Ctenophores
Ctenophora (comb jellies, sea walnuts)

Internal and External Features

Cydippida (Sea Gooseberries)

Ctenophores are transparent, gelatinous and biradiate coelenterates. They include the Cydippida or sea gooseberries, e.g.
Pleurobrachia and Hormiphora. The sea gooseberries are rounded, oval, egg-shaped or pyriform and are usually flattened in
one plane. The oval mouth is situated in the centre of the oral pole and the aboral or sensory pole contains the statocyst. The
following description is primarily of the Cydippid form, but will be modified for the other orders under their respective headings.

There are 8 equally spaced meridional rows of little ciliated plates extending from the aboral pole to near the oral pole. Each
plate is a transverse band of fused cilia, called a
comb or ctene. The plates are grouped together into plate rows (comb
rows, ribs
or costae). There is a pair of tentacles, each with its base enclosed inside a tentacle sheath, a deep pouch
containing the tentacle base fastened to its inner wall. The two tentacle sheaths are located at opposite points on the surface,
between two comb rows. The two tentacles are very long and very extensible solid filaments, each with a row of lateral
branches. Each tentacle can be completely retracted into its sheath.

The tentacles and the gastrovascular system impose
biradial symmetry upon these animals. There are thus two planes of
symmetry: 1. The transverse, tentacular or lateral plane – the vertical plane through the tentacle sheaths. 2. The vertical
sagittal, median or stomodaeal plane at right angles to the tentacular plane and in which lies the axis of the flattened
stomodaeum. The equatorial plane is across the body (and is not a plane of symmetry).

The comb rows are adradial and not along the principal planes. There are two comb rows per interradial quadrant formed by
the sagittal and tentacular planes. There are four subsagittal rows near to the two ends of the sagittal plane and four
subtentacular rows near the tentacular plane.

Cydippida: Gastrovascular System

The mouth is usually elongated in the sagittal plane and opens at the end of a tube (which is elongated sagittally and
flattened tentacularly) into the ectodermal
stomodaeum (pharynx). The stomodaeum has much-folded walls for digestion and
extends about two-thirds of the way to the aboral pole and opens into the oesophagus via a short constriction. The
oesophagus opens into the endodermal stomach (infundibulum, funnel) chamber. The stomach is flattened sagittally, that is at
right angles to the flattening of the pharynx.

The stomach opens into the gastrovascular system canals, which have a definite biradial arrangement. The roof of the
stomach opens into the aboral / infundibular canal that runs along the underside of the statocyst and gives out as 4 branches
the ‘excretory’ or anal canals (interradial). These canals terminate in ampullae, with two of them diagonally opposite each
other opening via ‘excretory’ pores, whilst the other two are blind. The ‘excretory’ pores eject indigestible matter and so are
actually anal pores.

The oral surface of the stomach opens into one pair of pharyngeal / paragastric canals that run orally, one along each
flattened surface of the pharynx and terminate blindly near the mouth.

From each side in the tentacular plane, the stomach opens into a large transverse canal, which leads into the horizontal
tentacular canal that terminates blindly in the tentacular sheath. Each tentacular canal gives off one pair of branches, or
interradial canals, that bifurcate to give a total of 8 canals, four in each half, that run to the inner side of each comb row to a
curved oral-aboral meridional canals running under each comb row.

Cydippida: Sensory Systems

The sensory pole contains the statocyst in its centre. The statocyst consists of a concave floor of tall ciliated epidermal cells
among which, at the 4 interradial points in this sensory floor, are 4 long s-shaped tufts of cilia (balancers) that connect to and
support a central rounded calcareous mass, the statolith. The whole structure is enclosed in a transparent dome, the cupule /
bell, which is made of modified fused cilia from the edge of the sensory floor (?) and constitutes an organ of equilibrium (?).
Around the statocyst are the four interradial ampullae of the anal canals. The sensory floor puts out ciliated furrows, two from
each balancer along the four interradii, one to the aboral end of the comb row pair in each quadrant. The sensory floor
continues as a long ciliated depression (polar plate, polar field) on each side of the sagittal plane. These polar fields possibly
have a sensory function (?).

O. Cydippida

The cydippids are globular, oval or pyriform and have two long tentacles with sheaths. The tentacles are fringed with lateral
filaments and possess colloblasts. Examples are Mertensia, Pleurobrachia, Hormiphora, Callianira and Lampetia. Lampetia is
able to evert its pharynx as a creeping sole.

O. Lobata

The Lobata have an oval body compressed in the tentacular plane and expanded in the sagittal plane into a rounded and
contractile oral lobe on each side of the mouth. The contractility of the oral lobe is brought about by a latticework of muscle on
its inner surface. The four subsagittal comb rows are longer than the four subtentacular comb rows. From the lower ends of
the subtentacular comb rows extend short or long, and sometimes spirally coiled, processes, the auricles. The auricles have
ciliated edges and project above the mouth, two on either side.

The tentacle sheaths are present only in the larvae, disappearing in the adult (during metamorphosis). The tentacles are
usually short and are situated more orally near the mouth. As a result of this shift in position the tentacular canals are
elongated. There is also a short row of tentacles in the ciliated auricular grooves.

There are no transverse canals. Four interradial canals spring from the stomach. The four subtentacular canals loop around
the edges of the auricles and anastomose with the oral ends of the two pharyngeal canals to form a ring around the mouth.
This ring is drawn into a loop in each oral lobe. The two subsagittal meridional canals of each side unite via sinuous loops
inside each oral lobe.

Examples of lobates are
Bolinopsis (= Bolina) with short auricular grooves; Mnemiopsis with very deep auricular grooves
extending to the statocyst;
Leucothea (= Eucharis); Eurhamphaea; and Ocyropsis (= Ocyroë) that has large muscular flapping
oral lobes used in swimming.

O. Cestida

The cestids have an elongated ribbon form, for example, Venus’s girdle (Cestum veneris). The body is very transversely
compressed and elongated in the sagittal plane to form a flattened gelatinous band up to 1.5 m long. The cestids swim by a
combination of comb rows and undulations of the body.

The four subtentacular comb rows are reduced to very short lengths, whilst the four subsagittal rows are elongated and run
along the entire aboral edge. Tentacle sheaths and tentacles are present, alongside the mouth. The tentacles are tufts of
filaments. There are also two rows of short tentacles in  grooves along the whole oral edge.

Each face of the band has 6 comb rows, 2 subsagittal meridional canals along the aboral edge, 2 subtentacular rows in the
middle, and 2 pharyngeal rows along the oral edge. All are horizontal and anastomose at the ends of the band. There are two
genera (?) found in Mediterranean and tropical waters:
Cestum is up to 1.5 m long and Velamen (=Vexillum, Folia) is up to 15
cm.

O. Beroida

The beroids are conical or thimble-shaped and compressed in the tentacular plane. They have a very large mouth and
pharynx. There are no tentacles and no tentacle sheaths (not even in the larva).

The polar fields are edged with branched papillae. There is a small stomach near to the statocyst. The stomach gives off 4
interradial canals. The meridional and pharyngeal canals give off numerous branched lateral diverticula that may anastomose
into a network. A ring canal runs along the mouth rim or around the mouth and unites the meridional and pharyngeal canals of
each half, or both halves in some species. The comb rows are of equal length and extend over half to almost the whole length.

An example of the Beroida is
Beroë, which is up to 20 cm in height and often pink in colour.

O. Platyctenea

The Platyctenea are oral-aborally flattened and oval in shape. They exhibit a creeping mode of life. Examples are Coeloplana,
Ctenoplana, Tjalfiella and Gastrodes. These ctenophores are 5-8 mm long (in the tentacular plane) with olive green, brown or
reddish dorsal colouring / patterning. The body consists of a central thick portion and two rounded thin lobes in the sagittal
plane. Situated in a notch between the lobes, at each end, is a large tentacle sheath and a long retractile tentacle edged with
filaments.

There is a statocyst in the centre of the dorsal surface and 8 prominent ciliated papillae encircle the pole. These papillae may
have a respiratory function (?). There are 8 short comb rows that connect to the statocyst via the ciliated furrows. The oral
surface is actually the everted pharynx and the ‘mouth’ is the opening to the inner pharynx. This inner pharynx has folded
walls and opens into a long oesophagus. The oesophagus opens into the rounded stomach, which is beneath the statocyst.

The stomach gives off two anal canals from its aboral surface. Each anal canal opens via a pore. Laterally the stomach gives
off six canals, four to the comb rows and two to the tentacular sheaths. These six lateral canals anastomose to form a network
in the periphery.

Ctenoplana is an example of the Platyctenea. Ctenoplana is planktonic and swims by folding its lobes together and uses its
combs or else flaps its lobes in and out. When resting on the bottom, the lobes are opened out flat and in this state
Ctenoplana may creep a little, or it may travel upside-down on the surface film by ciliary action. The gonads are four bilobed
masses in the walls of the subtentacular canals. Testes have only been observed so far (?) and each opens to the surface via
a duct and pore.

Coeloplana is another example that reaches up to 60 cm. It is a creeping ectocommensal on alcyonarians. Coeloplana has no
comb rows, but only traces of ciliated furrows. There are 12-60 erectile papillae on its aboral surface that lie approximately in
four rows overlying the meridional canals. Each papilla receives a branch from its underlying meridional canal. There may also
be ciliated papillae around the polar fields.

In both
Coeloplana and Ctenoplana each testis opens via its own duct and pore. In Coeloplana there are invaginated
epidermal sacs containing sperm, near to the openings of the testes These sacs presumably function as seminal receptacles
(?). Traces of such sacs also occur in
Ctenoplana. In Coeloplana there is an ovary and a testis in the wall of each meridional
canal. Developing eggs are attached to the oral (ventral) surface of the mother by a sticky secretion and each hatches into a
swimming cydippid larva with 8 comb rows. This larva settles and creeps about, the combs fall off and the stomodaeum everts
to form the ciliated ventral surface.

Tjalfiella is a flattened creeping platyctenean that is more or less sessile on the pennatulid Umbellula. Tjalfiella is flattened in
the oral-aboral direction and elongated along the tentacular axis with a tentacle and tentacle sheath at each end. Each end is
upturned and fused along the edges to form a ‘chimney’. There is a statocyst in the centre of the upper surface and no polar
fields, no ciliated furrows and no comb rows. The stomach gives off two large transverse canals that extend to the tentacle
bases and also give off two pairs of blind sacs (meridional canals ?) and one pair of canals that branch and extend to the
periphery and to the chimneys.

In
Tjalfiella there is an ovary and a testis in each of the 8 blind sacs, forming 8 bulges on the upper surface. On the aboral
surface there are ciliated epidermal sacs overlying the gonads. These sacs possibly function as seminal receptacles (?). The
eggs develop in brood pouches in the aboral surface and hatch into cydippid larvae. The larvae settle on a pennatulid host
and evaginating the stomodaeal wall to form the ventral surface.

The platyctenean
Gastrodes is a parasite in Salpa (a tunicate). It is a minute bowl-shape embedded in the host mantle. It
grows into a cydippid-type that leaves
Salpa and settles to the bottom (and sheds its comb rows, flattens out and everts its
pharynx). Eggs occur in the pharyngeal epithelium (ectodermal origin?) and hatch into planulae larvae that bore into a new
host.

Body Wall

The epidermis is syncytial or cuboidal-columnar and ciliated in certain regions. The epidermis is generally glandular and may
have gland cells interspersed with it. The epidermis may contain pigment granules or branched melanophores. The epidermis
has two types of sensory cell interspersed within it. The first type has several stiff bristles, whilst the second type has only one
sensory projection. Contractile sensory papillae may be present in some lobates, e.g.
Leucothea, and cestids. These papillae
are motile and extensible and have sensory and gland cells in their tips.

Ciliated furrows consist of rows of epidermal cells, each with 2 or 3 cilia. The areas between the combs of each row are
usually unciliated. In many Lobata a ciliated band connects adjacent combs.

Collenchyme

The collenchyme is an ectomesodermal gelatinous substrate containing scattered cells, connective tissue fibres, muscle
fibres, and possibly also nerve fibres (?). The collenchyme cells are all of ectodermal origin. The muscle fibres are derived
from amoeboid cells and are smooth and may also branch and anastomose.

There are usually longitudinal and circular muscle fibres beneath the epidermis and along the pharynx. There may be
sphincters around the mouth and statocyst. Radial fibres extend from the pharynx to the body wall. In the oral lobes of the
Lobata there is a crisscross arrangement of muscle fibres on the inner surface and the lobes are very contractile.

Tentacles

Each tentacle grows from its base in its sheath. The tentacles are solid, comprising an epidermis and a core. The epidermis
consists primarily of ‘lasso’ cells or colloblasts that are adhesive cells. Each colloblast consists of a hemispherical dome that
discharges sticky secretion involved in prey capture. The hemispherical dome is attached to the tentacle core via a contractile
spiral filament wound around a straight filament (the latter is derived from the cell nucleus). The tentacle core usually consists
of a central strand, which is possibly nervous (?), and a muscle sheath of mostly longitudinal fibres. The tentacle sheath is a
ciliated epidermal pouch.
The tentacle base is divided into three parts: a median region where the muscle bundles of the core originate, a pair of lateral
swellings that each contain the blind end of a tentacular canal, and an epithelium composed of colloblast mother cells.

Nutrition

The pharynx and certain areas of the stomach and gastrovascular canals are ciliated and generate a current through the
digestive tract. Other areas of the stomach and canals probably serve for intracellular digestion. Cell rosettes occur in the
gastrovascular canals. These consist of two circles of ciliated gastrodermal cells surrounding a small opening into the
collenchyme. The cilia of one circle beat towards the collenchyme, whilst those of the other circle beat towards the canal
lumen. These rosettes possibly have an excretory (?) or osmoregulatory function (?).

Ctenophores are carnivorous and eat small animals. Cydippids feed with their tentacles spread out in a fishing position and
catch prey by means of the sticky colloblasts and then the tentacle retracts and wipes the food onto the mouth rim.
Pleurobrachia eats small plankton, e.g. crab larvae, oyster larvae, copepods, fish eggs and arrowworms.

In the lobata,
Mnemiopsis leidyi uses ciliary action to bring prey into the auricular grooves where it is entangled by the short
tentacles, which pass the food into the four labial troughs in which it is conveyed to the mouth. In Mnemiopsis leidyi food
touching the oral surface is entangled in a mucous sheet and is also conveyed to the labial troughs by ciliary action and
hence on to the mouth. Typical prey includes mollusc larvae, copepods, minute plankton and debris.

Beroë has a large extensible mouth rim and is able to capture large prey, including other ctenophores and small crustaceans.
Tjalfiella is more or less sessile and has a functionless mouth. Instead food is ingested by the chimneys, which open into the
pharyngeal cavity. Some ctenophores, such as
Beroë, deploy toxins in prey capture.

Partial extracellular digestion occurs in the pharynx and is very rapid, with the pharynx emptying in 20-30 minutes into the
canals where food particles are phagocytosed for intracellular digestion. Indigestibles exit via the mouth or the anal pores.

Nervous System

The ctenophore nervous system is of the cnidarian type with a subepidermal plexus of multipolar cells and neurites. There are
strands of neurites (not true nerves) beneath the ciliated furrows and the comb rows and a ring of plexus around the mouth.
There is no CNS. Nerve fibres extend to the muscles.

Sensory Systems

The statocyst functions in comb row coordination and orientation of the animal with respects to gravity. There are also
sensory cells in the epidermis. Ctenophores are sensitive to chemicals, temperature and mechanical stimuli. The mouth rim is
especially chemoreceptive (especially in beroids).

In
Mnemiopsis the lips, auricles, the region around the main tentacles and parts of the oral lobes near to the mouth are
sensitive to clam juice. Stimulation of these areas with clam juice causes the ctenophore to stop briefly, by reversal of the
comb cilia, and then to secrete a mucous sheet to entangle food particles.

Comb rows may retract into the jelly when touched.

Many ctenophores rest (feeding posture?) by hanging vertically from the surface film, usually mouth uppermost and with the
combs inactive. If disturbed then they will resume this position by comb beats. A large disturbance, however, causes them to
rotate and swim rapidly downward, mouth first. Thus ctenophores swim downward in rough water (and possibly also in bright
light?).

Control of Comb Beat

Comb beat is under nervous control. Stimulation of the oral end causes cilia stoppage followed by reversal of beat.
Stimulation of the aboral end accelerates the normal beat. Displacements from a resting vertical position causes the cilia to
beat stronger on one side in order to restore the vertical position.

Cutting a comb row or a ciliated furrow across causes the two resulting segments to lose synchrony. Removal of the statocyst
results in a loss of coordination of the comb rows and an inability to maintain a vertical resting position.

Excretion

There is apparently no specialised excretory system, though perhaps the cell rosettes are excretory.

Respiration

There is no specialised respiratory system, though canal circulation may aid gas transport. Oxygen consumption is very low at
less than about 0.007 cm^3 per hour per g but is similar to that of other animals per g dry weight.

Reproduction

All ctenophores are hermaphroditic and many exhibit two periods of sexual maturity, one in the larval stage and one in the
adult stage, with the gonads degenerating between the two phases (dissogeny).

The gonads usually reside in the walls of the meridional canals as continuous or discontinuous bands, with the ovary on one
side and the testis on the other. Like gonads in adjacent canals face each other with the ovaries next to the principal planes.

The gametes exit through the mouth, except in
Coeloplana and Ctenoplana in which the testes open via ducts and aboral
pores. The gametes are possibly of endodermal origin (?).

Development

The sex cells are shed into the water and fertilised externally, except in Coeloplana and Tjalfiella which brood their young.
Cleavage is
biradial and the 8-cell stage is a curved plate of cells, the long axis of which becomes the future tentacular plane.
Cleavage is determinate and development is mosaic and results in a free-swimming cydippid larva. This larva resembles the
Cydippida adults and undergoes a variable degree of metamorphosis.
Gastrodes has a planula larval stage.

Regeneration

Ctenophores are fragile and so have high powers of regeneration. Removed parts are quickly replaced. Halves regenerate
with the half containing the statocyst regenerating faster whilst the other half regenerates the statocyst first. Portions may
close together without regenerating lost comb rows, which may sometimes be regenerated.

If
Lampetia is cut into three or more transverse pieces, then the more aboral piece regenerates faster. Separate pieces can
fuse together and if they are grafted with opposite polarity then each piece retains its own polarity. A grafted statocyst inhibits
regeneration of the host’s statocyst. The statocyst is dominant.

Asexual Reproduction

Asexual reproduction may possibly occur by fission (?). In Ctenoplana and Coeloplana small fragments detach as the animal
creeps and these fragments regenerate into new individuals.

Bioluminescence

Adults luminesce beneath their comb rows (from the outer walls of the meridional canals?). Pieces with four or more combs
luminesce. Luminescence begins in early cleavage stages and continues throughout life, but is only switched on after some
time in darkness.
The ctenophores or comb jellies are amongst some of the most beautiful of the oceans' jewels, coming in a wide variety of shapes and
sizes. Considered as 'jellyfish' by some, the ctenophores are very different from the true jellyfish (the scyphozoan cnidarians) - they
have little jelly, being more delicate and glass-like in appearance and being propelled, not be jet propulsion, but by 8 'comb rows' of
groups of cilia that propel the animal by beating in synchrony. These cilia tend to diffract incident light, creating a display of dancing
colours.