by adhering via the stump and secreting a new tube. Posterior portions may
regenerate if the cut is through the muscular region of the trunk. The decapitated trunk
regenerates a new anterior end. In Phoronopsis albomaculata part of the autotomized
anterior below the lophophore may constrict off and regenerate. P. psammophila
regenerates from pieces of the trunk. The pieces may first divide, each resultant fragment
regenerating.
Embryology
Cleavage is holoblastic and approximately equal and irregular or radial or spiral and
gives rise to a coeloblastula. The coeloblastula possesses an apical sensory plate with
apical cilia (flagella?). The vegetal pole invaginates to form a blastula. (The blastopore of
the blastula becomes the mouth and the archeneteron becomes the gut). The posterior of
the blastula elongates and the ciliated actinotroch larva escapes. The larva is planktonic,
oval and 1-5 mm long. It is equipped with a postoral ciliated ridge, 6-24 pairs of ciliated
tentacles (number depends on species) and a telotroch around the anus. The larva is
completely ciliated, but the telotroch is the main locomotory organ. After several weeks
as an actinotroch feeding on microorganisms, the larva develops an ectodermal
invagination or metasome pouch and definite tentacle buds. The larva sinks and
undergoes metamorphosis. Metamorphosis lasts 15-25 minutes. The metasome pouch is
everted. This contains the alimentary canal and becomes the adult trunk. The preoral lobe
shrinks, is cast off and ingested. The tentacles (ectodermal parts) are also cast off and
ingested. The definite tentacle buds increase in length and number. The anus is brought to
an anterior position. After metamorphosis the adult worm assumes a sedentary existence
and secretes a tube.
Ecology
All phoronids are marine but are absent from polar and subpolar waters. They occur in
the upper littoral zone. Their tubes occur on sandy bottoms, shells and pilings, etc. Two
species have their tubes enclosed in mollusc shell burrows or calcareous rocks. In
Phoronis australis the tubes occupy the interstices of the cnidarian Cerianthus.
Phoronids are sedentary, benthonic, upper littoral above 50 m) and tubiculous. The
secreted tube in which the worm lives is not fastened. The single tubes are imbedded
vertically in sand or mud. Forms with aggregated tubes occur on pilings and rocks, etc.
Tubes are secreted inside the burrow. Others permeate mollusc shells or calcareous rocks,
etc. Worms removed from their tube burrow into the bottom, rear-end first, secreting a
new tube that takes 2-3 weeks to thicken. P. hippocrepia colonies degenerate in winter,
leaving body fragments inside the tubes, which regenerate in favourable weather.
P. australis lives in tubes of the Cerianthus hydroid. This is the only commensal species
known. Phoronids have been found parasitised by gregarines in the intestinal epithelium,
forming cysts in the coelom. Trematodes have also been found in the coelom and ciliates
on the tentacles.
Chaetognaths (Arrowworms)
External features
Arrowworms are small, bilaterally symmetrical, enterocoelous, marine, mostly planktonic
and 1-10 cm in length. A few are over 100 mm long. There are about 70 known species.
These torpedo-shaped worms have 1-2 pairs of lateral horizontal fins supported by rays.
Chaetognaths have a horizontal tail fin, or caudal fin, also supported by rays. The
rounded head is armed with grasping spines and equipped on the posterior dorsal surface
with one pair of pigmented eyes. The fins act as flotation devices. A fold of the body wall
can be drawn over the dorsal and lateral surfaces of the head, like a hood. The mouth is
situated ventrally on the head and the anus is anterior to the tail fin.
The body is stiff and turgid and capable of slight bending. Some species are more flaccid.
Epipelagic species are colourless and transparent. Deeper water species may be pink,
orange or red. The body is regionated into head, trunk and tail. The head is rounded or
triangular. A neck demarcates the head from the trunk. The head possesses 2-4 short rows
of small anterior and posterior teeth or spines. Some species have only 1 pair of tooth
rows (the posterior rows), whilst other species have no teeth. The number of teeth
increases with age, until sexually mature and then the number decreases with senile loss.
The anterior teeth comprise a row of 3-10 short spines on each side of the anterior tip.
The posterior teeth are larger and number 30+ and are in a pair of rows curving from the
dorsal to the ventral surface of the head. There is one pair of vestibular organs behind
the posterior teeth on the ventral surface of the head. These consist of a transverse row of
papillae that may be borne on a ridge. One pair of vestibular pits may be present behind
the vestibular organs. These are glandular. Alternatively scattered gland cells may be
present in this region of the head. The pore of the retrocerebral organ opens on the
centre of the dorsal surface of the head. The ciliary loop may extend onto the posterior
part of the head up to this pore.
The vestibule is a ventral depression on the head that leads to the mouth. Grasping
spines (prehensile spines, seizing jaws) are borne on the sides of the head, 4-14 on each
side. These grasping spines are operated by powerful muscles and open and close to seize
prey. When the hood is drawn over the head, it protects the feeding apparatus when not
feeding and increases streamlining while swimming. In Spadella, the hood bears a
tentacle-like projection on each side.
The collarette is a stratified epidermal thickening on both sides of the neck. It may
extend onto the trunk and is present in most chaetognaths. In Pterosagitta draco, the
entire trunk and lateral fins are covered with collarette.
The fins are thin, transparent horizontal expansions for floating and equilibration. The
fins are not supplied with muscles and exhibit no swimming movements. In addition to
the postanal tail, 1-2 pairs of lateral fins occur on the trunk and tail, and may overlap the
trunk-tail boundary.
Arrowworms have pencils of bristles borne on small eminences that function as
tangoreceptors. Pterosagitta draco has one pair of especially large and long tufts of
bristles at about the middle of the trunk.
The female gonopores are a pair of small pores just anterior to the trunk-tail septum. The
seminal vesicles are visible as lateral bulges between the lateral and tail fins. The male
pores are possibly permanent, or possibly appear as temporary ruptures in the body wall.
Body Wall
The surface is covered by a very thin cuticle, which is thickened on the anterior of the
head and on the vestibule. The epidermis is mostly single-layered, but is stratified on the
collarette and on the dorsal surface of the head. Glandular areas lack cuticle and occur on
the hood lining and in the vestibular pits. There may also be 2 glandular tracts on the
head along the lines of hood attachment. These tracts converge to a glandular reservoir at
the tip of the head and occur in Eukrohnia and Heterokrohnia. There may also be a pair
of glandular canals in the neck, as in Bathyspadella.
Spadella has 4 parallel rows of mucous glands beneath the epidermis, which open via
pores above the level of the tail fin. In Spadella also, a large cement gland that fastens
eggs to objects encircles the female gonopore.
In the benthonic Spadella, adhesive papillae allow the animal to cling to objects. These
papillae are epidermal projections on the ventral surface of the tail. Each projection
consists of 6-10 tall epidermal cells whose swollen tips act as suckers. Alternatively, the
papillae are borne on fingerlike projections behind the posterior pair of lateral fins and
are operated by special muscles.
The head contains several skeletal pieces: plates, spines and teeth. Lateral and ventral
plates between the cuticle and epidermis support the teeth and spines and are sites of
muscle attachment. The grasping spines are made of chitin and their hollow interiors
contain pulp. This pulp is formed of epidermal cell extensions.
The basement membrane is very thin, but is thickened to form capsules for the eyes and
at the sides of the neck and sometimes also the adjacent trunk, where it forms skeletal
plates for muscle attachment. It also provides the fin ray extensions and is thickened near
the fins.
Retrocerebral Organ
The retrocerebral organ has an unknown function, but is possibly glandular. It consists of
a pair of sacs (or cell clusters in Spadella) imbedded in the posterior of the cerebral
ganglion, separated from the nervous tissue by a membrane. A common duct opens
behind the brain by the retrocerebral pore. Ganglion cell processes enter the sacs.
Ciliary loop (corona ciliata)
The ciliary loop is a dorsal strip of altered epidermis, which forms a short or long oval
with simple or sinuous contour. The long axis is usually parallel to the body axis, but may
be transverse as in Spadella. The anterior end is usually immediately behind the
retrocerebral pore. The ciliary-loop extends to the neck or far down on the trunk. The
histology of the ciliary loop is varied. It may consist of a ridge or groove of a variable
number of cells wide. It may also be adjacent to gland cells. The ciliary loop possibly
functions as a sense organ or as an excretory organ (a possible nephridium).
Musculature
The body wall musculature lies beneath the basement membrane. This muscle layer is
simple in the trunk and tail, where it comprises longitudinal fibres only, and also lacks an
inner peritoneal membrane. The body wall muscles are all striated.
In Spadella, there is an ectodermal mesenchyme layer between the basement membrane
and the muscle layer. In Spadella also, the muscle layer is divided into 4 thick
longitudinal bands, 2 dorsolateral and 2 ventrolateral, and there may also be 2 small, thin
lateral bands. There may also be a thin transverse layer inside these ventrolaterals, which
may occur throughout the trunk and / or tail or the anterior trunk, and is of taxonomic
importance.
The muscle fibres are similar to those of nematodes, consisting of a fibrillar part near the
basement membrane and a protoplasmic part facing the body cavity. The fibrillar part
consists of longitudinal fibrous plates or lamellae at oblique angles to each other and to
the body wall. This gives the fibres a feathery appearance in cross-section.
The head musculature is very complex. This has been best studied in Spadella and
Sagitta. Skeletal plates form a hardened head capsule for the attachment of these muscles.
Muscles of the Hood. An unpaired sphincter muscle pulls the hood forward (hood
protractor or protractor preputii). One pair of hood retractors (retractor preputii) occurs
in Sagitta. These retractors originate on the connective-tissue layer beneath the brain and
insert on the skeletal plates in the neck. In Spadella, superficial obliques are the main
hood retractors.
Muscles of the Teeth. Two pairs of small expander muscles (expansus superior and
inferior) run transversely in the anterior end of the head. These originate on a median
connective-tissue lamella and function as teeth erectors. One pair of small short obliques
(obliquus capitis brevis) pull down the anterior teeth. These originate on the same median
lamella as the teeth erectors.
Muscles of the Grasping Spines. Spreading open of the spines is not due to particular
muscles, but sue to alteration of head shape. In particular the lateral complex, bicornuate
muscles shorten and broaden the head. Closure of the spines is very fast. One pair of
adductors (adductor uncinorum) form conspicuous masses on the sides of the head. They
originate on the posterior parts of the lateral plates and insert on the spine bases.
General Dorsal Head Musculature. The central region of the head is occupied by
longitudinal oblique muscles (obliquus capitis longus) that insert on the anterior ends of
the lateral plates and converge toward the median line and then run to their attachments at
the posterior end of the head. These muscles shorten the head in the anteroposterior
direction. Curved hood retractors lie above the middle of the longitudinal obliques.
Transverse fibres of short oblique muscles at the tip of the head are followed by the teeth
expanders.
The large spine adductors occupy the sides of the head. A median triangle at the rear of
the head is formed from 2 superficial oblique muscles (obliquus superficialis) that help to
retract the hood. On either side of the triangle, dorsal transverse muscles (transversus
dorsalis) originate on the median lamella and insert on the posterior ends of the lateral
plates. These transverse muscles pull the ends of the lateral plates towards the median
line and hence assist spreading of the grasping spines.
The sides of the posterior end of the head contain the dorsal ends of the external neck
muscles (rectus colli externus) that insert on the skeletal plates at the sides of the neck.
These muscles run dorsoventrally and attach to the lateral plates. They produce nodding
motion of the head.
Muscles of the Mouth and Vestibule. These are seen in a ventral view of the head. The
teeth expanders are visible at the anterior end of the head. Behind these are one pair of
mouth constrictors (constrictor oris primus) that curve in front of the mouth and another
pair of mouth constrictors (constrictor oris alter) that run alongside the mouth opening.
The latter pair is absent in Spadella. Three pairs of muscles, visible in transverse sections,
dilate the mouth and vestibule: one pair of dilator vestibuli externus, one pair of dilator
vestibuli internus and one pair of dilator oris. This latter pair are absent in Spadella. The
external dilators are visible in external ventral view, lateral to the mouth constrictors.
Other Ventral Muscles. These run across the head, behind the vestibule. The bicornuate
muscles (bicornis) is a conspicuous sausage-shaped muscle that alters the shape of the
head. The massive lateral complex muscles (complexus lateralis) consist of several
bundles at the sides of the posterior part of the head. This muscle mass has dorsal
attachments to the spine adductors and lateral attachments to the ventral plates. Medially,
the bicornuate muscle is embedded in this muscle mass. The complexus lateralis changes
the shape of the head and thereby assists grasping spine and teeth operation. The
posterior ends of the hood protractors are visible on either side, behind the complexus
lateralis.
Neck Muscles. Two pairs of neck muscles produce nodding of the head: the rectus colli
externus and the rectus colli internus (located to the inner side of the rectus colli
externus, but concealed under the dorsal transverse). The internal neck muscles extend
from the skeletal plates of the neck to the lateral plates. On the ventral side of the neck is
the unpaired ventral transverse muscle (transversus ventralis) which attaches to the body
wall on either side.
Coelom
The coelom is not lined by definite peritoneum. A partition just behind the head and
another just behind the anus subdivide it into three compartments. The trunk and tail
coeloms are subdivided into paired lateral compartments by a median dorsoventral
double-walled mesentery. The trunk mesentery encloses the intestine between its two
walls. The mesentery is perforated to allow the lateral compartments to communicate.
The mesentery is not peritoneal, but is a continuation of the body wall basement
membrane, and is covered in a thin muscle layer.
In Sagitta, but not Spadella, the tail coelom comprises four compartments, divided by
median and lateral mesenteries. The Head coelom is reduced due to the head musculature
taking up the head space. The head coelom extends either side of the pharynx and into the
hood. The trunk coeloms also extend partway into the head.
The coelomic fluid is colourless and circulates in the trunk and tail, moving forward
along the inner surface of the body wall and backward along the median mesentery, by
cilia action.
Nervous System
The large brain or cerebral ganglion gives rise to a visible bulge on the dorsal surface of
the head. The brain gives out one pair of frontal commissures that proceed anterior,
outward and downward to the vestibular ganglia; one pair of circumenteric
connectives to the ventral or subenteric ganglion; one pair of optic nerves to the eyes
and one pair of coronal nerves to the ciliary loop.
The vestibular ganglia, either side of the mouth, are connected by the subpharyngeal
commissure. The vestibular ganglia give out one pair of frontal nerves to the posterior
teeth; one pair of large dorsal nerves that innervate some of the head muscles; one pair
of small mandibular nerves to the grasping spines and one pair of small vestibular
nerves to the vestibule. The frontal commissures give off one pair of nerves to the
pharyngeal ganglia, which gives off the lateral pharyngeal nerves.
The subpharyngeal commissure gives off one pair of very small labial nerves (that
possess minute ganglia) and an unpaired ventral pharyngeal nerve to the pharynx. The
arrangement of cerebral ganglion, frontal commissures, vestibular ganglia and
subpharyngeal commissure forms a central circumenteric nerve ring.
The subenteric ganglion gives off 12 pairs of small lateral nerves that form a plexus in the
lateral and dorsal walls of the trunk. It also gives off 2 large posterior nerves that form a
plexus throughout the trunk and tail. Thus, there is an epidermal plexus throughout the
trunk and tail that innervates muscles and tangoreceptors.
Sense Organs
Tactile bristles or tufts are arranged in longitudinal rows, inside the ciliary loop region
and may also occur on the fins. There are about 250 such bristles in Sagitta elegans
arranged in 6-7 rows. The ciliary pits in Spadella contain about 100 sensory bristles. In
Pterosagitta there is one pair of especially large lateral tufts on the middle of the trunk.
The ciliary loop detects water disturbances, but is possibly also tactile and
chemoreceptive. When Spadella pair for sperm transfer they often bring their ciliary
loops together.
There is one pair of eyes on the rear, dorsal part of the head, just beneath the epidermis.
The optic capsule is formed of basement membrane and is penetrated by the optic nerve.
Each eye is composed of pigment-cup ocelli. Sagitta has 5 combined pigment-cup ocelli
per eye, one large lateral ocellus and 4 small median ocelli in two tiers of two. The ocelli
pigment-cups are partially fused to give a rayed appearance (Usually three-rayed) of the
common pigment-cup. The cavity of each ocellus is filled with retinal cells that face the
pigment-cup (inverse type ocellus). The retinal cells terminate in a striated retinal club /
rod.
Locomotion
Spadella is benthonic and attaches to rocks and algae by means of adhesive papilla on its
tail. If disturbed, then it will swim and hide itself in bottom mud. Other species are
planktonic and float motionless, supported by their fins. When they sink, they swim in
short, swift forward darts, each covering about 5 cm, to gain lift and then glide.
Swimming is achieved by undulatory body waves and tail flirts.
Nutrition
Chaetognaths are carnivorous predators. They eat macroscopic zooplankton, including
copepods (the most common macroscopic zooplankton). Baby fish as large as themselves
may be swallowed whole, other chaetognaths and euphausiaceans are also eaten. The
arrowworm darts forward to seize its prey in its spines and the teeth also help to hold the
prey.
A sticky secretion entangles the prey and helps its passage to the posterior intestine,
where digestion takes place. The chaetognath gulps seawater after swallowing its prey,
which washes pharyngeal and anterior intestine secretions into the posterior intestine.
Digestion occurs at pH 6.4 and is extracellular. No proteolytic enzymes are apparent.
Indigestibles are defecated whole in about 3-4 hours or less.
The vestibule leads to the mouth, which is oval or T-shaped. The mouth can be projected
forward by muscular action when feeding (the vestibule disappears when the mouth is
projected). The vestibule leads to a muscular pharynx (oesophagus). The pharynx
consists of a luminal epithelium resting on a basement membrane, underlain by an inner
longitudinal muscle layer and then an outer circular muscle layer. The posterior of the
pharynx forms the pharyngeal bulb. The gut crosses the head-trunk septum as the
pharynx leads into the straight intestine. The intestine may possess a pair of diverticula in
its anterior part and may form a rectum in its posterior part. The diverticula project
forward to the head-trunk septum. The intestine contains absorptive and glandular cells in
its luminal epithelium and has a circular muscle layer outside the basement membrane.
The intestine opens via the anus, which is not equipped with special musculature.
Reproduction
Chaetognaths are protandric hermaphrodites and possess one pair of ovaries in the
posterior trunk, just anterior to the trunk-tail septum, and one pair of testes in the tail,
behind the septum. The ovaries mature after the tail coeloms fill with sperm.
Spermatogonia bud-off from the testes and undergo spermatogenesis in the tail coelom. A
sperm duct leads from each testis posteriorly to the seminal vesicles, while anteriorly the
sperm ducts open into the tail coelom along the midlateral line by way of a ciliated
funnel-shaped opening, the genital funnel (coelomostome). The filiform sperm circulate
in the tail coeloms and are swept into the sperm ducts by the genital funnel cilia. The
sperm accumulate in the seminal vesicles, which fill about every 12 hours. The
spermatophore forms inside the seminal receptacle and is emitted by rupture of the body
wall. The spermatophore has an adhesive disc for attachment to objects, or, in Sagitta, to
the fins of the same animal. Clusters of sperm are released from the spermatophore and
gain entry to the seminal receptacles where fertilisation occurs. Self-fertilisation is
apparently the norm in Sagitta.
Each oviduct consists of a tube within a tube. The outer tube is formed of cuboidal –
cylindrical epithelium, while the inner tube has a syncytial wall. The inner tube expands
posteriorly into the seminal receptacle, which stores sperm received at copulation. A
short vagina connects the seminal receptacle to the external female gonopore. The outer
tube possibly forms a temporary external pore for egg-laying. The oviduct forms a stalk
that attaches to each ripe egg. These stalks are hollow to allow the sperm access to the
egg. Fertilised eggs, each about 0.2 mm in width, are discharged singly into the sea. The
eggs float near the surface.
Cross-fertilisation occurs in Spadella cephaloptera, in which a spermatophore is placed
near the female gonopore of another animal. The pair remain together, antiparallel to
each other. They frequently bringing their ciliary loops into contact while the sperm enter
the seminal receptacle. Reciprocal exchange of sperm possibly takes place. Spadella lays
clusters of about 12-16 eggs every 8-10 days. Cement glands encircle the vagina and the
egg clusters are attached to seaweeds, etc.
In Krohnitta, the eggs are stuck together in gelatinous packets, attached to the worm’s
back near the trunk-tail septum. Pterosagitta draca produces floating gelatinous masses
containing 200-300 embryos. The numbers and timings of spawnings depend upon
species and geographical location.
Development
In both Sagitta and Spadella cleavage is equal, holoblastic, radial and indeterminate. No
yolk is evident in the eggs of Sagitta, whereas those of Spadella contain tiny yolk
granules. A spherical coeloblastula is produced, with a small central blastocoel and about
50 blastomeres. Embolic gastrulation produces a 2-walled gastrula. The embryo elongates
and after about 2 days it hatches into an adult-like transparent larva, about 1mm long. In
Sagitta the larvae float, whilst in Spadella they adhere to the substrate by means of
adhesive epidermal cells on the ventral trunk surface and a pair of adhesive tentacles on
the sides of the head. Development is direct and lacks a ciliated larval stage.
Regeneration
The fins and tail can be regenerated. Wounds close and clot. The head can also be
regenerated.
Ecology
Arrowworms are very common at all latitudes, through a range of depths and in all seas
at up to 200m depth. There are species-specific depth ranges that also depend on latitude,
and are determined by temperature. Most chaetognaths are warm-water and
epiplanktonic. However, cold and cool-water chaetognaths also occur.
Chaetognaths are eaten by medusae, ctenophores, young and small fish. As parasites,
amoebae have been observed in the tail coeloms, eating developing sperm. Thus, these
parasites flourish when the male worm is mature and are presumably transferred during
copulation. Flagellates have also been found in the intestine, as have trematodes and
nematodes. Gregarines have been found in the gut. Trematodes have also been observed
in the coelom and gonads. Larval cestodes and nematodes have been found in the coelom.
Copepods have been found attached near the female gonopores. Vesicles, of unknown
nature, have been seen rooted in the chaetognath body. Up to 4 concentric vesicles have
been seen, and as many as 10 vesicles on the body surface. These vesicles have no nuclei
or definite histological construction.
Sipunculida (Sipunculoids, Sipunculids; Sipuncula, Sipunculans):
Peanut Worms
There are about 250 known species of sipunculoid, in 13 genera. They range in depth
from intertidal down to 5000 m, and are found in all seas at all latitudes.
External Features
The sipunculoids are vermiform coelomates, and all are marine. The body is regionated
into a slender anterior introvert and a posterior plump trunk. Tentacular outgrowths,
lobulations, bulges or conical, branched, digitiform or filiform processes may encircle the
terminal mouth. There may be less than 10 of these outgrowths or many of them. The
conspicuous middorsal anus is situated on the anterior end of the trunk or on the introvert
(as in Onchnesoma). These worms are cylindroid in shape. The trunk usually terminates
bluntly or in a point, but there is a slender tail in abyssal species, as in Golfingia spp.
The introvert can be completely invaginated into the trunk by strong retractor muscles,
and is habitually run in and out rapidly. The mouth is on the terminus of the introvert on
the oral disc. The introvert may be shorter than the trunk, as in Sipunculus, and as little as
a quarter of the length, about the same length as the trunk, or longer than the trunk as in
Phascolion. In most species it ranges from a quarter to about the same length as the trunk.
The anterior end of the introvert may be globular.
Tentacles may be present. In Sipunculus, a flat tentacular fold, with a scalloped or
foliaceous margin, encircles the mouth. There may be 2-3 cycles of tentacles, and the
tentacles may be fused basally in pairs or clusters. In Dendrostomum, the tentacles are
dendritic with 4-8 main stems. The tentacles may form a crescent, rather than a complete
circle. Each tentacle bears a ciliated groove on the surface facing the mouth (usually the
inner surface). The number of tentacles increases with age.
The nuchal organ is located on the middorsal oral disc in Golfingia and some other
species, and is a bi- or quadrilobed ciliated cushion. Ventral to the nuchal organ (or in an
equivalent position if the nuchal organ is absent) there may be an unpaired or lateral
paired openings of the cephalic tube(s) that lead towards the brain and open into a large
pit around the brain.
There is usually a short smooth section behind the oral disc, called the collar. In
Phascolosoma, the anterior collar (cephalic collar) is a rounded ridge continuous dorsally
with the ends of the tentacular crescent, whilst the posterior or cervical collar is a thin
fold.
Tubercles, papillae or scales may cover the body surface. The introvert may be covered
by spines or thorns etc. and may have circlets of spines, thorns, hooks, etc. Phascolion
lives in empty gastropod and scaphopod shells and its trunk possesses hard thickenings
that function as holdfasts and grip the shell.
There may be a shield on the anterior end of the trunk, formed of calcareous plates,
hornlike plates or a calcareous cone situated dorsally and displacing the introvert
ventrally, or surrounding the anterior end of the trunk. Aspidosiphon also has a posterior
shield on the posterior end of the trunk, composed of a circular cap of radiating pieces.
Shields occur in those sipunculoids that live amongst dead coral rock.
Two nephridiopores open on the ventral trunk at about the level of the anus, but these are
usually invisible to the naked eye.
Sipunculoids range in size from less than 2 mm to about 60 cm in Sipunculus nudus.
Colouration is dull and uniform whitish, yellowish, bluff, grey, greyish brown, brown and
the surface elevations are often dark brown.
Body Wall
From the outermost to the innermost, the body wall layers are: cuticle, epidermis, dermis,
circular muscle layer, longitudinal muscle layer and peritoneum. There may also be a
diagonal muscle layer between the circular and longitudinal muscle layers. The cuticle
contains brown pigment granules and is often thick and lamellated on the trunk, but is
thin and homogenous on the tentacles and on the introvert. Scales and shield plates are
condensations in the cuticle.
The epidermis is cuboidal to columnar and ciliated on the upper tentacle surface, or
sometimes on both tentacle surfaces, and on the oral disc. It contains one-celled to manycelled glands, often on the papillae. Spines and thorns are generally hollow, straight or
curved and with a single or bifid tip. They may possess a comb of spinelets on their base.
Sensory buds occur on the epidermis, usually on the papillae, and maybe associated with
multicellular glands. Scales may cover the papillae.
The dermis comprises connective tissue and permeates the muscle layers and ranges from
very thin to thick. The dermis contains yellow-brown pigment cells. There may be
coelomic spaces or canals, especially in large, thick-walled species, in the dermis that
connect to the coelom by pores. These canals are lined by peritoneum. In Sipunculus
there are longitudinal canals underneath the longitudinal ridges on the trunk. The
peritoneum is composed of flattened cells with tufts of cilia.
Muscular System
The muscle system can be divide into three subsystems: the body-wall musculature, the
introvert retractors and small muscles that anchor the digestive tract. The introvert
retractors may be fused and connect the oesophagus to the longitudinal muscle stratum of
the trunk wall. There may be one pair (e.g. Xenosiphon) or two pairs (ventral and dorsal)
of introvert retractors, or a single muscle or one ventral and one dorsal retractor.
Contraction of the circular body-wall muscle increases coelomic pressure, everting the
introvert.
The longitudinal and circular muscle strata of the body wall may form separate bundles in
the trunk, of cross-striated muscle, that may underlay the longitudinal grooves visible on
the external surface of the body wall and may give the surface a pattern of squares or
rectangles.
There may be a spindle muscle attaching the intestine to the body wall near the anus and
throwing the intestine into a coil. Contraction of this muscle shortens the intestinal coil.
This muscle has a peritoneum covering.
The fixing muscles are short strands that anchor the digestive tract to the body wall. All
the muscles of sipunculoids are smooth muscle, comprised of fusiform cells, each about
1mm long and with a central nucleus.
Locomotion
Sipunculoids exhibit slow crawling outside their burrow. They attach their tentacular
crown to the substrate and pull the trunk forward. The worm will burrow in again when it
encounters a suitable substrate. Burrowing is affected by pushing and expanding
movements of the introvert. They burrow vertically in a few minutes and then proceeds in
any direction to produce a curved burrow with the oral disc protruded above the sand.
Burrowing proceeds in the following stages: 1. The introvert is run out and the tip
explores the substrate until a soft spot is found. 2. The introvert end is expanded and
pushed into the sand. 3. The trunk longitudinal muscles contract, bringing the body
forward. 4. The introvert is invaginated and everted repeatedly, penetrating deeper into
the substrate each time.
When undisturbed, the anterior end is thrust out of the burrow or refuge and the tentacles
are expanded for feeding and respiration and also to explore the surface. Upon
disturbance, the introvert instantly retracts and the trunk also contracts.
Sipunculus can swim by thrashing its body ends together, first to one side and then to the
other. The worm swims on its side by alternate contraction of the longitudinal muscles,
flexing alternately dorsally and ventrally.
If disturbed outside the burrow the worms assume a defensive attitude, motionless and
highly turgid with the longitudinal muscles contracted maximally. Sipunculoids exhibit
righting movements when placed dorsal-side down, for example. They flex their body
concave to the surface and fall to one side. This action is repeated until the animal is
right-side up.
Nutrition
Golfingia procera has been observed predating Aphrodite polychaete worms. When the
introvert contacts the Aphrodite, it is inserted into its body and sucks-out the contents.
Other species are possibly ciliary-mucous feeders (?). during burrowing substrate,
including sand and mud, are ingested and perhaps nutrient is obtained from the diatoms
and protozoa contained therein (?).
The digestive tract is recurved. The mouth opens into the oesophagus. A pharynx may be
present as a muscular beginning of the oesophagus. The oesophagus leads into the
intestine, which is a long tube that descends to the posterior trunk and then ascends. Both
intestinal limbs are spirally wound into a single coil. There may be a short section of
intestine at the end of the oesophagus (beginning of descending coil) with longitudinal
ridges projecting into the lumen and which may constitute a ‘stomach’. A straight rectum
opens via the anus.
There may be a blind secretory diverticulum, off the rectum, that possibly degenerates
with age. There may be one pair of bushy rectal glands or fingerlike rectal projections.
Wing muscles may be present, attaching the rectum to the adjacent body wall and serving
to dilate the rectum.
Enzymatic cells occur mostly in the descending intestine (and also the oesophagus) and
secretory ciliated pits may be present. A luminal ciliated groove runs from the mouth to
the rectal diverticulum and directs a current towards the anus. In Sipunculus the
connective tissue layer of the intestinal wall contains a sinus that is not connected to any
other cavities.
Tentacular System
The tentacular system is a system of cavities continuous with 1-2 tubular sacs adherent to
the outer surface of the oesophagus. In Sipunculus the tentacular fold houses a ring sinus
that communicates with the two long, blind tubular sacs. In Golfingia, each tentacle
contains three connected channels joined by a ring space around the digestive tract
connected to one tubular sac. In Dendrostomum one dorsal sac forms a network of
vessels over the oesophagus.
The tentacle system contains fluid and coelomocytes, but is not connected directly to the
main coelom. The sacs are contractile, compensation sacs for the tentacles and receive
the tentacular fluid when the tentacles contract and sends fluid back into the tentacles
when the tentacles expand. The fluid also circulates independent of tentacle movements
due to cilia action. Sipunculoids lack a circulatory system other than the coelom.
Coelom
The coelom extends from the beginning of the oesophagus to the posterior end of the
trunk. Mesentery remnants attach the anterior oesophagus to the retractor muscles. The
compensation sac(s) may be attached to the oesophagus by a mesentery. The spindle
muscle is attached to the midventral line of the ascending intestine by a mesentery.
Mesenterial strands containing muscle fibres also attach to the descending coil of the
intestine. There may be a series of short transverse peritoneal folds inside the trunk.
The peritoneum is made-up of flattened cells and granular cells bearing cilia tufts. Some
peritoneal cells may be altered into chloragogue cells and fixed urns. Chloragogue cells
occur especially on the oesophagus, intestinal coil and compensation sacs (both surfaces)
and are bulging clavate cells containing yellow granules in a reticular cytoplasm. Fixed
urns are found in the compensation sacs (both surfaces), on the mesenteries and on the
ascending intestinal coil. These bear tufts of cilia and may be borne on stalks. The stalks
may comprise several cells per urn, as in Sipunculus nudus.
The coelomic fluid contains free urns, balls of waste material, multinucleate giant
bodies, which may be plate-shaped and amoebocytes and red corpuscles. The red
corpuscles are biconvex nucleated discs 6-32 mm in diameter. Some multinucleate red
cells, up to 540 mm in diameter, also occur. The coelomic fluid and the contents of the
red corpuscles are pinkish or purplish and contain haemerythrin. There are about 105 red
corpuscles and a few thousand amoebocytes per mm3 of coelomic fluid.
The coelomic fluid may circulate due to the action of the ciliated peritoneum and the
fixed urns. In Golfingia vulgaris the current is forward along the dorsal body wall, back
along the median ventral wall and transverse from midventral to lateral. Contractions of
the gut and body wall muscles also circulate coelomic fluid.
Free urns have been seen in some genera, e.g. Sipunculus. These are thought to originate
from fixed urns and swim about in the coelomic fluid. In Sipunculus they are transparent,
globular, fluid-filled vesicles with a disciform ciliated cell at one end. Cytoplasmic
strands cross the vesicle. There are one or more stellate cells on the vesicle surface,
which are peritoneal cells. Free urns swim with the ciliated cell at the rear. The cilia
attract and accumulate particles (disintegrating coelomocytes, bacteria and injected dyes
and mammalian red cells). A viscous secretion traps the particles. The urns are positively
charged and attract negatively charged particles. The sipunculoid red cells are also
positively charged. The accumulated masses break-off and collect in the posterior coelom
as ‘brown bodies’, which include sporozoan spores and cysts and are voided by the
nephridia.
Excretion
The urns and the amoebocytes act as athrocytes (accessory excretory agents). The red
cells and chloragogue cells are also involved in excretion. Nitrogenous waste in the form
of ammonium salt, allontoic acid / allontoin accumulates in the red cells. There are 1-2
metanephridia with muscled walls that contract rhythmically under the control of a
secreted substance produced by the body wall and nerve cord. Each nephridium opens to
the coelom by a nephrostome.
Sipunculoids are ammonotelic as 83-90% of the nitrogenous waste is excreted in the form
of ammonia. These worms have been measured to excrete 1.18 to 1.32 mg of total
nitrogen per 100 g wet weight per 24 hours.
Sipunculoids are osmoconformers and have no osmotic control.
Respiration
There is no specialised respiratory mechanism, but red corpuscles and coelomic fluid
contain haemerythrin as does the nerve cord and the wall of the gut (Mr 66 000 – 119
000). This pigment is yellowish when deoxygenated and reddish when oxygenated. The
circulation of the coelomic fluid assists oxygen transport.
Circulatory System
There is no specialised circulatory system other than the coelom and tentacular system.
Nervous System
The nervous system is of the annelidan type. The brain is a bilobed mass situated dorsal
to the beginning of the digestive tract, just above the dorsal compensation sac. A pair of
circumenteric connectives connects to the ventral nerve cord, which is neither paired nor
segmented. The ventral nerve cord has a peritoneal covering and is midventral, extending
throughout the entire body. The cord gives out numerous lateral nerves that form
approximate rings innervating the muscles.
The brain gives out nerves to the nuchal organ and may also give out nerves to the
adjacent muscles. The circumenteric connectives give out nerves to the tentacles, to the
introvert retractor muscles, and a pair of pharyngeal nerves that give rise to a plexus in
the gut wall.
Sensory Systems
Neurosensory cells with a terminal bristle occur in the epidermis on the tentacular fold
and on the tentacles. Ciliated pits may be present on the tentacle fold and introvert.
These pits can be protruded into a papilla. The epidermis possesses fusiform buds of
elongated epidermal cells. The nuchal organ is supplied by highly branched nerves
(usually one pair of nuchal nerves) and possibly has a chemoreceptive function.
The one or two cephalic tubes that may be present are ciliated and open into a cavity
cupped over a forward projection of the brain called the cerebral organ. When there are
no cephalic tubes, the cerebral organ lies at the surface. The cerebral organ is separated
from the brain tissue by connective tissue and contains cells that have the appearance of
neurosecretory cells. The tentacular crown is also sensitive to bright light
Many sipunculoids have one pair of brown or black pigment-cup ocelli in the interior of
the brain, at the inner end of the ocular tubes. The ocular tubes usually contain lens-like
material.
There may also be a bundle of outgrowths of the anterior dorsal part of the brain above
the cephalic tube and cerebral organ. These outgrowths project into the coelom between
the brain and the dorsal wall of the introvert. These outgrowths may form a bunch of
papillae or long filaments and leaflike projections. They are separated from the brain by a
layer of connective tissue and are lined by peritoneum on their outer surface. They
contain nerve fibre tracts.
In Sipunculus nudus there is a pair of leaflike outgrowths into the base of the cephalic
tube. This structure has not been found in any other sipunculoid.
Reproduction
The gonads form an inconspicuous fringe on the coelomic wall at the origins of some or
all of the retractor muscles. The sex cells are shed into the coelom where they mature and
are then emitted through the nephridia. All species studied are dioecious and the sexes are
indistinguishable externally. Males often make-up less than 1% of the adult population.
Spawning occurs at nighttime in summer. The nephridia expand and take-up the mature
sex cells and then forcible eject them in cloudlike jets. The males may wave their raised
anterior ends about while ejecting sperm. Contact with the sperm stimulates the females
to spawn.
Development
Spiral cleavage results in a gastrula that develops into a trochophore. After several days
metamorphosis takes place. In Golfingia the trochophore elongates into a vermiform
larva that sinks to the bottom. After crawling on the bottom in caterpillar fashion, or after
swimming near the bottom by means of the metatroch for about one week, the tentacles
begin development from a pair of lateral projections. After about one month the larva is
adult-like and less than 1 mm long. There is no evidence of metamerism at any stage.
In Sipunculus nudus the larva is a very elongated trochophore about 1 mm long. In
Phascolosoma the trochophore develops into a planktonic pelagosphaera larva. This is a
transparent, spherical object 6mm in diameter. This larva sinks to the bottom and
develops a ciliated creeping foot.
Regeneration
Sipunculoids do not reproduce asexually, but the tentacles, posterior trunk end, introvert
and, in most species, the oral end regenerate.
Ecology
All sipunculans are marine and are found from the intertidal zone to deep waters (down
to 5000 m), at all latitudes. They are sedentary and form burrows in sand, mud, gravel,
shelly substrates, clefts in rock, porous lava, kelp holdfast tangles, eelgrass beds and
other algae, under rocks, among corals, in sponges and in empty shells and empty tubes.
The shield-containing species prefer cavities in decaying coral rocks and the shield
probably serves to close the cavity entrance when the worm is retracted. These coral-rock
dwellers have shorter more muscular introverts with stronger introvert hooks.
Phascolosoma lurco forms earthen burrows on the edge of mangroves above the reach of
high tide. Fish and people eat Sipunculoids.
Sipunculans house numerous types of parasite, including ciliates in the oesophagus,
gregarines in the intestine, coelom and in the red corpuscles. Vermiform parasites include
flatworms in the gonads, brain and intestine and nematodes fastened to the body wall of
the coelom. Parasitic copepods of sipunculans include the peculiar Siphonobius
gephyreicola on the retractor muscle, and its larvae in the coelom. Other copepods occur
in the coelom, attached by suckers, for example stalked suckers on their antennae.
Examples of commensal relationships with sipunculans include the relationship of
Aspidosiphon to solitary corals. The coral attaches to a snail shell occupied by
Aspidosiphon, or else Aspidosiphon selects a shell with a coral larva on it. The coral
overgrows the shell and possibly serves to expand the sipunculoid living chamber. The
sipunculoid moves the coral around and is given a safe refuge in return. Neither species
occurs by itself.
Golfingia hespera frequents burrows of other animals, including burrowing anemones,
brachiopod burrows, Cerianthus tubes and the tubes of annelids. Entoprocts are common
sipunculan commensals and cluster on the posterior end of the sipunculoid.
Syllis cornuta is a syllid polychaete occupies the excretory channel of Aspidosiphon and
Phascolion. Phascolion inhabits snail shells, the opening of which it plugs with sand
cemented together. Through this cement plug the sipunculoid maintains its main opening
and an excretory channel. Syllis benefits from the ventilatory currents and possibly
consumes food particles carried in these currents. Tiny bivalves also live in association
with Aspidosiphon and Phascolion.
Kinorhyncha (Echinodera)
External Features
Kinorhynchs are microscopic marine Aschelminthes less than 1 mm in length. They are
devoid of cilia and have a regularly segmented spiny or bristly body of 13-14 joints. The
round retractile head is covered with 5-7 circlets of spines, or scalids, that curve
backwards. There is a single neck segment, which is about the same width as the trunk.
The trunk is usually equipped with terminal spines and may have lateral and middorsal
spines.
The ventral surface of the trunk is flat, whilst the dorsal surface is arched and the sides
are more or less parallel. The posterior end is truncate or tapered. The cuticle is thick and
kinorhynchs are yellowish or brownish in colour.
The body joints, or zonites, represent superficial segmentation. The head is more or less
spherical and constitutes the first zonite. The mouth is terminal and central and borne on
a protrusible mouth cone. The mouth cone is armed with a circlet of spines or thorns –
the oral styles, and may have additional girdles of smaller spines or teeth. These spines or
teeth may also occur i9n the furrow between the cone and the head.
The scalids are hollow cuticular projections filled with epidermis. Each comprises a basal
socket and a scythe-like blade. The socket may possess one or more additional small
spines, separated from the blade by a circular fold. This circular fold imparts flexibility to
the scalid. There are 10-20 scalids per circlet. The scalids of successive circlets decrease
in size.
Some or all of the scalids in the most posterior circlets may be trichoscalids. Each
trichoscalid consists of a bristle borne on a cuticular plate or scale. In Echinoderes the
scales occur in sets of three, one before and one behind the middle scale bearing the
trichoscalid.
The head can be totally withdrawn into the 2nd or 3rd zonite. The 2nd zonite is the neck
and is covered by 8-16 large plates or placids. The placids may be thin and withdrawn
into the 3rd zonite, or thick and closed over the end, when the head is withdrawn into the
2nd zonite.
In addition to the head and neck there are 11 trunk zonites (12 in Campyloderes). Each
trunk zonite is flattened ventrally and arched dorsally. Each zonite also has and has a
midventral groove and is covered by one large dorsal or tergal plate and two ventral or
sternal plates.
The third zonite may be equipped with a closing apparatus. For example, in
conchorhagous forms, like Semnoderes, the third zonite possesses one pair of curved
lateral plates that close like the valves of a clamshell. In the homalorhagous group, e.g.
Trachydemus and Pychophyes, the third zonite possesses a ring made up of a large tergal
plate and three sternal plates.
Each zonite overlaps with the succeeding zonite by a thin cuticle fold that lends
flexibility. Similar folds also occur between plates. The trunk plates are usually thickened
anteriorly and striated posteriorly. The tergal and sternal plates may articulate laterally by
a peg-and-socket joint on each side.
Each trunk zonite usually possesses middorsal and lateral spines that are jointed onto the
cuticle, but not independently movable, or else are unjointed and toothlike. These form
more or less complete middorsal and marginal rows of spines or teeth. The spines or teeth
are hollow and filled with epidermis. Additional dense or scattered fine bristles occur
frequently.
The ventral surface of the 3rd or 4th zonite is equipped with a pair of adhesive tubes (cf.
Gastrotricha) each with a large gland cell at its base. The last tergal plate and sometimes
the lateral end spine, and also the last marginal spine when present, can be moved by
muscles. Similar, the median end spine of Centroderes and Campyloderes is also
movable.
Body Wall
The spines are hollow extensions of the cuticle, filled with epidermis. The epidermis is
more or less syncytial and projects into the pseudocoel as cushion-like thickenings,
especially at the bases of the scalids and as middorsal and lateral longitudinal thickenings
or chords. These chords have an enlargement in each zonite. Laterodorsal chords also
occur.
Musculature
The musculature is segmentally arranged. There is no definite subepidermal sheath.
Zonites 1 and 2 contain ring muscles, but these become dorsoventral bands in the trunk
zonites, running on each side of each zonite from the tergal to sternal plates. Contraction
of these muscles compresses the body and protrudes the head. Lateral trunk regions have
a similar series of diagonal muscle bands extending from the anterior edge of one zonite
diagonally forward to the preceding zonite.
There are segmented longitudinal muscle bands attached to the thickened anterior edges
of the cuticular plates. These muscles comprise one pair of dorsolateral and one pair of
ventrolateral muscles. These muscles split-up anteriorly to form the head retractors,
whilst posteriorly they operate the terminal spines. These head retractors comprise outer
and inner longitudinal bands, extending from the inner surface of the head cuticle and the
base of the mouth cone to various posterior levels of the trunk.
Kinorhynch muscle fibres insert on the cuticle and are striated, except for the ring
muscles of the head and neck.
Pseudocoel
The fluid-filled pseudocoel is spacious and situated between the gut and the epidermis.
The pseudocoel contains active amoebocytes that possibly originate from the gut wall.
Locomotion
Kinorhynchs do not swim, but exhibit worm-like movements in which the head
protrudes, the scalids grip the substratum, and the trunk is brought forward by
longitudinal muscle contraction while the head is withdrawn and thrust forward to begin
the next cycle. This locomotion is slow. The head also bends sideways in a searching
manner.
Kinorhynchs exhibit a protective reflex in which the head is withdrawn and protected by
the closing apparatus, while the kinorhynch lies motionless.
Nutrition
Kinorynchs eat detritus and microorganisms, especially diatoms. The mouth cone is
extended and the styles are spread open and thrust into the food material. Food is then
ingested by the sucking action of the muscular pharynx.
The digestive system of kinorynchs is similar to that of gastrotrichs and nematodes. The
mouth opens into the mouth cone cavity (the buccal cavity). This cavity is lined by a
syncytial epithelium that also clothes the outer surface of a thick cuticular ring, which
projects from the anterior end of the pharynx into the buccal cavity. This ring is called the
pharynx crown and forms the anterior end of the pharynx.
The pharynx is muscular and fusiform. The wall consists of a lumenal cuticular lining
overlying a syncytial epithelium overlying an outer thick layer of radial muscle fibres.
These radial muscle fibres are arranged in a series of rings. At the posterior end of the
pharynx the radial fibres form a sphincter. The pharyngeal lumen is rounded, flattened or
triangular. On the exterior of the pharynx is a layer of about 10 protractor muscle bands,
extending from the posterior end of the pharynx to the base of the mouth cone.
The rest of the gut is a straight tube regionated into the oesophagus, stomach-intestine
and end gut. The epithelium is cuboidal-columnar and non-ciliated and continuous with
the pharynx epithelium. The oesophagus is a slender tube with two dorsolateral and two
ventrolateral syncytial masses attached to it. These masses are possibly salivary glands.
The cuticle lining of the oesophagus is continuous with the pharyngeal cuticle. At the
oesophagus-stomach junction there are two or more pancreatic glands.
The widened stomach-intestine has no cuticle lining, no gland cells and an external
covering comprising a loose net of circular and longitudinal muscle fibres. The
longitudinal fibres continue over the oesophagus and end gut.
The gut is equipped with sphincters between the stomach-intestine and the end gut and
between the anterior expanded and posterior narrowed regions of the end gut. The
posterior end gut has a cuticle lining. The anus is terminal and situated between the tergal
and sternal plates of the last zonite.
Excretion
Either side of the intestine in the 10th zonite is a large multinucleate flame bulb. The
flame consists of one long flagellum or one long and one short flagellum. Each flame
bulb is connected to its own nephridiopore on the tergal plate of the 11th zonite via a short
tube equipped with driving flagella. Each nephridiopore is a sieve plate.
Nervous System
The nervous system is in close contact with the epidermis and it is hard to distinguish the
two histologically. The brain constitutes a circumenteric nerve ring encircling the base
of the mouth cone or anterior pharynx and may contact the head cuticle. Ganglion cells
are aggregated in the anterior, inner middle and posterior of the nerve ring, but are absent
ventrally.
In the anterior ganglionic mass the ganglion cells extend forward into epidermal cushions
at the scalid bases.
The nerve ring gives out a ventral ganglionated cord, which is in contact with the
epidermis in the midventral line inside the midventral groove. This cord possesses a
ganglion in the middle of each trunk zonite. The nerve cord between ganglia consists of
fibres only. Thus the nerve cord is segmented.
The lateral and middorsal epidermal chords also contain a ganglion cell mass in each
zonite, but these are not definitely connected into nerve cords. Ganglion cells also
apparently occur in the epidermis of each zonite between the chords.
Sensory Systems
Echinoderes has a pair of eyes, each comprising a cup-shaped pigment body enclosing a
lens body. Sensory bristles form longitudinal rows on the trunk. Each consists of a
cuticular extension around the plasmatic strand of an epidermal sensory cell.
Reproduction
Kinorhynchs are dioecious and the sexes are usually indistinguishable. In Trachydemus
and Pycnophyes only the males possess adhesive tubes and the edges of the plates of the
last zonite differ between males and females. In Echinoderes dujardinii, the females have
more end spines than the males.
The gonads are a pair of sacciform bodies that open to the outside separately, on the 13th
zonite. At the anterior end of each gonad is an apical cell that gives rise to the other
gonad cells (germ cells, nutritive cells in the female, and the epithelial wall of the gonad.
Initially, the ovary is a syncytium with ovocytic and nutritive nuclei. Later the ova
differentiate and absorb the nutritive syncytia. The posterior end of each ovary opens into
a short oviduct that opens to the outside via the genital pore. The oviduct may be supplied
by a dorsal diverticulum, which is the seminal receptaculum.
The wall of the testis is comprised of a flat epithelium and an inner spermatogonia layer
on the side next to the intestine. The interior is filled with developing spermatocytes at all
stages of development. The posterior end of each testis opens into a short sperm duct that
opens via the genital pore. The genital pore may be equipped with 2-3 penial spicules,
and sometimes additional hairs, spines or bristles. The sperm are large with a short tail.
Development
The youngest females contain sperm in their seminal receptacle and so there would
appear to be no parthenogenesis. Kinorhynchs reproduce at all seasons. The eggs hatch
into larvae that undergo a metamorphosis of several molts into the adult.
In cyclorhagous kinorhynchs, like Echinoderella, the larva is minute and has no definite
zonites, no head, no scalids, no placids, no pharynx and no anus. However, the spine
arrangement suggests that they are made-up of three zonites. The mouth is surrounded by
radiating grooves and the anterior half of the body is clothed with fine bristles. The
number of zonites increases as the larva passes through the Haploderes larval stages.
During these stages, other differentiations also occur, resulting in an adult.
Homalorhagous kinorhynchs have more advanced hatchlings with 6-7 zonites that look
more like adults. Pycnophyes develops through Leptodemus, Centrophyes and
Hyalophyes stages. The latter may reproduce sexually.
In general the juveniles have fewer zonites, a softer and more transparent cuticle and
fewer cuticular teeth, spines and bristles. Molting takes several days. The cuticle ruptures
in the mouth cone region and the whole cuticle is shed.
Classification
Kinorhynchs are divided into three groups: the Cyclorhagae, the Conchorhagae and the
Homalorhagae.
1. Cyclorhagae. Only the first zonite is retractable and the placids of the 2nd zonite close
over the first zonite when it is retracted. E.g. Echinoderes (which has 2 eyespots), and
Echinoderella (which has no eyespots), both of which have lateral and spines, but no
median end spines. Centroderes and Campyloderes both have a median end spine.
Centroderes is an Antarctic form with 14 zonites (the 14th zonite forms the base of the
end spine).
2. Conchorhagae. This group comprises a single genus, Semnoderes, in which the third
zonite has a closing apparatus comprised of a pair of shells.
3. Homalorhagae. The third zonite forms a closing apparatus comprised of one tergal
and three ventral plates. Homalorhagous kinorhynchs are less spiny and more truncate
at the ends. There are two genera in this group: Pycnophyes, which has lateral end
spines, and Trachydemus, which has no lateral end spines.
Ecology
All kinorhynchs are marine and live in mucky bottoms, feeding on detritus, or else live
among algae and feed on diatoms. They occur in the littoral benthonic zone. Kinorhynchs
are best known from European coasts, but have also been found off Zanzibar, Japan,
Antarctica and the Americas.
Some species contain zooxanthellae in their epidermis and elsewhere. These are devoured
by the pseudocoel amoebocytes during starvation. Kinorhynchs live from less than one
year to over one year. There are some 150 species recorded.
Nematomorpha (Horsehair Worms, Hair-Worms)
Nematomorphs are free-living as adults, but parasitic in arthropods as juveniles.
External Features
Nematomorphs are very filiform worms, being very slender and elongated. They may be
up to 0.5 to 1m or more in length and range in diameter from less than 1 mm to 3 mm.
The males are shorter, except in Nectonema in which the males are larger than the
females. Nematomorphs have an approximately uniform diameter throughout their length
and have rounded ends, except for Chordodes, which tapers anteriorly.
Hair-worms are yellowish, bluff or dark brown in colour. The anterior tip or calotte is
usually white and is usually bounded posteriorly by a dark ring. The mouth in is terminal
or ventral and situated in the calotte. The posterior end is rounded or lobulated into 2-3
caudal lobes, depending on genus and sex.
The anus (cloacal aperture) is terminal or ventral at the posterior end, anterior to the
caudal lobes if these are present. The posterior part of the males may be coiled or
ventrally curved as in nematodes.
In Chordotes a longitudinal mid-ventral groove marks the position of the ventral.
The cuticle is usually thick and may be rough, due to the presence of areoles – contiguous
rounded or polygonal areas in the cuticle. These areoles may project as papillae, each
with 0-2 bristles, or each may be perforated by a pore. The interareolar furrows may bear
bristles, warts or the pores of cuticular canals. Tracts of bristles, thorns or adhesive warts
may occur on the male cloacal region.
Nectonema has a double row of natatory bristles along the middorsal and midventral
lines, except at the ends of the body. These rows may appear laterally located due to body
torsion.
Body Wall
The outermost layer of the body wall is the cuticle. The cuticle consists of an outer
homogeneous layer, which is generally a thin layer, and an inner fibrous layer. In forms
with areoles the outer homogeneous layer is greatly thickened. The inner fibrous layer is
lamellate and consists of up to 45 strata. Beneath the cuticle is the epidermis. Beneath the
epidermis is a layer of longitudinal muscle.
The epidermis is a single layer of cuboidal-columnar cells. Nectonema has dark granules
between the cuticle and epidermis. Gordioids have a single ventral cord. Nectonema has
dorsal and ventral cords. The muscle fibres in Nectonema are of the coelomyarian type
found in nematodes.
Pseudocoel
A complete pseudocoel runs the length of the body, between the body wall and the
digestive tube. A septum at the level of the brain divides off a smaller anterior
pseudocoel. In gordioids, tissue and mesenchyme cells fill most of the pseudocoel, but
there are spaces, around the gut and the gonads, bounded by mesenchyme.
Locomotion
The males are more active than females and can swim for hours, or crawl on the bottom
by serpentine undulations. Females rarely, if ever, swim. When placed in dishes, gordions
tend to tangle themselves in knots (hence the name – gordions are named from the
Gordion knot).
Nutrition
The digestive tract is degenerate, at one or both ends, in both juveniles and adults, and it
is unlikely that food is ingested. A proper mouth may be present and the ‘pharynx’ is
usually a solid cord of cells. The intestine or midgut is a simple epithelial tube free in the
pseudocoel, which possibly serves an excretory function. The posterior gut receives the
genital ducts and opens via a cuticle-lined cloaca.
In Nectonema (a nectonematoid) a minute slender mouth opens into a cuticularised tube
that connects to the intestine. The intestine consists of 2-4 longitudinal cell rows or
syncytia. In nectonematoid intestine does not reach the cloaca. The cloaca serves a sexual
function only.
Nematomorphs presumably absorb nutrients through their body surface. The epidermis of
young gordioids possibly secretes digestive enzymes.
Excretion
The intestine possibly has an excretory function. There is no other excretory system and
metabolic wastes possibly diffuse out across the general body surface.
Nervous System
The nematomorph nervous system is similar to that of priapulids and echinoderids. It is
closely applied to the epidermis. There is a circumeneteric cerebral mass in the calotte
and a mid-ventral cord. The mid-ventral cord is divided into 3 tracts by glial partitions,
and contains several giant neurones. The two lateral tracts continue forward from the
brain as cephalic nerves into the calotte epidermis, where each gives rise to two ventral
epidermal nerves.
The cerebral mass gives out one pair of dorsal nerves to the ‘eye’. There is a posterior
thickening of the cord in the cloacal region, which sends nerves to the cloaca, sperm
ducts and caudal lobes.
Sensory Systems
The epidermis is equipped with sensory bipolar cells. Cuticular spines, bristles and warts
may also be sensory. Warts at the rear end of the male have a sensory role in copulation.
The ‘eye’ is a large sac in the calotte of Paragordius. There is a dark pigment ring behind
the sac, and the epidermis over the sac is transparent. It is not certain whether, or not, this
‘eye’ is actually photoreceptive.
Female nematomorphs release a pheromone sex attractant. The larvae hatch from the host
in damp conditions and so must presumably possess hygroreceptors.
Respiration
There are no special respiratory organs and gas exchange presumably occurs by diffusion
across the body wall.
Reproduction
The sexes are separate. Males can distinguish virgin from gravid females, but may mate
with a female already fertilised.
Gordioid reproductive system. The gonads are a pair of cylindrical bodies that run the
length of the body. In males, the gonads open separately into the cloaca via short sperm
ducts. The ovaries contain 3000-4000 lateral diverticula in the mesenchymal spaces. The
eggs mature in the diverticula and reenter the main ovary in the cloacal uterus region.
Each of the paired uteri opens separately into the common glandular antrum chamber via
an oviduct. The antrum opens into the cuticle-lined cloaca. The seminal receptacle is
situated beneath the intestine and extends anteriorly and opens into the antrum chamber.
Nectonematoid reproductive system. The nectanematoid males possess a single testis.
There is no definite ovary, in the females, but the ovocytes are free and fill the
pseudocoel, but are initially attached to the epidermis by strands. There is no cloaca, just
a short genital tube at the rear of the worm, which releases eggs to the exterior. The sex
cells are of mesenchymal origin.
The adult gordioids, when almost sexually mature, emerge from their insect hosts in
water, usually in spring / early summer and immediately copulate and lay long strings of
eggs in water. The eggs are cemented together, possibly by a secretion from the glandular
antrum.
The males seek out females and copulate by coiling their posterior ends around the
female. Sperm enter the cloaca and pass into the seminal receptacle of the female, where
fertilisation takes place internally.
Development
Cleavage is of a modified spiral type and holoblastic and gives rise to a blastula
(coeloblastula). The blastula develops into a gastrula, which develops into a larva that
resembles an adult priapulid or acanthocephalan. The larva enters a host and develops
into a juvenile worm over several weeks or months in the host pseudocoel, after molting.
Gordioid larvae undergo a brief free-living phase and then penetrate a small aquatic
animal host. They are able to develop to the juvenile stage in an insect, centipede or
millipede host. Nectonema infects a marine decapod crustacean host.
The larva leaves the host in damp conditions (the host is possibly induced to seek water)
and molts into the adult.
Classification
There are two orders of nematomorphs: Gordioidea and Nectonematoidea. There are
about 320 described species.
O. Gordioidea. The gordioids are fresh-water or terrestrial. They only have the ventral
epidermal cord and the pseudocoel is filled with mesenchyme. The gonads are paired.
E.g. Chordodes, Paragordius, Parachordodes, Gordionus and Gordius.
O. Nectonematoidea. This order consists of the single genus Nectonema, a marine pelagic
form. It has both ventral and dorsal chords, has an open fluid-filled pseudocoel and a
single gonad. Nectonema is pale, translucent and about 20 cm long.
Ecology
Nematomorphs occur throughout the world, in tropical and temperate climes and in all
aquatic habitats, including alpine streams.
Gastrotricha
External Features
Gastrotrichs are free-living microscopic worms. They range in length from 0.1 to 1.5
mm, but are usually <0.6 mm long. They inhabit fresh-water and salt-water. Gastrotrichs
are elongated, ventrally flattened and usually bristly or spiny. They glide on ventral cilia.
The head may be rounded and lobelike and is joined to the trunk via a constricted neck.
The posterior end is forked, pointed, rounded, truncate or drawn out into a slender tail.
There are 1-2 pairs of red pigment spots (ocelli?) on the sides of the head lobe in some
Macrodasyoidea. Some have 1-2 pairs of tentacles or palps.
The dorsal surface is convex or flattened. Some gastrotrichs are nearly circular, with
convex dorsal and ventral surfaces. Gastrotrichs are colourless and transparent; although
ingested food may give colour to the gut.
Cilia occur on the head lobe and usually also on the entire ventral trunk surface.
Macrodasys has one ventral broad ciliary band running the entire length of the animal.
Chaetonotus and Turbanella have 2 narrow ventral longitudinal bands of cilia. In
Dasydytes and Setopus small ciliated patches form two longitudinal ventral rows.
Thaumastoderma has transverse ventral bands of cilia. Cilia may be confined to the
anterior part of the trunk or they may be absent. The ventral trunk cilia usually continue
onto the ventral surface of the head lobe, which is ciliated on both surfaces with sparse
cilia or tufts, patches, transverse groups or partial girdles of cilia.
There may be very motile or immotile long cilia or bristles on the head, especially in the
chaetonotoids, which may have a sensory function. In freshwater genera there are often
2-3 tufts or short bands of these long cilia / bristles on each side of the head lobe, as in
Chaetonotus, Dasydytes, Ichthydium and Stylochaeta, and others.
In some Gastrotrichs there are 1-2 rows of long cilia on the dorsal surface of the head
lobe, as, for example, in Turbanella and Proichthydium. In Xenotrichula the cilia of the
ventral surface of the head and trunk form cirri that are employed in running about in a
similar manner to hypotrichous ciliates.
Body Surface
The body surface is covered in a thin cuticle, which usually forms surface structures such
as warts or scales (flat or overlapping) that may be smooth or each equipped with a
median keel or posteriorly curved spine. Spined scales are characteristic of the
chaetonotoids. The spines may subdivide into 4-5 prongs, as in Thaumastoderma and
Tetranchyroderma. Xenotrichula and Aspidiiophorous have double-decked stalked
spines, each consisting of 2 superimposed scales connected by a stalk. Dasydytes has
spines, but no scales.
Stylochaeta and Dasydytes have very long spines in bunches on the body sides. Neogossa
has such spines on its posterior end. Marginal scales may have longer spines. Marginal
spines are sometimes the only spines present.
Lepidodermella ( = Lepidoderma) has a complete covering of spineless scales. Plates
may be formed from the fusion of scales, especially on the ventral surface and the head
lobe of chaetonotoids. The head plates that are usually present in this instance are the
unpaired cephalion anterior to the mouth, unpaired hypostomium behind the mouth and
a pair of lateral pleurions. Each of the lateral pleurions may be subdivided.
Adhesive Tubes. Adhesive tubes are most abundant in the Macrodasyoidea, and are also
present in the Chaetonotoidea, except in the pelagic families Dasydytidae and
Neogosseidae, These adhesive tubes are cylindrical projecting cuticular tubes and each is
movable by one or more muscles and is supplied by a gland cell with 1-3 nuclei. These
tubes function to adhere the animal to objects.
There are up to 250 adhesive tubes in macrodasyoids, arranged in longitudinal series at or
near to the lateral margins, and in longitudinal or transverse rows or clusters on the
ventral surface of the head and in bunches on the tail forks or along the sides of unforked
tails.
In Turbanella, each lateral tube is accompanied by a long motile sensory cilium.
Chaetonotoids have 1-2 on each tail fork only. These may comprise the main part of the
tail fork or are terminal on the forks as toes.
Epidermis. The epidermis is syncytial and mostly thin, but is much thickened laterally in
macrodasyoids. Epidermal glands form the adhesive glands of the adhesive tubes and the
adhesive dorsal glands of the macrodasyoids. These dorsal glands are numerous
dorsolateral and rounded bodies of granular or homogeneous material, each with a pore.
There is no subepidermal muscle sheath, but delicate anucleate circular fibres, which are
continuous with the epidermis and situated in or just beneath the epidermis, give rise to
the muscles of the lateral adhesive tubes and moveable bristles.
Longitudinal muscles comprise the ventrolateral group that extends the length of the body
and operates the anterior and posterior adhesive tubes and inserts on the mouth region
and pharynx. There are also 2-6 smaller dorsal longitudinal bands, situated along most or
part of the dorsal wall and inserting on the mouth region and the pharynx. The
longitudinal muscle fibres are nucleated and smooth, except in Dactylopodalia, in which
they are striated.
Pseudocoelom
The pseudocoel consists of small spaces between the body wall and the viscera and has
no definite lining. In the macrodasyoids membranes derived from the epidermis divide
the pseudocoel. These membranes may contain muscle fibres. These divisions give rise to
a central compartment around the digestive tract and ripe eggs, and paired lateral spaces
containing the gonads. There are no free amoeboid cells in the pseudocoel.
Nutrition
The mouth is terminal or slightly ventral and may be bordered by numerous small curved
hooks. In chaetonotoids the mouth opens into a short cuticular buccal capsule bearing
longitudinal ridges and sometimes bearing projecting teeth. This buccal capsule is
protrusible to some extent and leads into the pharynx. In the macrodasyoids, the thinwalled distal end of the pharynx possibly comprises a buccal capsule.
The pharynx is very similar to the nematode pharynx. It is an elongated tube one sixth to
one third of the body length. It is usually equipped with 1-4 bulbous enlargements (4 in
Neogossea) and has a 3-angled lumen. In chaetonotoids there is one midventral angle and
2 dorsolateral angles, as in nematodes, but in macrodasyoids there is one middorsal and 2
ventrolateral angles.
The wall of the pharynx consists of columnar epithelium containing cross-striated radial
muscle fibres that are best developed in the bulbous regions. These fibres are part of the
epithelium. Some of the epithelial cells are modified into gland cells. In some
chaetonotoids, the pharynx has 2 pairs of large projecting cells, which possibly function
as salivary glands.
The pharynx lumen has a thin cuticular lining. The pharynx has an external covering of
thin circular muscle fibres, which are epithelial extensions that form an oral sphincter at
the mouth.
In macrodasyoids a pair of pharyngeal pores connect the pharyngeal lumen to the
outside. The posterior of the pharynx often projects into the midgut as a pharyngeal
plug. The pharynx leads into the midgut.
The gastrotrich midgut, or stomach-intestine, is a simple straight tube with no external
glands. It consists of a wider anterior stomach and a narrower posterior intestine,
although these regions are not definitely delimited. A single layer of cuboidal or
columnar epithelium lines the lumen. The midgut has no cuticle lining and has a thin
external covering of circular muscle fibres. Gland cells are apparently present, especially
in the anterior stomach, and these possibly secrete digestive enzymes.
The anus is ventral in macrodasyoids and situated between the bases of the tail forks, but
is terminal or slightly dorsal or slightly ventral in chaetonotoids. The anus is lined by
cuticle and is sometimes equipped by an anal sphincter muscle.
Digestion presumably occurs in the midgut lumen. The food comprises bacteria,
protozoans, diatoms and detritus, etc. Food is ingested by the sucking action of the
pharynx whilst the animal continuously moves about. Some chaetonotoids attach to the
substrate by the adhesive tubes of their tail forks and gather food particles by ciliary
action.
Excretion
Chaetonotoids have one pair of protonephridia, with one protonephridium either side of
the middle of the digestive tract. Each consists of a single non-nucleated flame bulb with
a very long flame. From each bulb a highly coiled tubule connects to the outside, via the
nephridiopore on the ventral side of the middle of the body. There is no urinary bladder.
The macrodasyoids have no protonephridia, but they have one or more pairs of granular
masses that open to the outside ventrally (ventral glands) and are possibly excretory.
Nervous System
Gastrotrichs have a relatively large brain consisting of a mass either side of the anterior
pharynx with a broad or narrow dorsal connection between them. The brain gives out a
pair of lateral nerves that contain ganglion cells and extend the length of the body.
Sensory Systems
The tufts of long cilia on the chaetonotoid head lobe are continuous with ganglion cells in
the brain and are presumably sensory. Single stiff or motile tactile hairs or bristles are
scattered over the body and are more numerous in the macrodasyoids.
The head possesses lateral sense organs. In chaetonotoids these are a pair of ciliated pits
on the head lobe just behind the most posterior ciliary tufts. In many macrodasyoids the
lateral cephalic sense organs are a pair of piston pits, in which the bottom of the pit
contains an unciliated projection, the piston. Disappearance of the pit and cilia and
elongation of the piston has given rise to the lateral tentacles or palps in some genera, e.g.
Thaumastoderma, Neogossea and Xenotrichula.
Pigment ocelli are found in a few species. These are aggregations of red pigment granules
inside some of the brain cells. Groups of highly refringent bodies found in some brain
cells or in the cells of the ciliated pits possibly have a static function.
Locomotion
Locomotion is by ciliary gliding, by use of the ventral cilia. Macrodasyoids can locomote
by ciliary gliding and leech-like movements. The anterior and posterior adhesive tubes
are employed in the leech-like mode of locomotion. They also exhibit long periods of
temporary sessility whilst attached to the substrate by their posterior adhesive tubes.
Reproduction
Gastrotrichs are thought to have been originally hermaphroditic, but the male system has
degenerated in Chaetonotoidea, which consist of females only (except for Xenotrichula)
that reproduce by parthenogenesis. The Macrodasyoidea are hermaphroditic, some are
protandric and Dactylopodalia has male, female and hermaphroditic individuals.
There are 1-2 ovaries comprised of cell masses, with no definite capsule, in the posterior
part of the body. These contain up to 25 ovocytes, the number is possibly fixed during
development of the embryo (?) as no mitoses have been observed. The eggs ripen and
become free in the uterus anterior to the ovary. In those forms in which the pseudocoel is
subdivided into central and lateral compartments, the larger eggs lie alongside the midgut
in the central compartment.
The macrodasyoids have a single oviduct with an enlarged anterior serving as a seminal
receptacle and a posterior thicker walled swelling that forms a copulatory bursa that
opens to the outside via the female gonopore. The female gonopore opens anterior to the
anus or in common with the anus. Turbanella has no copulatory bursa, however, and
Macrodasys has no seminal receptacle.
There is usually a yolk gland present as a single or paired mass of nutritive tissue.
Chaetonotoids have no definite oviduct, but have a posterior organ X that opens to the
outside via a ventral pore and is possibly a copulatory bursa. There are traces of testes in
some chaetonotoids.
In macrodasyoids the male system consists of a pair of testes or a single right testis. Each
testis is connected to the median male gonopore via its own sperm duct. There may be a
single male gonopore, but there is sometimes a pair of such pores when two testes are
present. The male gonopore(s) opens close to the female gonopore, or in common with it,
or the male gonopore is situated anterior to the female gonopore. Macrodasys and
Urodasys have a [penis-like structure.
Development
Lepidodermella squamatum is a fresh-water chaetonotoid. It lives for 8-21 days and lays
up to 5 eggs (usually 3-4). Apparently all chaetonotoids produce two types of egg (as in
rotifers): subitaneous and dormant. Both types of egg develop parthenogenetically. The
eggs are oval and enclosed in a shell, which is thicker in dormant eggs. The shell
possesses bristles or blunt spines, sometimes only on one side. The eggs are laid on the
surface film or near an object. They hatch in 1-3 days and the hatchlings resemble adults
and reach sexual maturity in about 3 days. The number of adhesive tubes may increase
after hatching.
In Neogossa, cleavage is holoblastic and determinate and gives rise to a coeloblastula.
During gastrulation two ventral cells enter the interior and possibly give rise to the
midgut. Cells are proliferated into the interior to form the pharynx and the hindgut. Two
ventroposterior cells give rise to the primordial germ cells that develop into the ovaries.
Classification and Ecology
There are about 450 species of described gastrotrich.
O. Macrodasyoidea. These gastrotrichs have elongated vermiform bodies with or without
a head lobe. They possess anterior, lateral and posterior adhesive tubes, pharyngeal pores
and a male reproductive system. They lack protonephridia. All macrodasyoids are marine
and live on shores amongst sand or in the vegetation zone. Macrodasyoids have been
found along European coasts, but possibly occur in other parts of the world. Examples of
macrodasyoid genera are: Cephalodasys, Lepidodasys, Acanthodasys, Macrodasys,
Urodasys,
Dactylopodalia
(Dactylopodella),
Turbanella,
Thaumastoderma,
Tetranchyroderma and Platydasys.
O. Chaetonotoidea. Chaetonotids are usually fusiform and possess a head lobe with tufts
of long sensory cilia or bristles. They lack pharyngeal pores. Chaetonotoids have no male
reproductive system and are parthenogenetic. Neodasys and Xenotrichula are exceptions,
as they possess a male reproductive system. There are 1-2 adhesive tubes on the tail forks
only, except in Neodasys in which there are lateral adhesive tubes, each of which is
accompanied by a long cilium. Chaetonotoids possess one pair of protonephridia. Most
are fresh-water and benthic, dwelling among vegetation in ponds, lakes, fouling material,
moss pools and bogs. Chaetonotus is very common in ponds. Neodasys is marine and
resembles macrodasyoids and has an elongated band-like body, lateral adhesive tubes,
adhesive tubes on tail forks and a male reproductive system. Xenotrichula is also marine,
but has the typical chaetonotoid appearance and possesses a head lobe, dorsal stalked
scales, adhesive tubes on the tail forks only and some male reproductive system is
retained. Examples of chaetonotoid genera are: Chaetonotus, Icthydium, Lepidodermella
(Lepidoderma), Polymerurus, Heterolepidoderma, Stylochaeta, Aspidophorous,
Proichthydium, Neogossea and Dasydytes.
Loricifera (loriciferans)
External Features
This phylum was discovered in 1983 and there are some 14-32 described species. Most
occur at 300-450 m in coarse marine sediments. Miciloricus hadalis occurs deeper than
8000 m. All are free-living.
A cuticular lorica encloses most of the body. These animals are 115-383m long, but are
composed of over 104 cells. Thus, loriciferans are minute but complex. The body is
regionated into the introvert (head), neck, thorax and loricate abdomen. The introvert,
neck and thorax can telescope into the abdominal lorica.
The mouth is at the end of the introvert (oral cone), which may be equipped with
protrusible oral stylets. Nine rings of spinelike scalids (x 200-400) of various shapes
protrude from the spherical head. The scalids have intrinsic muscles and in pliciloricids
some of the scalids are jointed near their bases. The first ring consists of anteriorly
directed clavoscalids, whilst the other 8 rings consist of posteriorly directed
spinoscalids. The number of clavoscalids differs between males and females in some
species.
The lorica consists of 6 plates and anteriorly directed spines around the base of the neck.
Though, in some species the lorica consists of 22-60 longitudinal folds.
Body Wall
The body surface is covered by cuticle. Beneath the body wall are cross-striated muscle
bands that can retract the anterior parts.
Body Cavity
The body cavity is thought to be a blastocoelom (pseudocoelom) and is spacious in
pliciloricids, but absent in Nanaloricus.
Locomotion
The larvae are equipped with a pair of posterior toes that are used in adhesion and
locomotion (swimming). Ball-and-socket joints connect the toes to the abdomen. Larvae
also have locomotory spines on the lorica (?). The adult stage is sedentary.
Excretion
Loriciferans have one pair of monoflagellate protonephridia located within the gonads.
Nutrition
The mouth opens into a long, tubular buccal canal. One pair of salivary glands open into
the buccal canal and the buccal canal is cuticle-lined, telescopically extrusible and may
also be equipped with accessory stylets. The buccal canal opens into a cuticle-lined
pharynx bulb, which in turn opens into a cuticle-lined oesophagus. The pharynx bulb wall
contains circular and longitudinal muscles. The oesophagus opens into a long midgut,
which opens into a short, cuticle-lined rectum. The rectum opens via the anus, which is
borne on the anal cone.
The oral stylets are used in piercing and sucking (?). Some loriciferans are known to eat
bacteria.
Nervous System
The CNS consists of a large, circumpharyngeal ganglion and smaller ganglia in various
parts of the body and a large ganglionated VNC.
Sensory Systems
Some scalids possibly have a sensory role (?).
Reproduction
Loriciferans are dioecious. The males have a different form to the females and a different
number of scalids. The male system consists of 2 dorsal testes in the abdominal body
cavity. The female system consists of one pair of ovaries (and probably also a seminal
receptacle?). Fertilisation is possibly internal (?).
Development
There is a free-living feeding larval stage, called the Higgins larva. The larva can only
withdraw its introvert, into its neck, and, in most species, it has a pair of toes (caudal
“feet”) at the posterior end of the abdomen. These toes are used in swimming and
adhesion (?).
The larva molts periodically into the adult, sometimes via a juvenile stage that resembles
a female loriciferan adult, but has no ovaries. Some deep-sea forms produce neotenous
larvae, which produce 2-4 more larvae by parthenogenesis. The larvae may encyst for a
time before metamorphosis.