Building Bodies of Jelly - Jellyfish
The images of the jellyfish above were created in Pov-Ray and represent a 'typical' jellyfish. Of course there
is really no such thing, jellyfish come in an incredible range of varieties and range in size from one centimetre
or less in diameter, to over two metres in diameter and some, like the Lion Mane's jellyfish, may reach half a
tonne in weight. Despite their beautiful and enchanting appearance, jellyfish are highly efficient predators. In
the early oceans on the primordial Earth the jellyfish were the top predators, and even today they are vastly
abundant and often swarm in thousands. To produce so much mass at such a prolific rate jellyfish must
clearly be
very efficient predators.

The term 'jellyfish' is an imprecise term that refers to an enormous variety of creatures from those animal
groups called the
cnidarians (which includes corals and sea anemones) and the ctenophores (comb
jellies). The jellyfish are those members of these two groups (which are sometimes collectively called the
coelenterates) which swim or float freely. These include the medusas, like the one above, named after the
Greek myth of a beautiful woman who was cursed by a jealous goddess and left with a hideous appearance
and writhing serpents for hair - these serpents have been compared to the jellyfish's tentacles. They also
include forms like the Portuguese Man O' War which consists of a colony of many individuals fused together
into a single organism. This page deals mostly with the medusae, and specifically those that are most
commonly found washed up on seashores - the
scyphozoan medusae (Scyphomedusae).

Jellyfish Anatomy

Look at the image above and the other viewpoints of the same model shown below (click on thumbnails to
enlarge) and then look at the labelled diagram of a similar jellyfish,
Aurelia, the Moon Jellyfish, and see if you
can use this diagram to identify some of the labelled structures on our 3D model!
Medusa: plan (exumbrella) view.
Medusa: underneath (subumbrella or oral) view.
Medusa: sideview.
Medusa: sideview
A labelled diagram of Aurelia, the Moon Jelly, probably so-named both for its whitish disc-like body and its
nocturnal habit of swimming near the surface of the sea. Identify as many of these labels as you can on
the 3D jellyfish medusa model before we look at what these structures are and what they do.
The main part of the medusa body is the bowl, dome, lantern, cuboidal, goblet, trumpet or disc-shaped bell
or umbrella. The domed surface of the bell, which is topmost, is called the
exumbrella, whilst the lower
surface, which ofter curves inwards, is called the
subumbrella. Contractions of the bell, cause it to pulse
and expel
jets of water from the cavity beneath the umbrella (the subumbrellar cavity) and these jets
propel the jellyfish along. It is often said that jellyfish are weak swimmers and at the mercy of the tides and
even that they can only swim upwards and sink downwards. However, much film footage clearly shows jellies
swimming horizontally as well as vertically and many are strong swimmers (particularly the agile Box Jellyfish), but not as strong as large fish and so they do sometimes get caught up in strong tidal currents, but they are better and more precise swimmers than most give them credit for. What they do lack in power they make up for in efficiency. Kinematic studies have shown that jellyfish swimming is extremely energy efficient. This is especially important as they must swim in order to feed. Indeed, the vortices generated by bell pulsation draw food towards the jellyfish! The bell contains a thick ring of strong muscle, called the
coronal muscle, that generates most of the power. Other more complexly arranged muscles assist the coronal muscle.

Hanging from the centre of the subumbrellar is a projection, called the
manubrium, which bears the mouth
at its terminus. The mouth is often surrounded by four
oral arms, though these may be absent and
sometimes number a multiple of four, such as 8, depending upon species. Hanging from the edge of the
underside of the bell, are the
tentacles. Some species lack the tentacles, some have hundreds of tentacles,
others only four tentacles, some species have very short tentacles (like Aurelia) others have tentacles many
metres in length.

How do jellyfish feed, grow and reproduce? How do jellyfish know which way is up? Where is the jelly?
Jellyfish diagram
Above: the jellyfish Cyanea. Note that the tentacles have been cut away from 4 of the 8 sectors for clarity. There
are several species of
Cyanea, Cyanea capillata is the lion-mane's jellyfish. There are 16 tentacles per cluster in
this specimen, but there may be as many as 150 per cluster. The lion's-mane has 8 lobes and 8 rhopalia and the
bell diameter may reach 2.5 metres! The colour varies from yellowish to deep red or reddish-purple. The tentacles
Cyanea can be up to 10 to 30 metres or more in length and are very sticky, and they can be fanned-out to form
a massive fishing net that the jellyfish trawls through the water. Click the image to enlarge.
Cyanea lamarckii is up
to 15 cm or more in diameter (I once saw what was almost certainly a specimen of this jellyfish some two to three
feet across) and in this species the 8 primary lobes are divided into pairs of secondary lobes which divide at the
edge into pairs again, making a total of 32 lobes.
Cyanea lamarckii is whitish-blue in colour (the specimen that I
saw was a striking translucent sea-blue colour).
Where is the jelly?

The bell of the jellyfish is essentially two layers of cells, one on the outside surface, called the epidermis,
and another which follows the lining of the subumbrella as it extends down the pendulous protuberance and
enters the mouth, at which point the cell lining takes on different characteristics and is called the
gastrodermis. This inner cell layer, or gastrodermis, continues to line the stomach. The stomach in Aurelia,
and in our model jellyfish, is divided into a central chamber and four pouches coming off the sides. These
pouches contain the gonads (reproductive organs that produce sperm and/or egg cells). The gonads are
visible in our model as the four pinkish horseshoe shaped structures in the centre of the bell.

Beneath these two layers of cells, the epidermis and the gastrodermis, the main bulk of the jellyfish is made
up of a jelly-like substance called mesogloea. In some tiny jellyfish, the mesogloea may be little more than a
thin sheet, but in large jellyfish it becomes a thick mass. Cells that develop from the epidermis and/or
gastrodermis of the developing baby jellyfish, migrate into the jelly (especially in the larger types) and form
muscle and nerve cells as well as wandering amoeboid cells, that resemble
amoebae and wander around the
body. Thus, the mature animal (especially in the larger jellyfish) contains more than simply two layers of cells!

Radial Symmetry and the number 4

Jellyfish of the medusa type we are considering here, have what we call radial symmetry - meaning they are
essentially circular (or spherical). A human, on the other hand, has bilateral symmetry - meaning that your
body is in two mirror halves and has a definite front end and back end. Jellyfish are also built on around the
number 4, with most of their structures occurring either in 4s or in multiples of 4, such as 8 or 16 etc. Thus,
tentacles may number 4, 8, 16, ..., to 8 x 40 = 320 or more. Our model has 4 gonads, 32 (8 x 4) lappets (the
crinkly projections along the bell margin), 4 oral arms, etc.

Knowing which way is up and where things are

The rhopalia (singular rhopalium) are the small pink structures, 8 in number in our model, which can be
seen located around the bell margin at regular intervals, between lappets. These are sensors. Each
rhopalium contains a gravity sensor, which allows the jellyfish to tell which way is up and which way down,
and to know how much its body is tilted. These organs may also contain what look like olfactory (smell)
sensors and in some species each rhopalium has a tiny eye. These eyes may be simple light sensors, or
they may be complex eyes equipped with a lens. Some jellyfish do not have eyes, but even these can detect
light by other means.

Scyphozoan jellyfish avoid bright sunlight, descend deeper into the water at midday and in darkness, but
surface in the morning or late afternoon and during cloudy days. Thus, most jellyfish medusae prefer twilight
or diffuse light, though some do prefer sunlight. Medusae also descend into the water during rough and
stormy weather.

What no brain?

Jellyfish have no obvious brain, as a large mass of nerve cells, but they clearly possess sophisticated
computers. What some do have is a marginal
nerve ring which connects to the rhopalia and little ganglia
(dense balls of nerve cells) each associated with one rhopalium, and they also have a nerve net. The
, or plexus, is a network of neurones (not true nerves) that cover the subumbrella (and sometimes a
nerve net or plexus that covers the exumbrella) just beneath the surface. These structures function as a
sophisticated computer, not as complex or as sophisticated as the mammalian brain, but sufficient for the
medusa's needs. Note: true highly integrated nerve rings are found in
hydromedusae and cubozoa (e.g. sea
wasp)though not in most scyphomedusae (true jellyfish like
Aurelia and Cyanea) although the nerve net
tends to form circular bands overlying the coronal swimming muscle in scyphomedusae also.

Nerve rings occur in cubomedusae (cube or box jellyfish, e.g. the Sea Wasp) and also in the related hydromedusae (not considered true jellyfish by zoologists, due to their much thinner layer of mesogloea giving them a more glassy appearance, but colloquially also often referred to as 'jellyfish'). In jellyfish like Aurelia and Cyanea there is no compact nerve ring, but neurones do form circular networks around the bell margin, what we may call a neuronal ring. This, along with the rhopalia, is as close as jellyfish get to a 'brain' but is not a true brain since it does not consist of centralised ganglia (though each rhopalium may be innervated by a ganglion). Each rhopalium acts as a pacemaker to synchronise swimming muscles to produce coordinated and appropriately-timed bell pulsations when swimming. Mathematical models demonstrate that having several pacemakers connected in series, in this fashion, improves the degree of synchrony and the precision of the pacemaker system. At any one moment, one rhopalium dominates the others, but this changes randomly in the absence of stimulation. Otherwise, the most strongly stimulated rhopalium becomes dominant.

Common myth - jellyfish have no skeleton

Whilst it is true that jellyfish are soft to the touch and have no hard bony parts, they do have the mesogloea.
Stiffening fibres traverse the jelly and in some jellyfish, the mesogloea can form hardened plates, rather like
cartilage, that hinge together. These plates provide support for the animal and the muscles may attach to
these plates, so they function as a skeleton. Obviously, the jellyfish skeleton of jelly, of more or less
firmness, is not as hard as the mammalian bony skeleton, nor as hard as the cartilagenous skeleton that
sharks have, but it is still a skeleton, albeit more or less soft, and is sufficient for the jellyfish which does not
move the bulk of its body quickly in complex ways and can rely on the surrounding sea water to buoy up and
support its body.

Making a living - the Jellyfish's Sting

The tentacles, and sometimes other surfaces of the jellyfish, are armed with stinging cells called
nematocysts. These nematocysts are grouped into stinging batteries. Each cell, when triggered by the
touch of potential prey (or a predator), discharges a tiny thread which is a miniature harpoon that impales
the victim and injects venom. A prey item, such as a fish, will be injected with dozens of these harpoons.
Other nematocysts discharge sticky threads to trap the prey. There are many different types of nematocyst
found in the coelenterates, and which type or types an individual has depends upon species.

Each tentacle can be moved by its own muscles, as can the oral arms. The tentacles and/or oral arms pass
the captured food to the mouth. Once in the stomach, the food is digested into a broth within about six hours.
This liquid is transported around the animal by the
circulatory system. This consists of radial canals that
radiate away from the stomach and then connect to the
ring canal (if present) shown as a pink ring in the
model, and then back to the stomach, with remaining waste being carried out through the mouth (jellyfish
have no separate anus!). These canals together with the stomach (gastric cavity) form the
gastroendodermal system. In some jellyfish the stomach gives off complex branching canals, in others just
four straight radial canals are apparent. The gastrodermis lines these canals and each cell possesses a
flagella, a long (but microscopic) whip-like structure that stirs up the water, creating specific currents that flow
in the desired direction, transporting the broth around the body, along with sea water that enters through the
stomach. This circulatory system probably also transports dissolved oxygen around the body and removes
waste, including carbon dioxide.

Aurelia feeds in a different way. Tiny planktonic creatures, including molluscs, crustaceans, eggs, minute
worms and larvae, collect on the exumbrella surface, where they become trapped in mucus. Tiny beating
hairs (cilia or flagella) carry the food-laden mucus to the edge of the bell, where it collects in eight masses (in
the centre of the lappets) where it is licked off by the oral arms and carried by tiny hairs along a groove that
runs along the inside of each arm, through the mouth and into the stomach. The food is partly digested by
the stomach and then carried along 8 straight (ad)radial canals, along the ring canal, and back to the
stomach along the branched radial canals. Outward currents generated by tiny beating hairs on the oral
arms, carry the waste out through the mouth, as inward currents bring more food in. This is very efficient, for
a single
Aurelia medusa can clear the plankton from 700 ml of water in less than one hour and it doesn't
have to do very much, just wait for the food to stick to its body as it swims past!

The stomachs of jellyfish generally consist of four pouches or less-distinct lobes (the
coronal stomach)
radiating from a
central stomach. The 4 muscular septa which divide the stomach into its 4 chambers are
pierced by a circular opening (septal ostium), forming a ring sinus which connects the 4 chambers together.
The inner edge of each septum is equipped with 2 rows of gastrodermal tentacles, called
gastric filaments,
which consists of  a mesogloea core lined by gastrodermis equipped with nematocysts and gland cells. The 8
rows of gastric filaments project into the central stomach. This gut pattern occurs in the adults of some
species, whilst in others (including
Aurelia and Cyanea) it occurs only in the scyphistoma larval stage and is
modified in the adult: the septa disappear, leaving a central stomach which may be slightly scalloped into
four chambers. The gastric filaments now spring from the stomach floor, in rows or groups in an interradial
position (in-between the four main radii).

Most jellyfish, however, are fierce hunters, trapping and eating animals as large as fish. The huge Lion
Mane's jellyfish has a vast tangle of tentacles that sweep the oceans like fishing nets, spanning an area the
size of a tennis court. No wonder these Lion Mane's jellies often reach half a tonne in weight! In some
jellyfish, the oral arms are highly branched feathery structures, whilst in others these arms fuse to form a
conical structure, which may be truly massive in some jellyfish, and which contains hundreds of frilly mouths!

Reproduction - churning out new jellyfish

Most jellyfish are dioecious, meaning that individuals are either male or female, but some species are
hermaphrodite (having both male and female gonads). Jellyfish typically ripen in spring and summer. The
eggs develop either in the gonads, or in pockets on the oral arms (after being released from the gonads
through the mouth) depending on species. Each egg produces a tiny larval creature, called a
planula, which
escapes and swims away with the help of tiny beating hairs (cilia) that cover its surface. The planula is either
hollow or solid. After a short planktonic existence, during which the planula may travel great distances, the
planula attaches to a solid surface, such as a submerged rock, and develops into a small trumpet-shaped
organism called a
scyphistoma. In some species the planula puts out stolons (shoots) which bud new
scyphistomes at intervals and then detach. If the scyphistoma moves about, stolons may detach and each
fragment can regenerate into a new scyphistoma. In the related Stauromedusae, in which the adult is sessile
and attached to the substrate by a stalk, the planula is vermiform (worm-like) and may put out 1 to 4 stolons
may detach as vermiform creeping larvae which eventually develop into stalked sessile larvae.

The scyphistoma develops tentacles around the mouth which is on its top (apical) surface. These tentacles
catch tiny food items, with the help of nematocysts, and so the scyphistome eats and feeds, rather like an
upside-down jellyfish stuck to the rock, but no more than a few centimetres long. The scyphistoma may
bud-off new scyphistoma (asexual reproduction) or grow stolons which bud off new scyphistomae. In winter
or early spring, the scyphistoma starts to split up into a stack of discs, rather like a stack of plates, a process
strobilation. This stack of disks is called a strobila. One by one each disc detaches from the end of
the strobila and become a tiny jellyfish, slightly different from the mature form, and called an
Depending on species, the scyphistoma may bud off one ephyra and then regenerate its tentacles before
later budding off another ephyra, and so on. This is called monodisk strobilation. Others undergo polydisk strobilation, in which the scyphistoma fragments into a stack of plate-like structures, the most distal (tipmost) detaching first ad the more basal develop. Ephyra detachment involves muscular constriction.

Each ephyra is only a few millimetres in diameter, but will feed and grow, and if it survives then it will become
a mature jellyfish, possibly weighing as much as half a tonne. Scyphistomae may live for several years,
strobilating each winter, and feeding each summer. In this way, each scyphistoma is like a jellyfish factory,
churning out dozens of jellyfish! Note that the life-cycle of some jellyfish is very different from that just
described, and indeed is unknown for many.

Where to see more jellyfish

It is impossible to do justice to the diversity, complexity and beauty of jellyfish in a couple of pages! However,
a search on Google will reveal dozens of stunning photographs. One of the best accounts ever written about
jellyfish, including many beautiful diagrams, is that given by Libbie Henrietta Hyman in her 1940 volume 1 of
The Invertebrates (unfortunately not in print at the moment!). Libbie Hyman was one of the greatest
zoologists of all time and motivated by the sheer appreciation of the beauty of living things to produce one of
the best series of zoology books ever written. The standard of this work is an example to all scientists and is
one of the best scientific works ever produced. It is unfortunate that she never lived long enough to complete
her review of the invertebrates, but then that's hardly surprising when one considers how many different
types of invertebrate there are! There are more living wonders on Earth than any individual can ever live
long enough to see, study and appreciate, but just to see some of these creatures is well worth the while! If
you don't get the chance to travel and see these wonders or maybe you can't travel to see these wonders,
there are a lot of ways to still see these creatures. Visit you local library or rent textbooks, or search the Web
where there are lots of resources. One other way could be to check out your local aquarium.

The life-cycle of a scyphozoan such as
Aurelia. Click images to enlarge.
ephyra model

Above and below: a Pov-Ray model of an ephyra larva of a jellyfish like the moon jellyfish.

ephyra anatomy
jellyfish v2
jellyfish swarm
Above: a strobila strobilating. This is an example of polydisk
strobilation, in which the scyphistoma has constricted itself into a
series of developing plate-like structures which are released at the
tip as young ephyrae. The ephyrae are voracious feeders, eating protozoans (and/or small hydromedusae and ctenophores in Aurelia). Food is caught by the lappets, entangled in mucus and then the lappets curve inwards to deposit the food on the manubrium. Flagellary ('ciliary') currents on the exumbrella carry small food particles to the margin where the lappets can reach them and similar currents on the subumbrella drive particles from the margin to the manubrium. In the gastrovascular canals of the ephyra, currents pass down the lappets along the dorsal surface and centrally along the floor.
Comment on this article!
jellyfish planview, Pov-Ray model
jellyfish oral view, Pov-Ray model
aurelia sideview, Pov-Ray model
aurelia sideview, Pov-Ray model
A 3D computer (Pov-Ray) model of the Lion's Mane jellyfish, Cyanea capillata. These organisms are phenomenal
fishing machines, trawling the seas for fish and other prey when they extend and spread their vast net of tentacles.

The model illustrates the 8 primary lappets of the bell margin, the 4 frilly oral arms and 8 V-shaped clusters of
tentacles (16 tentacles per cluster in this case, though this number is highly variable in life). The fishing tentacles
are being deployed.
Article updated:
9 Jan 2016
30 Jan 2016
31 Jan 2016
25 Dec 2016
3 Feb 2018
29 Dec 2018
5 Aug 2019
10 Aug 2019
11 Aug 2019
12 Aug 2019
13 Aug 2019

strobila, Pov-Ray model
strobila, Pov-Ray model
Above: a model of the moon jellyfish, Aurelia.
Aurelia, Pov-Ray model
Aurelia jellyfish cutaway diagram
Jellyfish cutaway diagram to label
Label your own jellyfish diagram!
Scyphistoma, Pov-Ray model

Above: a scyphistome feeding. This polyp-stage larva developed from a planula which attached to the rock.

Above: subumbrella view of Aurelia. Each of the 4 gonads occur in the stomach floor and hang down into
the subumbrella cavity. Many jellyfish have subumbrellar funnels, 4 deep pits or invaginations in the
subumbrella (which occur on the interradii) of unknown function, though water flows in and out of them as
the bell pulses (so they may be respiratory, excretory or chemo/thermosensory). In other jellyfish,
Aurelia, the funnels disappear during the course of development to be replaced by shallower
depressions, called
subgenital pits. These are visible in the diagram as a small circle in the subumbrella
inside each arc-shaped gonad.

The most familiar jellyfish, especially to those living in temperate regions, belong to the order
Semaeostomeae. This includes
Aurelia (e.g. Moon Jellyfish) and Cyanea (Lion's-mane Jellyfish).
Aurelia medusa, submumbrella view
Aurelia, submubrella view, unlabeled
The cnidarians (a type of colenterate) are diverse organisms and include hydra, sea anemones, true
corals and the true jellyfish. The true jellyfish, like
Aurelia, are so-called because the bulk of their bodies
are composed of gelatinous or cartilaginous
mesogloea (shown in blue in the above diagram). Jellyfish
alternate between attached and often stalked larvae (see below) which resemble hydra (the polyp stage)
and the sexually-reproducing and free-swimming medusa (named after the woman of Greek legend who
had her hair turned into snakes by goddesses envious of her beauty, in reference to the 'tentacled-head'
appearance of jellyfish). The medusae of the more familiar jellyfish are dome, saucer or bell-shaped with a
number of
tentacles hanging down from the edge. The upper surface is the exumbrella, whilst the
undersurface is referred to as the
subumbrella. The mouth hangs down from the subumbrella, on a
pendulous structure called the
manubrium. The four corners of the mouth are typically drawn-out into
oral arms.
The model below is an older Pov-Ray jellyfish computer model, which illustrates the main
anatomical features.
Cyanea jellyfish, PovRay model
Lion's-mane jellyfish, Pov-Ray model
Jellyfish lifecycle
Above: a 'typical' scyphozoan jellyfish life-cycle. In reality, there are many differences depending on species. The more ancient trend is for external fertilisation: eggs and spermatozoa being shed into the sea at the same time (as may be determined by a lunar cycle, for example) to increase odds of fertilisation. However, there has been a tendency for some species to evolve internal fertilisation, with eggs being retained in the ovaries or in brood pouches in pockets formed by the oral arms. In some cubozoa (Marques et al. 2015) there is even courtship and the male uses his tentacles to manipulate the female and transfer spermatophores (packets of spermatozoa) to her, which she take inside. Other modifications include the direct metamorphosis of the planula into an ephyra, as in Pelagia missing out the scyphistoma and strobila stages.
The Importance of Sleep: Sleep and Vision in Jellyfish

Cubomedusae, such as Chironex fleckeri, the Sea Wasp, are particularly active swimmers, capable of swimming at up
to 0.5 m/s and changing direction with considerable agility. The box-shaped body of the cubomedusae, up to 24 cm in
diameter in
C. fleckeri, has four pedalia, or muscular fleshy pads, one at each corner of the bell and each bearing
one or more tentacles. Midway between each pedalium is a rhopalium (sensory stalk) hanging down inside an
indentation in the bell margin. Each rhopalium bears a gravity-sensing statocyst and a cluster of 6 eyes (making 24
eyes in total). The eyes are of four different types: two pit-eyes, two slit-eyes and two lens-bearing camera-type eyes
of different sizes. The camera type eyes are complex, each equipped with a cornea, pupil, lens, a vertebrate-like
retina and pigment layer. This suggests that they are capable of image formation. Kavanau (2006) suggests that
these eyes are used in hunting for prey. The more hunting these jellyfish do, the more sleep they need, whereas
those which are 'hand-fed' in aquariums may need no sleep at all (for as long as 9 months in one case). During sleep
they may lie motionless on the sea floor, with their tentacles touching the bottom. They may also rest and perhaps
sleep upside-down, with their tentacles and captured food in the cavity of the bell. Individuals tracked in nature may
sleep for 2 to 15 hours each day. The hypothesis is that jellyfish rest upside-down whilst they remove food from their
tentacles and digest their food.

This rest-and-digest behaviour may have evolved into sleep in the Cubomedusae because they need to rest their
nervous systems. Their central nervous system consists of a nerve ring connecting the rhopalia and pedalia. The
notion is that the vast computational demands placed upon the visual systems of actively hunting cubomedusae
prevents their nervous systems from carrying out general housekeeping tasks, including memory formation (which
requires changes to the synapses). Visual processing is certainly a very demanding computational task (which is why
our computers have dedicated video cards) and processing images would probably saturate much of the capacity of
jellyfish nerve nets. The mammalian brain faces similar problems: it was discovered only a few years ago that during
sleep glymphatics open-up more to flush the brain, washing it and removing toxic waste products and leaked
neurotransmitters that accumulate during the day. (Glymphatics are channels around the smaller blood vessels of the
brain which carry circulating cerebrospinal fluid, CSF). Sleep is also evidently important for memory formation in
mammals. Combined with these recent and important studies in jellyfish, it would seem that the 'mysteries' of sleep
are indeed unraveling!

Cnidarians are of course worthy of study in their own right, but they have also been extremely useful model systems in biology and biomedical sciences. They have shed light on the evolution and development of animals, as well as the functioning of nervous systems (they were among the first nervous systems studied in depth) and they have even inspired robotics due to the efficiency of their locomotion. They are also important ocean predators. It is a pity that funding for this 'grass roots' science is increasingly hard to obtain, since those who control the finances want research to be increasingly justified by a narrow, and often commercial scope. By asking scientists to justify their research by immediate economic or medical value, we are in danger of missing out on important basic understanding. There should always be some funding set aside for 'pure science'.

References and Bibliography

Kavanau, J.L., 2006. Is sleep’s ‘supreme mystery’ unraveling? An evolutionary analysis of sleep encounters no
mystery; nor does life’s earliest sleep, recently discovered in jellyfish.
Medical Hypotheses. 66(1): 3-9.

Marques, A.C., J. Garcia and C. L. Ames, 2015. Internal fertilization and sperm storage in cnidarians: a response to Orr and Brennan. Trends in Ecology 30(8): 435-436.

Hyman, L.H. 1940. The Invertebrates: Protozoa through Ctenophora. McGraw-Hill Book Company, inc. New York and London.

Types of Scyphomedusae

For those who are curious, the following is a detailed account of selected scyphozoans, to give the interested reader a better feel for jellyfish!

Note: the positions of organs and appendages in medusae are described according to the radius they lie along. In Aurelia, the four perradii are inline with the four stomach or gastric pouches (gastric pockets), the corners of the square mouth and the four oral arms (the oral arms may branch). The four interradii are situated between the perradii and align with the septa (the tissue in between the four gastric pouches) and the subumbrellar funnels. The adradii are situated between the perradii and the interradii. Additionally, a structure may be centripetal (towards the centre of the bell) or centrifugal (towards the margin) or on the margin (marginal). For appendages, proximal refers to the basal part of the appendage near to the main body, distal to the other end further from the main body.

1. Somaeostomes

Moon jellyfish (Aurelia aurita)

Moon jellyfish, cutaway

Somaeostomesinclude the Moon Jellyfish (Saucer Jellyfish) Aurelia aurita ( = Aurellia aurita). The medusa of this jellyfish reaches up to 25 cm in diameter. The bell is divided into 8 principal velar lappets alternating with 8 marginal sensory appendages (rhopalia and associated structures) one between each pair of velar lappets. Short hollow tentacles arise at the bell margin (just above the bell edge) and these alternate with an equal number of small marginal lappets. Each marginal sensory appendage or rhopalium is situated between two narrowed marginal lappets, called rhopaliar lappets. Each is also equipped with an ocellus (simple eye) of ectodermal origin at its proximal end, which faces outwards and also a pigment-cup eye of endo- and ectodermal origin on the inner surface, facing inwards towards the oral arms. Many somaeostomes lack ocelli, however. Typical of scyphomedusae, a hood of tissue covers  the rhopalium on the upper (exumbrella or outer) side. This hood is equipped with an exumbrella pit lined by sensory epithelium whose cells have sensory cilia. This is situated proximally on the top of the hood or just behind (above) it. Typically, a similar ciliated pit is located on the subumbrella surface at the base of the rhopalium, a subumbrella pit (I am not certain if this latter occurs in Aurelia). In fact, there are two exumbrella sensory pits in Aurelia: an outer pit as just described (at the top of the hood) and an inner pit on the proximal end of the rhopalium just beneath the hood. The end of each club-shaped rhopalium is filled with weighty crystals (statoliths) and acts as a gravity sensor ('organ of equilibrium'). The rhopalia are situated on the inter- and perradii.

Moon jellyfish, cutaway

The corners of the four-sided central mouth protrude as four thick gelatinous oral arms (mouth arms). Each oral arm has a gutter or trough along its length on the inner (centripetal) surface facing the mouth. The edges are frilly, giving 8 frilly edges in total, and lined by small tentacle-like protuberances. The oral arms are perradial. The four horseshoe-shaped interradial gonads open via canals on the stomach floor where eggs pass out from the mouth to the oral arms where they are brooded in brood pouches formed by the frilly margins until they develop into free-swimming ciliated planulae which then escape.

Jellyfish typically generate vortices as they swim. locomotion in Aurelia aurita has been studied in detail. Contraction of the bell generates a toroidal starting vortex. The recovery stroke (bell expansion) generates a stopping vortex of opposite rotational sense. Starting and stopping vortices merge in a side-wards oriented supervortex which induces flow of surrounding sea water into the subumbrellar cavity, from which food is captured by the tentacles and oral arms, and also downstream to produce thrust. With the correct spacing, the generation of such vortices by pulsatile swimming can generate as much as 150% of the thrust of a continuous jet. Trainling vortices also form in the centre of the toroidal vortices, as water is accelerated, but these reduce efficiency. Pulsed jets produce fewer trailing vortices than continuous jets, so pulsed jetting also increases efficiency (i.e. reduces drag and reduces energy expenditure). Jellyfish swimming may not especially fast or powerful but it is remarkably energy efficient.

Lion's-Mane Jellyfish (Cyanea capillata)

Similar vortices also occur in the Lion's-Mane Jellyfish (Cyanea capillata) which also serve to drive food towards the mass of tentacles. Cyanea is also a somaeostome. This species occurs in several 'varieties' which are sometimes designated as separate species. This is a North Atlantic coldwater jellyfish. The varieties are distinguished by geographical distribution, size and colour, but these are variable and not stable characteristics. The Arctic form, Cyanea capillata var. arctica will be described in detail and occurs off the North American coast, north of Cape Cod, in summer. This is the largest known species of jellyfish reaching a bell diameter of 230 cm (though such giants are not common). The bell is usually a rich brown and yellow but very variable in colour. The muscles and tentacles are a rich rosin or yellow colour. The margin of the lenticular (lens-shaped) bell is divided into 8 main lobes by 8 deep adradial clefts. A median cleft in each main lobe results in 16 lobes, but these are further divided by smaller notches, one on either side of each median cleft, to give a final total of 32 lappets. The 8 median clefts (per- and interradial) each contain a hollow sensory rhopalium covered above by a web of tissue between adjacent lappets. The club has a ventral swelling covered in wart-like protuberances and papillae then extends into a distal tube ending in a knob containing statoliths. There are no ocelli. There are 8 adradial crescents on the subumbrella, midway between the margin and centre of the bell, with the horns facing outwards, bearing 5 concentric rows of tentacles in each. the innermost row contains the oldest and longest tentacles. There are about 800 tentacles in total. The tentacles are hollow and highly contractile, extending for a length of about 25 times the bell's diameter when fully extended (suggesting a maximum length of about 58 m (but more typically up to 37 m).

Four long perradial oral arms extend from the four-cornered mouth which is situated in the centre of the subumbrella. The edges of these arms are greatly folded to form curtain-like oral fringes, hanging down beneath the bell and about as long as the bell is wide.

Each of the four interradial gonads resides in a complexly-folded pouch on the subumbrella floor. Clusters of gastral cirri (gastric filaments) project from the bases of the gonads into the stomach cavity. The coronal muscle forms a ring centrifugal to the gonads and is about as wide as one 8th the bell radius. It is made up of 16 trapezoidal circular muscle blocks, those 8 in the rhopaliar radii being about half as wide as the other 8. The stomach is a central lenticular chamber giving off 16 radial pouches on its outer side and numerous branched canals which ramify throughout the bell without anastomosing. The 8 pouches in the rhopaliar radii are about half as wide as the other 8. The canals are continuous with the hollow interiors of the tentacles and rhopalia stalks and with the gonads. There is no radial canal as there is in Aurelia.The stomach has deep clefts in its aboral floor.

Cyanea jellyfish, bell structure

The diagram above illustrates the coronal muscle, radial muscles and circulatory canals.

The eggs of Cyanea capillata var. arctica are orange and pass from the ovary into the gastric cavity, to which the gonads are connected, and out through the mouth to be brooded in the folds of the oral arms and escape as planulae. The planulae eventually settle via their anterior end and open a mouth at their posterior. The scyphistoma initially develops 4 tentacles, then later 15-20 tentacles in total and may put out stolons from which secondary scyphistomae may develop. after a variable amount of time, but as short as 18 to 20 days post-attachment strobilation occurs. The young ephyra are only 3.5 mm in diameter and have a 4-cornered mouth and a lenticular stomach with 16 radiating pouches. The four lips will eventually develop into oral arms. The young medusa, about 7 mm in diameter, rarely surfaces and will spread its oral fringes out over the bottom and sides of an aquarium and is largely sedentary. The young medusae and scyphistomae feed on protozoa, starfish larvae and mollusc larvae.

The variety ferruginea is thought to be a variant of the arctica and is found off the North Pacific coasts of America and Asia and reaches 45 cm in diameter with a yellow/orange bell. The stomach and their radial pouches are light brown, the gonads yellow and the tentacles reddish.

Cyanea capillata var. capillata has a bell diameter usually between 50 and 120 cm. A young medusa at 13 mm in bell diameter has about 7 tentacles in each adradial cluster, the middle one being the longest and oldest. By a diameter of 86 mm this has increased to 63 tentacles in each cluster. The bell, palps (oral arms) and tentacles are reddish or yellow-brown and the gonads a red or rose colour. This form occurs in the English Channel, North Sea, and coast of Norway in summer and autumn. Although this form may wash ashore on the British isles, the largest species of jellyfish found off the coasts of Britain is Rhizostoma pulmo since the largest arctic form of Cyanea capillata does not occur in this region. This explain why the largest species of Britain's coasts is often said to be the Lion's-Mane Jellyfish. The variety postelsii is thought to be a local variety of capillata found in the North Pacific to the Aleutian Islands to Oregon.

Cyanea lamarckii (= Cyanea capillata var lamarckii)

The taxonomy of Cyanea capillata is not settled.The definition of an 'animal species' is a population of animals that can interbreed to form fertile offspring (or else reproduces asexually). To what extent the forms of Cyanea interbreed is not known. If they can interbreed, then they are strictly varieties or subspecies; if not then they are distinct species. Cyanea lamarckii (The Bluefire Jellyfish or Blue Jellyfish) has a blue, whitish or violet bell (occasionally yellowish) and oral arms, the bell being darkest at the centre. It typically reaches a bell diameter of about 15 cm. It has about half as many tentacles as the Lion-Mane's Jellyfish (var. capillata) and larger gonads for its size.

A mystery solved?

I once found a blue 'Cyanea capillata' that measured about 1 m in bell diameter stranded off the coast of wales. There are occasional reports of other similar strandings. This could be a very large Cyanea lamarckii or a blue form of Cyanea capillata (var. capillata) or perhaps they do interbreed? In any case, it was a remarkably beautiful specimen.

Cyanea capillata var. fulva (= Cyanea fulva)

A small yellowish form, found off the North American coast from south of Cape Cod to the Carolina coast. The bell diameter rarely exceeds 20 cm.

Cyanea capillata var. nozakii

Similar to var. fulva, but milk-white and found in the inland Sea of Japan.

Cyanea capillata var. versicolor

A pink form and perhaps a southern variety of Cyanea fulva. found off Cape Hatteras (North Carolina) to south Florida.

2. Rhizostomes

Rhizostome jellyfish

Above: the rhizostome medusa Mastigias (based on Hyman, 1940, The Invertebrates). The mouth arms bear frilly mouths and end in terminal appendages. Printable version.

Rhizostome medusae have no marginal tentacles and have numerous mouths on 8 fleshy adradial branched oral arms. The lips of the mouths are bordered by minute mobile tentacles. The mesogloea is tough and these jellyfish often reach a large size. The following description applies to the mediterranean Rhizostoma pulmo. Rhizostomes are generally tropical water jellyfish, but Rhizostoma pulmo extends far into temperate waters and is the largest jellyfish found off the coast of the British isles (this is var. octopus = Rhizostoma octopus) and is also called the Barrel jellyfish or Dustbin-lid Jellyfish on account of its size with the bell frequently reaching 60 cm in diameter.This variety is found along the coasts of France, England, Scotland, Belgium, Holland and Germany. Rhizostoma pulmo has 80 lappets (96 to 112 in var. octopus in which the lappets are also more pointed). The bell is pyriform and higher than a hemisphere and usually no more than 15 cm, in diameter, but occasionally as much as 60 cm (swarms of medusae of the larger size have occurred off the coast of England). The exumbrella bears stinging or nettling-warts, giving it a granular texture. There are 8 rhopalia, each bearing an orange-coloured mass (statoliths) but lacking ocelli. There is a triangular ciliated exumbrella sensory pit above the rhopalium and a subumbrella ciliated sensory pit on its lower surface. Each rhopalium is flanked by a pair of narrow lanceolate (pointed and elongated) rhopalar lappets. There are 8 velar lappets in each octant (64 in total) and 16 rhopalar lappets giving (16 + 64) 80 lappets in total.

In the centre of the subumbrella is is the arm-disk, a raised ring of tissue supporting the massive oral arms. The young medusa usually loses the primary central mouth, but sometimes this persists in the centre of the arm-disk. The arm-disk is 4-sided proximally, but widens distally to an 8 to 16-sided disc bearing 8 pairs (16) short scapulets which are frilly appendages concealed within the subumbrella space bearing fringed secondary mouths on their upper convex edges. From the ring of scapulets and arm-disk hang the 8 oral arm. There is a proximal unfrilled upper section to the arms and a more proximal or lower frilly section. Each frilly segment bears 3 longitudinal wings, so this region of the arms is Y-shaped in cross-section, with a central arm canal and the two arms of the Y directed outwards (centrifugally). These frilly wings bear numerous mouths whose lips are fringed by a row of short flexible knobbed tentacles armed with nematocysts and mucus-secreting cells to aid in food capture. Each arm bears a distal pendant club-shaped structure, the terminal club, an appendage of unknown function which is triangular to 3-rayed in cross-section with a central canal. These appendages are thinnest in their middle and are widest proximally (or distally in var. octopus). the upper arm section is longer than the winged part (shorter in var. octopus). The arms are about as long as the bell is wide.

In some rhizostomes, including Mastigias, the four genital sacs fuse centrally to form a single cruciform chamber situated between the arm-disk and the stomach and not connected to the stomach. The four subgenital pits also fuse into a cruciform subgenital porticus below the stomach. In this case the arm-disk is reduced to four thick perradial columns alternating with gaps into which each chamber of the genital sac opens via a genital pore (genital ostium) in each lobe of the subgenital porticus. The stomach is cruciform (giving off 4 perradial pouches). In Rhizostoma, the genital sacs open into the stomach and gametes can then be shed via the secondary mouths (see below).

The interradial gonads are invaginated (as in Aurelia) and not protruding (as in Cyanea) and beneath each is a narrow subgenital pit in the subumbrella. The gonads are enclosed in genital sacs and connect to the stomach. The cruciform (cross-shaped) stomach gives out 16 radial canals, 8 to the rhopalia and 8 to the bell margin. The outer halves of these canals are joined together by a network of anastomosing canals. The powerful swimming coronal muscle consists of 16 deltoid blocks of circular muscle alternating with radial canals. There is no distinct ring canal, but the interconnecting canals are widest along the innermost edge of the network. Four canals extend into the arm disk and fork to give 8 canals, one extending into and along each oral arm and sending out branches to the scapulets. (This pattern of canals is modified if the central primary mouth persists in the adult).

The disc mesogloea is milky to creamy-yellow to rusty-yellowish and translucent. the marginal lappets are cobalt blue/blue/violet and the sensory clubs of the rhopalia tips are orange (due to orange statoliths). The frilled mouths are orange/yellow/brown-yellow as are the terminal clubs on their outer surface. The gonads are yellowish. This form is abundant in the Mediterranean in summer.

Presumably, vortices generated by bell pulsations drive feeding currents towards the secondary mouths on the oral arms and scapulets (when the latter are present).

Rhizostoma pulmo jellyfish

Above: Rhizostoma pulmo from the Mediterranean (based on: Mayer, 1910. Medusae of the world vol. 3). Note the distal ends of the frilly scapulets just visible beneath the bell. These are born on arc-shaped appendages that branch off from the arm-disc/arm-base mass and arc upwards and outwards and bear frilly mouths on their upper convex edges. These effectively catch any food carried up into the bell (presumably by vortices generated by bell contraction). Note also the distal frilly mouth-bearing region of the oral arms and the orange pigment on the outer club of the arm appendages.

The life-cycle of the northern European variant of Rhizostoma pulmo, that is Rhizostoma octopus, has recently been reported (Holst et al. 2007. DOI 10.1007/s00227-006-0594-8). In this form (but apparently not the Mediterranea Rhizostoma pulmo in the strict sense) the males and female medusae can be distinguished by the colour of their gonads, visible through the translucent bell: brown in females when the eggs are ripe, and whitish-blue in males, when these are also ripe. (Rhizostoma is, like most jellyfish, dioecious, that is the sexes are separate.) The released eggs (each one-tenth of a millimeter in diameter) are not brooded in the oral arms but sink to the bottom of the water column (at least in an aquarium tank) and develop into planulae after two days. The planula is light brown and up to 0.15 millimeters in length and has a flagellated ectoderm (the planula is 'ciliated'). The anterior end is broader and equipped with nematocysts. The planulae swam for one to five days, under controlled conditions, rotating about their long axis as they do so (presumably the flagella are arranged in spiral rows along the length of the planula). The planula then attaches and develops into the scyphistoma. The scyphistoma initially has four primary tentacles and at 12 days post-attachment measured 0.5 millimetres in height and after two years, 2.3 millimeters but with up to 24 tentacles each up to 20 mm in length. A small fraction reproduced asexually, chiefly by budding off podocysts (from the foot of the attached scyphistoma). Presumably, a new scyphistoma 'germinates' from each podocyst. The spherical podocysts were up to 0.5 millimeters in diameter and encased in a chitinous shell. During strobilation the tentacles are resorbed and up to 5 discs are formed, each disc detaching to become an ephyra.

3. Coronatae

The coronate medusae (crown jellyfish) are less familiar to the casual observer beach comber, as they mainly occur in deep waters, though Nausirhoe, Linuche and Atorella occur in surface waters. These jellyfish are also mostly small, most being less than 5 cm in diameter, though some are medium-sized and reach 15 cm in diameter. They are characterised by the coronal groove: a circular groove on the exumbrella which demarcates the upper bell from the lower bell, as if the jellyfish was wearing a crown (corona). The bells are conical, domed or flattened with a scalloped edge.

Coronate jellyfish

Above: Nausithoe globifera as described by F. S. Russell in 1956. Below: with labels. This specimen was 22 mm (almost one inch) in diameter. I think this jellyfish looks rather like a flying saucer! The dashed circular lines represent the coronal muscle. Printable version.

Coronate jellyfish

The lower bell is a circlet of pedalia separated by radial grooves. (Pedalia are sometimes thought of as 'tentacle feet' but some bear a rhopalium instead). The radial grooves are inline with the marginal lappets. Each pedalium bears either a single solid tentacle or a rhopalium. Each rhopalium is situated in a groove between a pair of lappets. In coronate medusae the rhopalia may or may not bear ocelli. The rhopalia, tentacles and pedalia are in multiples of four. For example,  Nausithoe and Linuche (Thimble Jellyfish) have 8 rhopalia, 8 tentacles and 16 pedalia and is found in the Bahama-Florida region and similar waters. Periphylla periphylla (= Periphylla hyacinthina) the Helmet Jellyfish, has 16 lappets, 16 pedalia, 4 rhopalia and 12 tentacles and a purple dome-shaped bell. It emits red flashes of bioluminescent light. Bioluminescence is typical among coronate medusae and is used to startle potential predators. Note that in each case the number of tentacles + rhopalia equals the number of pedalia.

The four-angled mouth has a simple border (no oral arms etc.) and is borne on the end of a pendant cylinder called the manubrium. Four subumbrella funnels surround the mouth. These are funnel-shaped pits in the subumbrella of coronate medusae (and also in Cubomedusae and Stauromedusae, see below) but only occur in young stages of somaeostomes and rhizostomes, being replaced in the adults of these forms with the four subgenital pits below the gonads. The subumbrella funnels have no known function. Hyman (1940) suggests they may have a respiratory function, though they could also be secretory. One would certainly expect sea water to circulate within them as the bell pulsates, however, they may serve a mechanical function facilitating bell contraction? Some coronate medusae lack these structures (e.g. Atolla bairdii). The central stomach is located high in the bell and is divided peripherally into four gastric pouches lined by gastric filaments (also called digitelli, these are small subumbrella tentacles, typically armed with nematocysts in jellyfish and used in food processing they secrete digestive enzymes and help phagocytose the products of digestion and perhaps guard the entrance to the jellyfish from invaders). The gastric filaments are arranged in rows called phacellae. The filaments project into the stomach and are borne on four gelatinous dividing walls or septa which occupy the space between the gastric pouches. Each septum (there are four) is triangular, with the narrow apex pointing inwards towards the stomach and expand outside the central stomach to form the claustrum, a dividing membrane which narrows the conduits connecting the gastric pouches to the rest of the canal system to narrow slits called gastric ostia. These gastric ostia connect the stomach to the 'coronal stomach' a wide ring canal or sinus in the lower half of the bell, which gives off canals into the pedalia and marginal lappets. The septa connect to the exumbrella at four points, called the septal nodes or cathammata. The powerful ring of coronal muscle which contracts the bell is formed of 16 deltoid blocks of circular muscle in Nausirhoe. Excretory openings, connecting the gastrovascular system to the outside, are present in some species, e.g. there may be numerous openings or pores on the subumbrella where tentacular canals cross the ring canal, when the latter is present.

The life-cycle is poorly known in many coronatae (and jellyfish in general) but in Linuche the eggs are shed into the sea where they develop into swimming planula. There are 8 crescentic or U-shaped gonads on walls of the septa. The scyphistome of Nausithoe is known and consists of a clustered colony of trumpet-shaped polyps, several branching from one or more common basal stems. Each polyp has an expanded oral end fringed with short tentacles. The scyphistome is found in several habitats, including inside sponges. It undergoes polydisk strobilation to produce several ehphyrae.

Nausithoe has the following arrangement:

  • 4 perradial and 4 interradial rhopalia
  • 8 adradial tentacles
  • 8 adradial gonads
  • 16 pedalia
  • 16 marginal lappets
  • 4 interradial septa bearing gastric filaments (gastric cirri)
  • 4 perradial gastric ostia connecting to a wide ring sinus
  • 16 radial canals, one supplying each tentacle and each rhopalium
  • an outer ring canal connecting the radial canals

Atolla has a more complex form. The bell is flat and discoid with a lens-shaped corona. In Atolla bairdii there is a smooth circular ridge beneath the coronal groove, beneath which is a another shallow groove separating the ridge from the pedalia. There are two alternating rings of pedalia, the upper ring bearing tentacles (one solid tentacle per pedalium), each of the lower a rhopalium instead. The arrangement in Atolla bairdii is as follows:

  • 9 or more tentacles (16 to 32, and the same number of rhopalia)
  • pedalia in two alternating rings
  • twice as many marginal lappets as rhopalia
  • 8 adradial bean-shaped gonads around the base of the manubrium
  • 8 excretory openings - slits in the subumbrella, centripetal to ring of gonads
  • 8 radial muscles between the gonads on the subumbrella floor
  • The coronal muscle consists of an inner ring of circular muscle, divided into sectors, and an additional outer ring of continuous circular muscle projecting from the subumbrella as a thick annular ridge
  • There are no subumbrellar funnels
The stomach and gonads are red; the outer ring of circular muscle is dark red. The mesogloea is milky-blue and translucent and the central exumbrella has rust-coloured patches. In young specimens the gonads occur in 4 interradial pairs, before separating and swinging into the adradial position.

Coronate jellyfish

Above: Atolla bairdii. Note the upper row of tentacular pedalia and the lower alternating row of rhopalar pedalia and the marginal lappets. Printable version.

Other well-studied genera genera include Periphylla, Atorella and Periphyllopsis.

Coronate jellyfish

Above: Atolla wyvielli (based on: Mayer, 1910. Medusae of the world vol. 3). Printable version.With characteristic wide radial notches or furrows in the lenticular crown. Up to 73 mm diameter with 22 to 28 tentacles. Note the small marginal lappets.

4. Cubomedusae (Box Jellyfish)

Box Jellyfish are sometimes included in the Scyphozoa, along with the somaeostomes, rhizostomes and coronate jellyfish. Others consider them separate from the Scyphozoa (true jellyfish), Hydrozoa (hydroids), Anthozoa (anemones and true corals) as the Cubozoa. In a number of regards they are more advanced than other scyphozoans and I currently regard them as a highly evolved group of scyphozoans. They occur in warm tropical and subtropical waters, in harbours and bays and also in the open sea, but chiefly in shallow water. Occasionally they are found washed ashore on temperate coasts. Genera include Carybdea (illustrated below), Tripedalia, Chirodropus, Chironex (Chironex flickeri is the Sea Wasp) and Chiropsalmus (Fire Medusa). Given that some have lethal or potentially lethal stings, such as  Chironex and Chiropsalmus, the name Sea Wasp is often applied to all the cubomedusans.

The bell is very translucent and colourless or blueish. Generally only the tentacles generally have appreciable colour. Most are small (2 to 4 cm in bell height) but some are moderately large at 10 to 25 cm in height with tentacles up to about 3 m in length. The bell can pulsate very fast at 120 to 150 per minute.

Box jellyfish
Printable version

The cubomedusan has a cuboidal bell with four flattened (perradial) sides and a simple margin (i.e. with no lappets). However, the bell margin folds in to create a shell-like rim called a velarium. The four corner edges are interradial and bear, just above the margin, a tentacle or tentacle cluster. Each tentacle is borne on a tough blade-like pedalium. There are four pedalia and tentacles in Carybdea, one per corner. Tripedalia has three pedalia/tentacles in each corner whilst Chiropsalmus has a thick pedalium in each corner which branches into smaller pedalia, each of which bears a tentacle. these 'hand-like pedalia give Chiropsalmus the common name of Four-handed Box Jellyfish. Each tentacle bears rings of nematocysts.

The four rhopalia are perradial, with one in the midline of each side face of the box towards the bell margin, situated in a niche and  just above a thickened gelatinous fold, called a frenulum. The four frenula support the velarium. The gastrodermis contributes to the tentacles, velarium and rhopalia. there is a short quandrangular manubrium hanging down from the top of the bell deep inside the deep subumbrella cavity. This bears the mouth which opens into the central stomach at the top of the bell. Four subumbrellar funnels encircle the base of the manubrium. The four gastric pouches are perradial (one in each face) and connected to the central stomach by a gastric ostium or pore passing through septa on either side. Each of the four septa extend from the exumbrella to the subumbrella and each carries a pair of attached sheet-like gonads along the height of the bell. Each septum encloses one subumbrellar funnel. In Chiropsalmus and Chirodropus each gastric pouch evaginates on its upper surface to give a pendant subumbrellar sac, hanging down within the subumbrella cavity. Where each septum joins the central stomach, it gives off a U-shaped bunch of gastric filaments. Each gastric pouch gives off a canal to each rhopalium which forks at each perradial fold (the perradial fold is an extension of tissue continuous with the frenulum) and exits an opening in the septum at the bell margin to form a ring canal. A canal is also given off to each tentacle and blind canals or sinuses supply the velarium.

Box jellyfish

Label your own cubomedusa diagram!

Each rhopalium has a gravity-sensing statolith and ocelli looking inwards into the subumbrella (note that the bell is highly translucent). In Carybdea each rhopalium has 6 eyes, 2 larger eyes equipped with lens and thus more sophisticated than the ocelli of other schyphozomedusae, as well as 2 pairs of pigment-cup ocelli.  Box Jellyfish are strong and very maneuverable swimmers and it has been suggested that they use their visual system to track shoals of fish, fish being their chief prey. The nervous system of cubomedusans is also more advanced, paralleling that in some hydromedusae, by possessing a definite nerve ring near the bell margin, which loops upwards to contact each rhopalium. Jellyfish might not have anatomical brains, consisting of aggregated neuronal ganglia, but their neural nets can process information. The presence of lens-bearing eyes suggests that cubomedusans are extracting some positional information about objects in their visual field. This intensive image processing apparently pre-occupies their nerve nets during active hunting and periods of hunting are followed by periods of sleep, perhaps enabling the nervous system downtime for housekeeping maintenance. This is a convincing argument for the evolution of sleep.

Little is known about cubomedusan reproduction and development. A free-swimming planula develops from the egg and attached to form the polyp (a scyphostoma or cubostoma) which has been observed to completely metamorphose into a single medusan, without strobilation, prompting some to reclassify the cubomedusans as Cubozoa rather than as Scyphozoa. However, Strachler-Pohl and James, 2005 (Mar. Bio. 147: 1271-1277)  observed an additional optional second mode of development in Carybdea marsupialis, in which the polyp metamorphoses into the medusa, but leaves behind part of itself as a remnant, suggestive of monodisc strobilation (when a strobila buds off a single ephyra as occurs in some schyphomedusae). This lends weight to the idea that the cubomedusans are a specialised and highly evolved offshoot of scyhphomedusae, whether we decide to classify them separately or not.

5. Stauromedusae (Stalked Jellyfish)

The stauromedusaeare also sometimes classed separately from other jellyfish, but appear to be derived from other scyphomedusae. Stauromedusae live upside-down and attached to the substrate via either a stalk (pedicel or peduncle) ending in an adhesive foot (pedal disc) or via an adhesive spot in the center of the exumbrella in those stalkless species. These flower-like forms are small to medium in size, approaching several cm in diameter. They are usually green or brown in colour, but pink, orange, blue and violet forms also occur. Some absorb chromoplasts from their algal substrates through the pedal disc to assume the same colour as vegetation they are attached to. They occur in colder coastal waters, in bays and sounds, etc. Some forms of the genus Lucernaria have been found at abyssal depths and at hydrothermal vents (Lutz et al. 1998, Deep-sea Research II 45: 329-334). They cannot swim, but some can move in a hydra-like fashion (see: hydrozoa) whilst others are permanently attached to their substrates (seaweeds, shells, rocks, etc.) by secreting a hard chitinous adhesive.

Stalked jellyfish

Above: Haliclytus; note the central mouth surrounded by four oral lobes and the attachment stalk ending in an adhesive disc and the 8 arms bearing bunches of 100 to 200 hollow tentacles, each tentacle with a terminal knob (equipped with nematocysts). The disc is 20 to 30 mm in diameter. A gonad is visible extending into each arm. Printable version.

These jellyfish are sessile medusae and trumpet or goblet-shaped and resembling polypoids. This is an interesting example of the twists and turns evolution can take to find a niche. Jellyfish are particular amongst cnidarians for having sexual mature free-swimming medusae and sessile polypoid larvae. Here we have an adult form that has reverted to a polypoid-like existence, but is of course a sexually mature adult. Haliclystus expands orally with the subumbrella uppermost and tapers aborally to the stalk (pedicel or peduncle). The bell margin is either circular and fringed with tentacles, or 8-sided and drawn into 8 adradial arms (or 4 bifurcated arms as in Kishinouyea) each ending in a bunch of short capitate tentacles (capitate = clubbed or equipped with a terminal knob). Depending on species (and presumably age of the individual) there may be between 20 and several hundred tentacles per cluster.

Stalked jellyfish

In each perradius and interradius there is a marginal body (x 8 in total) called a rhopalioid. This is not a true rhopalium, since it lacks the specialised rhopalial sensory systems, but is a reduced tentacle which serves an adhesive function and is also called an anchor or colletocystophore. At least, the anchor consists of glandular tissue, but Hyman (1940) could observe no use for them in Haliclystus, the anchors did not respond to touch and the tentacle clusters are used for anchorage following detachment and during locomotion. Thus, the function of the 'anchors' remains enigmatic. Each is a horseshoe or oval-shaped adhesive cushion.The coronal muscle, no longer required for swimming, is reduced to a thin marginal band. There are radial epidermal muscle fibres and both the subumbrella and exumbrella nerve nets are well developed.

The four-cornered mouth (flanked by four small oral lobes)  opens into a short quadrangular manubrium, which is surrounded by four subumbrellar funnels at its base and opens into the central stomach. The central stomach gives off four perradial lobes (gastric pockets or gastric pouches) separated by 4 interradial septa. Each septum encloses one of the subumbrella funnels which are interconnected through pores in the septa, called septal ostia, to form a ring sinus which gives off canals to the rhopalioids, arms and tentacles. Where the septa joins the central stomach it gives off gastric filaments. The septa also contain septal muscles and manufacture nematocysts. The 8 gonads (or 4 horseshoe-shaped pairs) are elongated bodies borne on the faces of the septa and each gonad extends into one arm. The septa extend to the pedal disc and may fuse in the center to divide the stalk canal into 4 separate canals, as in Haliclystus. Some forms, such as Craterolophus and Halimocyathus, have 4 claustra: longitudinal partitions between the septa along much of their length. These divide off the outer compartment of each gastric pouch for most of the animal's length, but the claustra do not reach to the base of the bell and here the outer compartments are in direct communication with the rest of the stomach. Haliclystus lacks claustra. Haliclystus has been observed to feed on small crustaceans which it catches with its tentacles and then bends the arm inward to deposit the food on the manubrium / oral lobes.

Other forms differ: Lipkea has no anchors and no terminal knobs on its tentacles. Depastrum has no rhopalioids, 16 tentacle clusters and a simple bell margin.

Gametes are shed through the mouth at night, where fertilisation is apparently external. The fertilised eggs develop into non-ciliated vermiform planulae (vermiform = worm-like) that creep along before attaching at their anterior ends, developing a mouth and sometimes budding out secondary larvae as planula-like stolons. The polyp metamorphoses directly into the adult. Some consider the adult to be a modified scyphistoma, but it is perhaps more likely a modified medusa that failed to detach from the strobila following monodisc strobilation. interestingly, Haeckel (1880) described four specimens of three genera of medusae he classified as stauromedusae: Tessera, Tesserantha and Tessaria (though he considers that the three may be different developmental stages of the same species) but which were free-swimming and not sessile. These lacked attachment stalks, but did have a hollow tubular protuberance in the center of the exumbrella. The tentacles were solid and lacked terminal knobs. The reasons for classifying these as stauromedusae are not clear. To my knowledge these forms have not been reported  by any other biologist and they remain an enigma.