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).
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 and inside these ventrolaterals, which may occur throughout the trunk and / or tail or the anterior trunk, and are 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 due to alteration of head shape. In particular the
lateral complex and the 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.
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.
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
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
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.
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. Generally it appears that the body flexes in the vertical
direction, undulating vertically, but this is too quick to appear more than a tremble to the naked human eye. The fins are generally considered to be
flotation devices, but they presumably prevent rolling and may possibly act as hydrofoils in lift generation, along with the general body surface.
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.
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.
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.
The fins and tail can be regenerated. Wounds close and clot. The head can also be regenerated.
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.
Recently, bioluminescence has been discovered in two species of chaetoganth (one reported as recently as 2010). In Caecosagitta macrocephala bears a
luminescent organ on the ventral edge of each anterior lateral fin. Eukrohnia fowleri has a luminescent organ running across the centre of the tail fin, on
both the dorsal and ventral surfaces. This luminescence is known to serve a protective function, as these chaetognaths release luminescent clouds when
startled, but they also serve other functions. The luminescent particles are packed in a hexagonal arrangement inside the organs (a feature unique to
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 (taste sensors). 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.
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
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 retrocerebral organ has an unknown function [check for updates on this], 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.
(Diagram based on: Hyman,1959. The
Invertebrates (vol V): Smaller Coelomate Groups;
Note: constrictor oris primus = primary mouth constrictor; constricter oris alter = additional mouth constrictor. The bicornuate
muscle is named for its shape, like a double-horned structure.