Cnidarians exhibit primary radial symmetry. In the Hydrozoa and Scyphozoa all diameters are apolar (i.e. have
like ends). Furthermore, any two diameters at right angles to each other are also alike, dividing the animal into
symmetric halves. The Anthozoa are either biradial or bilateral. Biradial Anthozoa have apolar diameters with
the sagittal axis aligned along the long axis of the mouth and defines two symmetric halves that differ
distinctively from the transverse axis, which is at 90o to the sagittal. In bilaterally symmetric Anthozoa the
sagittal axis is heteropolar (i.e. it has unlike ends) and so the animal has distinct dorsal and ventral surfaces
and the only exact plane of symmetry is the sagittal plane.
The cnidarian polyp consists of a base, a stalk and the oral end. The base has a rounded or pointed end
thrust into the substrate or alternatively an adhesive pedal disc, or an adhesive skeletal secretion or root-like
stolons. The column, stem or stalk forms a cylinder. The oral end of the hydrozoan polyp is an elongated
vase-shaped hydranth bearing a circular mouth. In the Anthozoa the oral end is an expanded oral disc and the
mouth is often elongated with a ciliated groove (siphonoglyph) at one or both ends.
The oral end bears tentacles, though tentacles are absent in minute polyps, such as Microhydra and
Protohydra, and are also absent in some parasitic hydroids, e.g. Hydrichthys, and also in rhizostome medusae,
the anemone Limnactinia and some modified morphs in polymorphic species. The tentacles are irregularly
scattered or more usually arranged in one or more circlets. The tentacles may be hollow or solid and are
armed with nematocysts.
The polyp wall consists of epidermal and gastrodermal epithelia with mesogloea inbetween the two epithelia.
The mesogloea is a thin sheet of cement, constituting a mesenchymal or fibrous connective tissue. For a
detailed study of the cnidarian body wall see the hydra.
Hydroid polyps possess a tubular coelenteron partially divided by 4 septa. In Anthozoa the pharynx comprises
a stout tube hanging down from the mouth into the coelenteron and in some anthozoans septa attach the
pharynx to the body wall.
There are various types of modified polyp in polymorphic species. Tentaculozooids have no mouth and no
coelenteron and have a tentacular function. Gonozooids have no tentacles and no digestive tract and have a
reproductive function. Siphonozooids have an emphasis on current-producing devices.
Gonophores bud from stolons, hydranths, the hydrocaulus or from modified hydranths (gonozooids or
blastostyles) or often in a definite sequence borne on the gastrozooids, with the oldest situated most basally.
Gonozooids form from gastrozooids by reduction and loss of the tentacles, closure of the mouth, and reduction
of the gastrovascular cavity. Gonophores may have a periderm covering, called the gonotheca.
A gonangium consists of blastostyle covered by gonotheca and one to many gonophores. The oldest
gonophore bud occurring near to the apex. The gonotheca may be smooth, spined or ribbed. A gonangium
with or without protective modifications, e.g. phylactocarps, is a modified hydrocladium (branch) or its
The medusa is the active locomotory stage and the sexually mature form.
The medusa body consists of a deep to shallow bowl of gelatinous nature, the bell or umbrella. The convex
aboral surface is the exumbrella and the concave oral surface is the subumbrella. Sense organs and
tentacles are borne on the umbrella rim. The mouth hangs down from the centre of the subumbrella on the
end of a tubular (endoderm-lined) projection called the manubrium, which leads into the gastric cavity or
stomach in the bell centre. In Hydromedusae the stomach is usually a simple chamber. In Scyphomedusae the
stomach periphery may be divided into 4 perradial gastric pouches by 4 interradial septa. The stomach gives
off the gastrodermal canals: four, or a multiple of four, radial canals and the ring or circular canal in the bell
The medusa has tetramerous symmetry and the radial canals, tentacles and sense organs are all arranged
tetramerously. There are four perradii at right angles to each other, and in line with the four radial canals. The
interradii comprise the sectors between the perradii. In between each perradius and interradius is an adradius.
Ectodermal epithelium lines both outer surfaces of the bell. Endodermal epithelium lines the manubrium and
the radial and ring canals. There is a thick layer of jelly (mesogloea or collenchyme) between the ecto- and
endodermal epithelia. Hydromedusae possess a velum: a circular shelf projecting inward from the bell margin
and partially cutting off the subumbrellar space. Medusae possessing a velum are called veiled or craspedote
medusae. Scyphomedusae have no velum and are acraspedote. The velum functions with the circular muscle
band in swimming.
Some have undergone loss of tentacles, mouth and reproductive system to form a locomotory bell or float for
the polyp colony. Others have lost their non-reproductive structures and have no independent life.
Cnidaria are at the tissue grade of construction (i.e. they lack organ body parts made up of several different
cell types) and the sense organs of the medusae are considered to be the only organs proper.
The epidermis (ectoderm) is cellular or syncytial. When cellular the cells may be cuboidal to columnar or flat
(as on the exumbrellar) or slender and elongated (as in anemones). A cuticle covering may be present, or
alternatively the epidermis may be ciliated (as in anemones) or flagellated (as in some medusae). The
epidermal cell cytoplasm consists of strands with fluid-filled spaces between. The bases of the epidermal cells
are fastened to the mesogloea by pseudopodial processes. On exposed parts of hydroid polyps and
medusae (except the exumbrella) the cell bases give off two long strands that run longitudinally and external
to or else embedded in the mesogloea. These processes contain a contractile fibre (myoneme) and the cells
are therefore epitheliomuscular cells. In hydras each epidermal cell gives out two opposite basal strands. In
some Trachylina, most Scyphozoa and in Anthozoa the amoeboid epidermal cell bases lack contractile
Nematocysts and cnidoblasts occur on the epidermis of the tentacles, oral regions, and also sometimes on
the stems of polyps and sometimes on the exumbrella of medusae. The nematocysts are often in wart-like
clusters or ascending tracts.
Epidermal gland cells occur on the tentacles, oral regions, pedal disc, on the pharynx of anthozoans and on
exposed parts of the column. The pedal disc epidermis of hydra consists entirely of gland cells with muscular
basal extensions (glandulomuscular cells). Mucous cells provide attachment, protection and are involved in
entanglement of prey and debris and provide lubrication to assist swallowing of food. Granular cells are
common in the pharyngeal lining and are sparse on the surface epidermis.
In Trachylina, Scyphozoa and Anthozoa the epidermal muscle fibres are usually independent fibres sunk into
a subepidermal position. These may form a layer or may form bundles close to or embedded in the
mesogloea. The muscle bundles may have a mesogloeal core. Epidermal / subepidermal muscle fibres may
be fastened to the surface of scallops or folds of the mesogloea, an arrangement which serves to increase
the number of fibres. In polypoids the muscle is of the smooth type, whilst the circular muscle fibres of the
velum and subumbrella in medusae are cross-striated.
Sensory cells are common in the epidermis of the tentacles and oral regions, and may occur singly or in
patches (often in medusae). Each sensory cell terminates in one or more bristles or a bulb or a long motile
flagellum and gives out a projection to the nerve plexus.
Interstitial (indifferent) cells are undifferentiated cells scattered among the epidermal supporting cells. These
cells can differentiate into nematocysts or sex cells or into other cell types during regeneration. The body wall
thus consists of three strata: the outer supporting cells and glandular and sensory cells, the nervous stratum
and the muscle stratum. Beneath these is the mesogloea.
The gastrodermis is an endoderm of cuboidal to columnar nutritive cells and interspersed glandular and
sensory cells. Some of the nutritive cells have myonemal basal extensions and are nutritive-muscular cells.
The myoneme extensions form circular muscle in polypoids. Medusae usually lack gastrodermal muscle
extensions or a gastrodermal muscle layer. Circular and longitudinal gastrodermal muscle occurs in
anthozoans and these are more strongly developed than the epidermal muscle and may be separate from the
gastrodermis. In Anthozoa the gastrodermal muscle attaches to mesogloeal folds or plates. The gastrodermis
is columnar and often folded where digestion occurs and contains food vacuoles and each nutritive cell
usually possesses two flagella.
Mucous gland cells are very abundant near the mouth and are flagellated and have muscular basal
extensions. Granular gland cells occur in the gastrodermis and are possibly involved in enzyme secretion (?).
These granular cells may be flagellated and their bases may or may not reach the mesogloea.
Nematocysts occur on certain gastrovascular structures in Scyphozoa and Anthozoa, such as the gastric
filaments, septal filaments and acontia and are otherwise absent from the gastrodermis. The subgastral nerve
net is less developed than the epidermal one.
Zoochlorellae are found in the gastrodermis of the green hydra and yellow zooxanthellae in corals and
The mesogloea is the layer between epidermis and gastrodermis. In hydrozoan polyps the mesogloea is a
thin gelatinous membrane devoid of cells and fibres, forming a mesolamella. In medusae the mesogloea is
gelatin-like and constitutes the bulk of the animal. In hydromedusae it contains few cells and fibres of
unknown origin (?) and is a mesogloea proper. In scyphomedusae the mesogloea contains fibres and
amoeboid cells and constitutes a collenchyme. All medusae also possess a mesolamella between the
mesogloea and the epi- and gastrodermises. In medusae (Aequorea, Rhizostoma) the mesogloea contains no
gelatin or mucin and so is not connective tissue of classical nature. Anthozoa contain a cellular mesogloea or
mesenchyme. This consists of stellate amoeboid cells in a jelly matrix or in a fibrous connective tissue, often
comprising several layers of different orientation.
Sensory cells (palpocils) are common in the epidermis of tentacles and oral regions. They occur singly, or, as
is often the case in medusae, they occur in patches. Each sensory cell terminates in one or more bristles or a
bulb or a long motile flagellum. A basal process connects each sensory cell to the nerve plexus.
Interstitial (indifferent) cells are scattered among the epidermal supporting cells. These are undifferentiated
cells that may differentiate into nematocysts, sex cells or other cells during regeneration. Thus, the covering
layer comprises three strata: the outer stratum of supporting, glandular and sensory cells, the nervous
stratum and the muscle stratum, backed by mesogloea.
The gastrodermis (endoderm) consists of cuboidal-columnar nutritive cells, some of which have myoneme
basal extensions and are therefore nutritive-muscular cells. The myoneme extensions are circular in
polypoids, circular and longitudinal in Anthozoa, but are usually absent from medusae. In Anthozoa the
gastrodermal musculature is more strongly developed than the epidermal musculature and may be separate
from the gastrodermis and adherent to mesogloeal folds or plates. Glandular and sensory cells also occur in
Medusae are 95-96% water (increasing to 98% in brackish water) and less than 1% organic material, the rest
being salts. The organic material is mostly a low nitrogen protein, possibly chitin (?).
The gut constitutes an epithelial sac that may be subdivided, by septa in polyps, or into a central stomach
and radial and marginal canals in medusae. In scyphozoa the gastric filaments contain nematocysts and are
borne on septa. In Anthozoa the free edges of the septa form a sinuous cord (septal filament) containing
gland cells and nematocysts. The digestive tract is continuous between individuals in colonial forms.
Cnidarians are carnivorous and feed on animals and bits of animals, etc. The tentacles bear nematocysts
and produce adhesive secretions to immobilise the prey. The tentacle(s) bends towards the mouth, the
mouth opens and the food is grasped by the mouth rim. Food is swallowed, aided by mucous secretion from
the gastrodermis of the manubrium or pharynx, and sometimes by ciliary action, and sometimes by muscular
action. The body is highly distensible and cnidarians can swallow large objects. Some anthozoans feed by a
mucous-ciliary method in which mucous strands entangle particles that are then conveyed to the mouth by
Digestion is extracellular by enzyme secretion and intracellular by phagocytosis. Alkali proteolytic secretions
are produced by the gastrodermis (and septal filaments in Anthozoa) and is rapid, taking 8-12 hours. The
resultant particles are phagocytosed into food vacuoles inside the gastrodermal cells, in which the secretion
changes from acid to alkaline. Most cannot digest starch inside these vacuoles. Intracellular digestion is
slower, taking several days. Waste is ejected through the mouth. Excess nutrients are stored in the
gastrodermis as fat and glycogen.
In polyps the muscular system consists of an outer cylinder of (irregularly spaced) longitudinal fibres formed
from the base of the epidermis and an inner cylinder of circular fibres comprising the base of the
gastrodermis. The longitudinal fibres bend the body and tentacles and contract the animal. The circular
fibres extend the animal and tentacles and produce peristaltic waves employed in swallowing food and in
In medusae the gastrodermal muscle is nearly or totally absent and the epidermal cylinder is reduced to
longitudinal fibres in the tentacles and manubrium, and to radial fibres in the subumbrella, and to circular
epidermal muscle cells in the velum and subumbrella.
In Anthozoa the epidermal muscle is reduced to fibres in the tentacles and oral disc. Longitudinal bands
strengthen the anthozoan gastrodermal layer. In Scyphozoa and Anthozoa, rather than forming cylinders, the
muscles may be festooned on mesogloeal plates that penetrate the epithelia, or the muscles may be sunk
into the mesogloea as bundles.
In pedal and oral discs the epidermal fibres are radial, whilst the gastrodermal fibres are circular.
There is no specialised respiratory mechanism. The thin body wall facilitates diffusion and the tentacles
increase surface area for exchange. In the gastrodermal cilia/flagella drive currents through the
gastrodermal system. Body movements also mix the gastrovascular contents. In medusae there are flagellar
currents in the canal system and in Anthozoa the ciliated siphonoglyph generates a water stream.
Scyphozoa possess four deep pits in the subumbrella, of unknown function, that are possibly respiratory.
Respiration is aerobic, but the rate is very low in medusae because of their very small organic content. In
Anthozoa respiration is low when the animal is contracted and increases when the animal is extended, and
requires clean aerated water.
There are no special excretory structures. In some hydromedusae and scyphomedusae, each radial canal
opens near to the tentacles via a pore that has been seen to eject non-nutritive particles and possibly has
an excretory function. The gastrodermis may therefore have an excretory function.
In some Siphonophora there is a mass permeated by gastrodermal canals beneath the float. This mass
possibly has an excretory function (?) and contains guanine crystals.
In the Anthozoa the septal filaments and septa are excretory and contain xanthine crystals. In Alcyonium and
some hydroids amoeboid cells transport waste to the exterior.
Urea, uric acid, creatine and creatinine are absent. Xanthine and guanine are found in the gastrodermis and
may be waste products. Anemones and medusae excrete ammonia. In anemones ammonia accounts for
77-100% of the nitrogenous waste, and there are also traces of urea and uric acid.
Sensory nerve cells in the epithelia connect to the subepithelial plexus, outside the muscle layer. This plexus
may be connected to the less-developed sub-gastrodermal plexus, which is known to exist in some Cnidaria
and possibly exists in all cnidarians. Nerve cells exist in the mesogloea and may serve to connect the two
In hydromedusae the plexus is limited to the subumbrellar surface and connects with two nerve rings in the
bell margin. The upper marginal nerve ring communicates with the marginal sense organs, whilst the lower
ring controls the velum ring muscle. In scyphozoans there are no marginal nerve rings. Ganglia near each
rhopalium connect to the subumbrellar plexus.
In general there is a diffuse radial nervous system, which is synaptic, and conducts equally well in all
directions. In anemones stimuli conduct longitudinally more readily than transversely. Cnidarians are
characterised by autonomy, or the independence of parts. Isolated portions maintain their behaviour.
Isolated tentacles and acontia move as normal when stimulated and pedal discs creep about. There is little
or no development of a CNS. Characteristic reflexes include contraction of anemones, righting of medusae
and opening of the mouth and pharynx when the tentacles are stimulated by food. Generally, however, there
are few reflexes.
Cnidarians characteristically possess independent effectors. Nematocysts, gland cells and cilia react
independently of the nervous system. It is thought that the longitudinal muscles of anemone acontia also
react independently of the nervous system. The nerve plexus conducts without decrement or fatigue at
signal velocities of 7-120 cm/s (cf. 12 500 cm/s in humans).
The coelenterates are jelly-like animals and consist of
two phyla: the Cnidaria and the Ctenophora (comb
jellies). On this page and the links below we look at the
Cnidaria. Cnidarians are divided into at least three major
groups (classes): Scyphozoa, Hydrozoa and Anthozoa.
The Scyphozoa are the jelly-fish proper. The sexually
mature form of the scyphozoans is the free-swimming
tentacled saucer-, bowl-, bell-, disc- or umbrella-shaped
tentacled medusa, named after the Medusa of ancient
Greek myth whose hair was turned into a mass of
tentacle-like serpents (and who symbolised the night-sky).
Medusae are the sexually mature forms, but these are
produced by a smaller form which reproduces only
asexually and which is sessile (remains attached to the
substrate), called the polyp stage. This alternation of
generations, between asexual polyps and sexual medusae
is characteristic of cnidarians.
Hydrozoans differ in that the dominant form is the hydroid
and in some, like hydra opposite, there is no medusa
stage, rather gonads develop directly on the polyp. (Some
regard the gonads as reduced medusae that remain
attached to the polyp). Polyps are typically attached to the
substrate by a stalk and usually do not swim (though they
frequently crawl, glide or float and some can swim) and
have tentacles around the mouth at the top of the
column-like body. In Hydrozoa the polyps are also called
hydroids. Both polyps and medusae are typically
carnivorous, catching prey with their tentacles that have
cells equipped with stinging organelles called nematocysts
and/or entangle prey in various ways. The tentacles of
cnidarians, in both polyps and medusae, are muscular
and capable of varied movements.
Polyps like hydra also reproduce asexually by budding. In
many hydroids, the polyps do not completely separate
after budding, but remain physically attached, forming a
hydroid colony. The nervous systems of such polyps are
typically connected, such that the colony functions as a
single organism. In Obelia, the polyps are connected by
tubes called stolons and the whole is enclosed in a
common skeleton, consisting of a tubular, chitinous
sheath. This pattern of producing larger more complex
organisms by repeating individuals or modules has
happened many times throughout evolution. Once we
have a modular organism, it is possible for some of the
'individual' polyps or modules to take on different
functions by division of labour. For example, in Obelia,
some individuals have tentacles to catch food, whilst
others are involved only in producing tiny medusae by
asexual budding (the medusae eventually detach and
have free-living existences for the purpose of
reproduction). In some forms, other individuals may
specialise in colony defense.
This phenomenon whereby evolution built-up more
complex systems by duplicating simpler components, and
specialising them, happened throughout nature on many
levels. In DNA, a gene may accidentally become
duplicated, freeing up one copy to mutate randomly
(possibly passing through many dysfunctional states
before arriving at a new useful state) without impairing
function. Metameric segmentation, as seen for example in
annelid worms and vertebrates appears to involve a
duplication of individuals (segments) into a chain. Some of
the segments can then group together to form specific
functions (such as those segments that make up the
Siphonophores are a group of Hydrozoa in which the
individuals of the colony (zooids) are derived from both
medusoid and hydroid types. One or more medusoids
may form floats or swimming bells for propulsion and all
the zooids are connected by a stalk, which may be
flattened to a disc as in the Portuguese Man o' War
(Physalis). Siphonophores and free-living hydrozoan
medusae are sometimes called jellyfish, though zoologists
reserve this title for the large scyphozoan medusae which
have large amounts of 'jelly' (mesogloea). Many
cnidarians are bioluminescent, like the siphonophore on
the left. Siphonophores may have quite a number of
different zooid types with different functions.
Velella or By-the-wind Sailor, was formerly classed as a
siphonophore, but neurological studies suggest that it is a
single polypoid zooid with a number of mouths that floats
upside-down on the water. This organism reminds us how
difficult it can be to distinguish an 'individual' from a colony
of connected individuals.
Hydroid colonies can take on a variety of forms, from
encrusting to branching and tree-like, to hydrocorals with
massive calcareous and stony exoskeletons (the millepore
and stylasterine corals).
The third group of cnidarians, the Anthozoa, include
solitary sea anemones and colonial soft corals and horny
corals like sea fan, sea pens and sea whips and the true
or stony corals. Some solitary forms are also worm-like. In
anthozoans the medusoid stage is completely absent.
These will be discussed in a future article.
Also coming soon - a detailed discussion of cnidarian
nervous systems. These are among the simplest animal
nervous systems known, but still remain only very
incompletely understood. They represent a good subject
of study for those wishing to understand biological
information-processing systems and artificial intelligence
A scyphozoan medusa (jellyfish).
Hydra - a solitary polypoid hydroid.
Obelia - a tree-like hydroid colony.
|Cnidaria - many ways of building bodies