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zygote, fertilized egg cell that results from the union of a female gamete (egg, or ovum) with a
male gamete (sperm). In the embryonic development of humans and other animals, the zygote
stage is brief and is followed by cleavage, when the single cell becomes subdivided into smaller
cells.
The zygote represents the first stage in the development of a genetically unique organism. The
zygote is endowed with genes from two parents, and thus it is diploid (carrying two sets of
chromosomes). The joining of haploid gametes to produce a diploid zygote is a common feature
in the sexual reproduction of all organisms except bacteria.
The zygote contains all the essential factors for development, but they exist solely as an encoded
set of instructions localized in the genes of chromosomes. In fact, the genes of the new zygote
are not activated to produce proteins until several cell divisions into cleavage. During cleavage
the relatively enormous zygote directly subdivides into many smaller cells of conventional size
through the process of mitosis (ordinary cell proliferation by division). These smaller cells,
called blastomeres, are suitable as early building units for the future organism.

Encyclopædia Britannica (3)
fertilization (reproduction)
seed and fruit (plant reproductive part)
zygote (cell)
Table of ContentsfertilizationArticleMaturation of the egg- Egg surface- Egg coatsEvents of
fertilization–- Sperm–egg association–- Specificity of sperm–egg interaction- Prevention of
polyspermy- Formation of the fertilization membra...- Formation of the zygote
nucleusBiochemical analysis of fertilizationAdditional ReadingCitations
Article
Maturation of the egg
Egg surface
Egg coats
Events of fertilization
Sperm–egg association
Specificity of sperm–egg interaction
Prevention of polyspermy
Formation of the fertilization membrane
Formation of the zygote nucleus
Biochemical analysis of fertilization
Additional Reading
Citations
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fertilization
Primary Contributor: Alberto Monroy, M.D.
ARTICLE
from the
Encyclopædia Britannica
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fertilization, /EBchecked/media/126478/A-sperm-cell-attempting-to-penetrate-an-egg-tofertilize
/EBchecked/media/126478/A-sperm-cell-attempting-to-penetrate-an-egg-to-fertilizeunion of a
spermatozoal nucleus, of paternal origin, with an egg nucleus, of maternal origin, to form the
primary nucleus of an embryo. In all organisms the essence of fertilization is, in fact, the fusion
of the hereditary material of two different sex cells, or gametes, each of which carries half the
number of chromosomes typical of the species. The most primitive form of fertilization, found in
micro-organisms and protozoans, consists of an exchange of genetic material between two cells.
The first significant event in fertilization is the fusion of the membranes of the two gametes
resulting in the formation of a channel that allows the passage of material from one cell to the
other. Fertilization in advanced plants is preceded by pollination, during which pollen is
transferred to, and establishes contact with, the female gamete or macrospore. Fusion in
advanced animals is usually followed by penetration of the egg by a single spermatozoon. The
result of fertilization is a cell (zygote) capable of undergoing cell division to form a new
individual.
The fusion of two gametes initiates several reactions in the egg. One of these causes a change in
the egg membrane(s), so that the attachment of and penetration by more than one spermatozoon
cannot occur. In species in which more than one spermatozoon normally enters an egg
(polyspermy), only one spermatozoal nucleus actually merges with the egg nucleus. The most
important result of fertilization is egg activation, which allows the egg to undergo cell division.
Activation, however, does not necessarily require the intervention of a spermatozoon; during
parthenogenesis, in which fertilization does not occur, activation of an egg may be accomplished
through the intervention of physical and chemical agents. Invertebrates such as aphids, bees, and
rotifers normally reproduce by parthenogenesis.
In plants certain chemicals produced by the egg may attract spermatozoa. In animals, with the
possible exception of some coelenterates, it appears likely that contact between eggs and
spermatozoa depends on random collisions. On the other hand, the gelatinous coats that surround
the eggs of many animals exert a trapping action on spermatozoa, thus increasing the chances for
successful sperm-egg interaction.
The eggs of marine invertebrates, especially echinoderms, are classical objects for the study of
fertilization. These transparent eggs are valuable for studies observing living cells and for
biochemical and molecular investigations because the time of fertilization can be accurately
fixed, the development of many eggs occurs at about the same rate under suitable conditions, and
large quantities of the eggs are obtainable. The eggs of some teleosts and amphibians also have
been used with favourable results, and techniques for fertilization of mammalian eggs in the
laboratory may allow their use even though only small numbers are available.
Maturation of the egg
Maturation is the final step in the production of functional eggs (oogenesis) that can associate
with a spermatozoon and develop a reaction that prevents the entry of more than one
spermatozoon; in addition, the cytoplasm of a mature egg can support the changes that lead to
fusion of spermatozoal and egg nuclei and initiate embryonic development.
Egg surface
Certain components of an egg’s surface, especially the cortical granules, are associated with a
mature condition. Cortical granules of sea urchin eggs, aligned beneath the plasma membrane
(thin, soft, pliable layer) of mature eggs, have a diameter of 0.8–1.0 micron (0.0008–0.001
millimetre) and are surrounded by a membrane similar in structure to the plasma membrane
surrounding the egg. Cortical granules are formed in a cell component known as a Golgi
complex, from which they migrate to the surface of the maturing egg.
The surface of a sea urchin egg has the ability to affect the passage of light unequally in different
directions; this property, called birefringence, is an indication that the molecules comprising the
surface layers are arranged in a definite way. Since birefringence appears as an egg matures, it is
likely that the properties of a mature egg membrane are associated with specific molecular
arrangements. A mature egg is able to support the formation of a zygote nucleus; i.e., the result
of fusion of spermatozoal and egg nuclei. In most eggs the process of reduction of chromosomal
number (meiosis) is not completed prior to fertilization. In such cases the fertilizing
spermatozoon remains beneath the egg surface until meiosis in the egg has been completed, after
which changes and movements that lead to fusion and the formation of a zygote occur.
Egg coats
The surfaces of most animal eggs are surrounded by envelopes, which may be soft, gelatinous
coats (as in echinoderms and some amphibians) or thick membranes (as in fishes, insects, and
mammals). In order to reach the egg surface, therefore, spermatozoa must penetrate these
envelopes; indeed, spermatozoa contain enzymes (organic catalysts) that break them down. In
some cases (e.g., fishes and insects) there is a channel, or micropyle, in the envelope, through
which a spermatozoon can reach the egg.
The jelly coats of echinoderm and amphibian eggs consist of complex carbohydrates called
sulfated mucopoly-saccharides; it is not yet known if they have a species-specific composition.
The envelope of a mammalian egg is more complex. The egg is surrounded by a thick coat
composed of a carbohydrate protein complex called zona pellucida. The zona is surrounded by
an outer envelope, the corona radiata, which is many cell layers thick and formed by follicle cells
adhering to the oocyte before it leaves the ovarian follicle.
Although it once was postulated that the jelly coat of an echinoderm egg contains a substance
(fertilizin) thought to have an important role not only in the establishment of sperm-egg
interaction but also in egg activation, fertilizin now has been shown identical with jelly-coat
material, rather than a substance continuously secreted from it. Yet there is evidence that the egg
envelopes do play a role in fertilization; i.e., contact with the egg coat elicits the acrosome
reaction (described below) in spermatozoa.
Events of fertilization
Sperm–egg association
The acrosome reaction of spermatozoa is a prerequisite for the association between a
spermatozoon and an egg, which occurs through fusion of their plasma membranes. After a
spermatozoon comes in contact with an egg, the acrosome, which is a prominence at the anterior
tip of the spermatozoa, undergoes a series of well-defined structural changes. A structure within
the acrosome, called the acrosomal vesicle, bursts, and the plasma membrane surrounding the
spermatozoon fuses at the acrosomal tip with the membrane surrounding the acrosomal vesicle to
form an opening. As the opening is formed, the acrosomal granule, which is enclosed within the
acrosomal vesicle, disappears. It is thought that dissolution of the granule releases a substance
called a lysin, which breaks down the egg envelopes, allowing passage of the spermatozoon to
the egg. The acrosomal membrane region opposite the opening adheres to the nuclear envelope
of the spermatozoon and forms a shallow outpocketing, which rapidly elongates into a thin tube,
the acrosomal tubule that extends to the egg surface and fuses with the egg plasma membrane.
The tubule thus formed establishes continuity between the egg and the spermatozoon and
provides a way for the spermatozoal nucleus to reach the interior of the egg. Other spermatozoal
structures that may be carried within the egg include the midpiece and part of the tail; the
spermatozoal plasma membrane and the acrosomal membrane, however, do not reach the interior
of the egg. In fact, whole spermatozoa injected into unfertilized eggs cannot elicit the activation
reaction or merge with the egg nucleus. As the spermatozoal nucleus is drawn within the egg, the
spermatozoal plasma membrane breaks down; at the end of the process, the continuity of the egg
plasma membrane is re-established. This description of the process of sperm-egg association,
first documented for the acorn worm Saccoglossus (phylum Enteropneusta), generally applies to
most eggs studied thus far.
During their passage through the female genital tract of mammals, spermatozoa undergo
physiological change, called capacitation, which is a prerequisite for their participation in
fertilization; they are able to undergo the acrosome reaction, traverse the egg envelopes, and
reach the interior of the egg. Dispersal of cells in the outer egg envelope (corona radiata) is
caused by the action of an enzyme (hyaluronidase) that breaks down a substance (hyaluronic
acid) binding corona radiata cells together. The enzyme may be contained in the acrosome and
released as a result of the acrosome reaction, during passage of the spermatozoon through the
corona radiata. The reaction is well advanced by the time a spermatozoon contacts the thick coat
surrounding the egg itself (zona pellucida). The pathway of a spermatozoon through the zona
pellucida appears to be an oblique slit.
Association of a mammalian spermatozoon with the egg surface occurs along the lateral surface
of the spermatozoon, rather than at the tip as in other animals, so that the spermatozoon lies flat
on the egg surface; several points of fusion occur between the plasma membranes of the two
gametes (i.e., the breakdown of membranes occurs by formation of numerous small vesicles).
Specificity of sperm–egg interaction
Although fertilization is strictly species-specific, very little is known about the molecular basis of
such specificity. The egg coats may have a role. Among the echinoderms solutions of the jelly
coat clump, or agglutinate, only spermatozoa of their own species. In both echinoderms and
amphibians, however, slight damage to an egg surface makes fertilization possible with
spermatozoa of different species (heterologous fertilization); this procedure has been used to
obtain certain hybrid larvae.
The eggs of ascidians, or sea squirts, members of the chordate subphylum Tunicata, are covered
with a thick membrane called a chorion; the space between the chorion and the egg is filled with
cells called test cells. The gametes of ascidians, which have both male and female reproductive
organs in one animal, mature at the same time; yet self-fertilization does not occur. If the chorion
and the test cells are removed, however, not only is fertilization with spermatozoa of different
species possible, but self-fertilization also can occur.
Prevention of polyspermy
Most animal eggs are monospermic; i.e., only one spermatozoon is admitted into an egg. In some
eggs, protection against the penetration of the egg by more than one spermatozoon (polyspermy)
is due to some property of the egg surface; in others, however, the egg envelopes are responsible.
The ability of some eggs to develop a polyspermy-preventing reaction depends on a molecular
rearrangement of the egg surface that occurs during egg maturation (oogenesis). Although
immature sea urchin eggs have the ability to associate with spermatozoa, they also allow
multiple penetration; i.e., they are unable to develop a polyspermy-preventing reaction. Since the
mature eggs of most animals are fertilized before completion of meiosis and are able to develop a
polyspermy-preventing reaction, specific properties of the egg surface must have differentiated
by the time meiosis stops, which is when the egg is ready to be fertilized.
In some mammalian eggs defense against polyspermy depends on properties of the zona
pellucida; i.e., when a spermatozoon has started to move through the zona, it does not allow the
penetration of additional spermatozoa (zona reaction). In other mammals, however, the zona
reaction either does not take place or is weak, as indicated by the presence of numerous
spermatozoa in the space between the zona and egg surface. In such cases the polyspermypreventing reaction resides in the egg surface. Although the eggs of some kinds of animals (e.g.,
some amphibians, birds, reptiles, and sharks) are naturally polyspermic, only one spermatozoal
nucleus fuses with an egg nucleus to form a zygote nucleus; all of the other spermatozoa
degenerate.
Formation of the fertilization membrane
The most spectacular changes that follow fertilization occur at the egg surface. The best known
example, that of the sea urchin egg, is described below. An immediate response to fertilization is
the raising of a membrane, called a vitelline membrane, from the egg surface. In the beginning
the membrane is very thin; soon, however, it thickens, develops a well-organized molecular
structure, and is called the fertilization membrane. At the same time an extensive rearrangement
of the molecular structure of the egg surface occurs. The events leading to formation of the
fertilization membrane require about one minute.
At the point on the outer surface of the sea urchin egg at which a spermatozoan attaches, the thin
vitelline membrane becomes detached. As a result the membranes of the cortical granules come
into contact with the inner aspect of the egg’s plasma membrane and fuse with it, the granules
open, and their contents are extruded into the perivitelline space; i.e., the space between the egg
surface and the raised vitelline membrane. Part of the contents of the granules merge with the
vitelline membrane to form the fertilization membrane; if fusion of the contents of the cortical
granules with the vitelline membrane is prevented, the membrane remains thin and soft. Another
material that also derives from the cortical granules covers the surface of the egg to form a
transparent layer, called the hyaline layer, which plays an important role in holding together the
cells (blastomeres) formed during division, or cleavage, of the egg. The plasma membrane
surrounding a fertilized egg, therefore, is a mosaic structure containing patches of the original
plasma membrane of the unfertilized egg and areas derived from membranes of the cortical
granules. The events leading to the formation of the fertilization membrane are accompanied by
a change of the electric charge across the plasma membrane, referred to as the fertilization
potential, and a concurrent outflow of potassium ions (charged particles); both of these
phenomena are similar to those that occur in a stimulated nerve fibre. Another effect of
fertilization on the plasma membrane of the egg is a several-fold increase in its permeability to
various molecules; this change may be the result of the activation of some surface-located
membrane transport mechanism.
Formation of the zygote nucleus
After its entry into the egg cytoplasm, the spermatozoal nucleus, now called the male pronucleus,
begins to swell, and its chromosomal material disperses and becomes similar in appearance to
that of the female pronucleus. Although the membranous envelope surrounding the male
pronucleus rapidly disintegrates in the egg, a new envelope promptly forms around it. The male
pronucleus, which rotates 180° and moves towards the egg nucleus, initially is accompanied by
two structures (centrioles) that function in cell division. After the male and female pronuclei
have come into contact, the spermatozoal centrioles give rise to the first cleavage spindle, which
precedes division of the fertilized egg. In some cases fusion of the two pronuclei may occur by a
process of membrane fusion; in this process, two adjoining membranes fuse at the point of
contact to give rise to the continuous nuclear envelope that surrounds the zygote nucleus.
Biochemical analysis of fertilization
Many of the early studies on biochemical changes occurring during fertilization were concerned
with the respiratory metabolism of the egg. The results, however, were deceiving; the sea urchin
egg, for example, showed an increased rate of oxygen consumption as an immediate response to
either fertilization or parthenogenetic activation, in apparent support of the idea that the essence
of fertilization is the removal of a respiratory or metabolic block in the unfertilized egg.
Extensive comparative studies have shown that the increased rate of oxygen consumption in
fertilized sea urchin eggs is not a general rule; indeed, the rate of oxygen consumption of most
animal eggs does not change at the time of fertilization and may even temporarily decrease.
At the time of fertilization the egg contains the components required to carry out protein
synthesis, and hence development, through an early embryonic stage called the blastula. Most
immediate post-fertilization protein synthesis is directed by molecules of ribonucleic acid,
known as messenger RNA, that were formed during oogenesis and stored in the egg. In addition,
protein synthesis up to the blastula stage (up to a much earlier stage in the mammalian embryo)
is directed by the cell components called ribosomes, which are present in the unfertilized egg;
new ribosomes, as well as molecules of another type of RNA involved in protein synthesis, and
called transfer RNA, are synthesized at a later stage in embryonic development (gastrulation).
Eggs fertilized and allowed to develop in the presence of the antibiotic actinomycin, which
suppresses RNA synthesis, not only reach the blastula stage but their rate of protein synthesis is
the same as that in untreated embryos.
Unfertilized sea urchin eggs, as well as those of other marine animals studied thus far, have a
very low rate of protein synthesis, suggesting that something in the unfertilized egg inhibits its
protein synthesizing machinery. Since the rate of protein synthesis increases immediately
following fertilization, it may depend on some change in, or removal of, an inhibitor. In the sea
urchin egg, for example, the low efficiency of the protein synthesizing apparatus apparently
depends on certain properties of the ribosomes. Most of the ribosomes found in an unfertilized
sea urchin egg are single ribosomes (so-called monosomes); soon after fertilization, however, the
single ribosomes interact with messenger RNA molecules thus giving rise to the polyribosomes,
which are the active units in protein synthesis. This process also occurs in eggs of a few other
marine animals that have been studied. The protein-synthesizing inefficiency of unfertilized seaurchin-egg ribosomes is caused by an inhibitor that is associated with them and interferes with
the binding of messenger RNA molecules to the ribosomes; the inhibitor is removed almost
immediately following fertilization, perhaps by enzymatic breakdown.
It thus appears that at least in the sea urchin egg the overall rate of protein synthesis is controlled
at the ribosome level and that the first step in the activation of protein synthesis following
fertilization is the “turning on” of the ribosomes.
In vertebrates such as amphibians, activation of protein synthesis takes place at the onset of egg
maturation, apparently initiated by the action of a hormone, progesterone. The effect of
progesterone is
Cnidaria
From Wikipedia, the free encyclopedia
(Redirected from Cnideria)
Jump to: navigation, search
Cnidaria
Pacific sea nettles, Chrysaora fuscescens
Scientific classification
Domain:
Eukaryota
Kingdom:
Animalia
Phylum:
Cnidaria
Hatschek, 1888
Subphylum/Classes[3]
Anthozoa—corals and sea anemones
Medusozoa—jellyfish:[1]
Cubozoa—box jellyfish, sea wasps
Hydrozoa—hydroids, hydra-like
animals
Scyphozoa—true jellyfish
Staurozoa—stalked jellyfish
Unranked, may not be scyphozoans[2]
Myxozoa—parasites
Polypodiozoa—parasites
Cnidaria ( /naɪˈdɛəriə/ with a silent c) is a phylum containing over 10,000[4] species of
animals found exclusively in aquatic and mostly marine environments. Their distinguishing
feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies
consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of
epithelium that are mostly one cell thick. They have two basic body forms: swimming medusae
and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles
that bear cnidocytes. Both forms have a single orifice and body cavity that are used for digestion
and respiration. Many cnidarian species produce colonies that are single organisms composed of
medusa-like or polyp-like zooids, or both. Cnidarians' activities are coordinated by a
decentralized nerve net and simple receptors. Several free-swimming Cubozoa and Scyphozoa
possess balance-sensing statocysts, and some have simple eyes. Not all cnidarians reproduce
sexually. Many have complex lifecycles with asexual polyp stages and sexual medusae, but some
omit either the polyp or the medusa stage.
Cnidarians were for a long time grouped with Ctenophores in the phylum Coelenterata, but
increasing awareness of their differences caused them to be placed in separate phyla. Cnidarians
are classified into four main groups: the almost wholly sessile Anthozoa (sea anemones, corals,
sea pens); swimming Scyphozoa (jellyfish); Cubozoa (box jellies); and Hydrozoa, a diverse
group that includes all the freshwater cnidarians as well as many marine forms, and has both
sessile members such as Hydra and colonial swimmers such as the Portuguese Man o' War.
Staurozoa have recently been recognised as a class in their own right rather than a sub-group of
Scyphozoa, and there is debate about whether Myxozoa and Polypodiozoa are cnidarians or
closer to bilaterians (more complex animals).
Most cnidarians prey on organisms ranging in size from plankton to animals several times larger
than themselves, but many obtain much of their nutrition from endosymbiotic algae, and a few
are parasites. Many are preyed upon by other animals including starfish, sea slugs, fish and
turtles. Coral reefs, whose polyps are rich in endosymbiotic algae, support some of the world's
most productive ecosystems, and protect vegetation in tidal zones and on shorelines from strong
currents and tides. While corals are almost entirely restricted to warm, shallow marine waters,
other cnidarians live in the depths, in polar seas and in freshwater.
Fossil cnidarians have been found in rocks formed about 580 million years ago, and other fossils
show that corals may have been present shortly before 490 million years ago and diversified a
few million years later. Fossils of cnidarians that do not build mineralized structures are very
rare. Scientists currently think that cnidarians, ctenophores and bilaterians are more closely
related to calcareous sponges than these are to other sponges, and that anthozoans are the
evolutionary "aunts" or "sisters" of other cnidarians, and the most closely related to bilaterians.
Recent analyses have concluded that cnidarians, although considered more "primitive" than
bilaterians, have a wider range of genes.
Jellyfish stings killed several hundred people in the 20th century, and cubozoans are particularly
dangerous. On the other hand, some large jellyfish are considered a delicacy in eastern and
southern Asia. Coral reefs have long been economically important as providers of fishing
grounds, protectors of shore buildings against currents and tides, and more recently as centers of
tourism. However, they are vulnerable to over-fishing, mining for construction materials,
pollution, and damage caused by tourism.
Contents
[hide]
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







1 Distinguishing features
2 Description
o 2.1 Basic body forms
o 2.2 Colonial forms
o 2.3 Skeletons
o 2.4 Main cell layers
o 2.5 Cnidocytes
o 2.6 Locomotion
o 2.7 Nervous system and senses
o 2.8 Feeding and excretion
o 2.9 Respiration
o 2.10 Regeneration
3 Reproduction
o 3.1 Sexual
o 3.2 Asexual
4 Classification
5 Ecology
6 Evolutionary history
o 6.1 Fossil record
o 6.2 Family tree
7 Interaction with humans
8 Notes
9 Further reading
o 9.1 Books
o 9.2 Journal articles
10 External links
[edit] Distinguishing features
Further information: Sponge, Ctenophore, and Bilateria
Cnidarians form an animal phylum that is more complex than sponges, about as complex as
ctenophores (comb jellies), and less complex than bilaterians, which include almost all other
animals. However, both cnidarians and ctenophores are more complex than sponges as they
have: cells bound by inter-cell connections and carpet-like basement membranes; muscles;
nervous systems; and some have sensory organs. Cnidarians are distinguished from all other
animals by having cnidocytes that fire like harpoons and are used mainly to capture prey but also
as anchors in some species.[5]
Like sponges and ctenophores, cnidarians have two main layers of cells that sandwich a middle
layer of jelly-like material, which is called the mesoglea in cnidarians; more complex animals
have three main cell layers and no intermediate jelly-like layer. Hence, cnidarians and
ctenophores have traditionally been labelled diploblastic, along with sponges.[5][6] However, both
cnidarians and ctenophores have a type of muscle that, in more complex animals, arises from the
middle cell layer.[7] As a result some recent text books classify ctenophores as triploblastic,[8] and
it has been suggested that cnidarians evolved from triploblastic ancestors.[7]
Sponges[9][10]
No
Cnidarians[5][6] Ctenophores[5][8] Bilateria[5]
Yes
No
Yes
No
Cnidocytes
No
Colloblasts
Digestive and
No
Yes
circulatory
organs
Two[5] or
Number of main
Two, with jelly-like layer between them
Three
Three[7][8]
cell layers
No, except that
Cells in each
Homoscleromorpha have Yes: inter-cell connections; basement membranes
layer bound
basement membranes.[11]
together
No
Yes
Sensory organs
Number of cells
(Not
Many
Few
in middle "jelly"
applicable)
layer
Cells in outer
(Not
layers can move
Yes
No
applicable)
inwards and
change functions
Simple to
No
Yes, simple
Nervous system
complex
Mostly
Mostly
Mostly
None
Muscles
epitheliomuscular myoepithelial
myocytes
[edit] Description
[edit] Basic body forms
Aboral end
Oral end
Mouth
Oral end
Aboral end
Exoderm
Gastroderm (Endoderm)
Mesoglea
Digestive cavity
Medusa (left) and polyp (right)[6]
Oral end of actinodiscus polyp, with close-up of the mouth
Adult cnidarians appear as either swimming medusae or sessile polyps. Both are radially
symmetrical, like a wheel and a tube respectively. Since these animals have no heads, their ends
are described as "oral" (nearest the mouth) and "aboral" (furthest from the mouth). Most have
fringes of tentacles equipped with cnidocytes around their edges, and medusae generally have an
inner ring of tentacles around the mouth. The mesoglea of polyps is usually thin and often soft,
but that of medusae is usually thick and springy, so that it returns to its original shape after
muscles around the edge have contracted to squeeze water out, enabling medusae to swim by a
sort of jet propulsion.[6]
[edit] Colonial forms
Tree-like polyp colony[6]
Cnidaria produce a variety of colonial forms, each of which is one organism but consists of
polyp-like zooids. The simplest is a connecting tunnel that runs over the substrate (rock or
seabed) and from which single zooids sprout. In some cases the tunnels form visible webs, and in
others they are enclosed in a fleshy mat. More complex forms are also based on connecting
tunnels but produce "tree-like" groups of zooids. The "trees" may be formed either by a central
zooid that functions as a "trunk" with later zooids growing to the sides as "branches", or in a zigzag shape as a succession of zooids, each of which grows to full size and then produces a single
bud at an angle to itself. In many cases the connecting tunnels and the "stems" are covered in
periderm, a protective layer of chitin.[6] Some colonial forms have other specialized types of
zooid, for example, to pump water through their tunnels.[12]
Siphonophores form complex colonies that consist of: an upside-down polyp that forms a central
stem with a gas-filled float at the top; one or more sets of medusa-like zooids that provide
propulsion; leaf-like bracts that give some protection to other parts; sets of tentacles that bear
nematocytes that capture prey; other tentacles that act as sensors; near the base of each set of
tentacles, a polyp-like zooid that acts as a stomach for the colony; medusa-like zooids that serve
as gonads. Although some of these zooids resemble polyps or medusae in shape, they lack
features that are not relevant to their specific functions, for example the swimming "medusae"
have no digestive, sensory or reproductive cells. The best-known siphonophore is the Portuguese
Man o' War (Physalia physalis).[12][13][14]
[edit] Skeletons
In medusae the only supporting structure is the mesoglea. Hydra and most sea anemones close
their mouths when they are not feeding, and the water in the digestive cavity then acts as a
hydrostatic skeleton, rather like a water-filled balloon. Other polyps such as Tubularia use
columns of water-filled cells for support. Sea pens stiffen the mesoglea with calcium carbonate
spicules and tough fibrous proteins, rather like sponges.[6]
In some colonial polyps a chitinous periderm gives support and some protection to the
connecting sections and to the lower parts of individual polyps. Stony corals secrete massive
calcium carbonate exoskeletons. A few polyps collect materials such as sand grains and shell
fragments, which they attach to their outsides. Some colonial sea anemones stiffen the mesoglea
with sediment particles.[6]
[edit] Main cell layers
Cnidaria are diploblastic animals, in other words they have two main cell layers, while more
complex animals are triploblasts having three main layers. The two main cell layers of cnidarians
form epithelia that are mostly one cell thick, and are attached to a fibrous basement membrane,
which they secrete. They also secrete the jelly-like mesoglea that separates the layers. The layer
that faces outwards, known as the ectoderm ("outside skin"), generally contains the following
types of cells:[5]




Epitheliomuscular cells whose bodies form part of the epithelium but whose bases extend
to form muscle fibers in parallel rows.[15] The fibers of the outward-facing cell layer
generally run at right angles to the fibers of the inward-facing one. In Anthozoa
(anemones, corals, etc.) and Scyphozoa (jellyfish), the mesoglea also contains some
muscle cells.[6]
Cnidocytes, the harpoon-like "nettle cells" that give the phylum Cnidaria its name. These
appear between or sometimes on top of the muscle cells.[5]
Nerve cells. Sensory cells appear between or sometimes on top of the muscle cells,[5] and
communicate via synapses (gaps across which chemical signals flow) with motor nerve
cells, which lie mostly between the bases of the muscle cells.[6]
Interstitial cells, which are unspecialized and can replace lost or damaged cells by
transforming into the appropriate types. These are found between the bases of muscle
cells.[5]
In addition to epitheliomuscular, nerve and interstitial cells, the inward-facing gastroderm
("stomach skin") contains gland cells that secrete digestive enzymes. In some species it also
contains low concentrations of cnidocytes, which are used to subdue prey that is still
struggling.[5][6]
The mesoglea contains small numbers of amoeba-like cells,[6] and muscle cells in some
species.[5] However the number of middle-layer cells and types are much lower than in
sponges.[6]
[edit] Cnidocytes
A hydra's nematocyst, before firing.
"trigger" cilium[6]
Firing sequence of the cnida in a hydra's nematocyst[6]
Operculum (lid)
"Finger" that turns inside out
/ / / Barbs
Venom
Victim's skin
Victim's tissues
These "nettle cells" function as harpoons, since their payloads remain connected to the bodies of
the cells by threads. Three types of cnidocytes are known:[5][6]



Nematocysts inject venom into prey, and usually have barbs to keep them embedded in
the victims. Most species have nematocysts.[5]
Spirocysts do not penetrate the victim or inject venom, but entangle it by means of small
sticky hairs on the thread.
Ptychocysts are not used for prey capture — instead the threads of discharged
ptychocysts are used for building protective tubes in which their owners live. Ptychocysts
are found only in the order Cerianthria, tube anemones.[6]
The main components of a cnidocyte are:[5][6]




A cilium (fine hair) which projects above the surface and acts as a trigger. Spirocysts do
not have cilia.
A tough capsule, the cnida, which houses the thread, its payload and a mixture of
chemicals which may include venom or adhesives or both. ("cnida" is derived from the
Greek word κνίδη, which means "nettle"[16])
A tube-like extension of the wall of the cnida that points into the cnida, like the finger of
a rubber glove pushed inwards. When a cnidocyte fires, the finger pops out. If the cell is a
venomous nematocyte, the "finger"'s tip reveals a set of barbs that anchor it in the prey.
The thread, which is an extension of the "finger" and coils round it until the cnidocyte
fires. The thread is usually hollow and delivers chemicals from the cnida to the target.


An operculum (lid) over the end of the cnida. The lid may be a single hinged flap or three
flaps arranged like slices of pie.
The cell body which produces all the other parts.
It is difficult to study the firing mechanisms of cnidocytes as these structures are small but very
complex. At least four hypotheses have been proposed:[5]




Rapid contraction of fibers round the cnida may increase its internal pressure.
The thread may be like a coiled spring that extends rapidly when released.
In the case of Chironex (the "sea wasp"), chemical changes in the cnida's contents may
cause them to expand rapidly by polymerization.
Chemical changes in the liquid in the cnida make it a much more concentrated solution,
so that osmotic pressure forces water in very rapidly to dilute it. This mechanism has
been observed in nematocysts of the class Hydrozoa, sometimes producing pressures as
high as 140 atmospheres, similar to that of scuba air tanks, and fully extending the thread
in as little as 2 milliseconds (0.002 second).[6]
Cnidocytes can only fire once, and about 25% of a hydra's nematocysts are lost from its tentacles
when capturing a brine shrimp. Used cnidocytes have to be replaced, which takes about 48 hours.
To minimise wasteful firing, two types of stimulus are generally required to trigger cnidocytes:
their cilia detect contact, and nearby sensory cells "smell" chemicals in the water. This
combination prevents them from firing at distant or non-living objects. Groups of cnidocytes are
usually connected by nerves and, if one fires, the rest of the group requires a weaker minimum
stimulus than the cells that fire first.[5][6]
[edit] Locomotion
Chrysaora quinquecirrha ("sea nettle") swimming
Medusae swim by a form of jet propulsion: muscles, especially inside the rim of the bell, squeeze
water out of the cavity inside the bell, and the springiness of the mesoglea powers the recovery
stroke. Since the tissue layers are very thin, they provide too little power to swim against currents
and just enough to control movement within currents.[6]
Hydras and some sea anemones can move slowly over rocks and sea or stream beds by various
means: creeping like snails, crawling like inchworms, or by somersaulting. A few can swim
clumsily by waggling their bases.[6]
[edit] Nervous system and senses
Cnidaria have no brains or even central nervous systems. Instead they have decentralized nerve
nets consisting of : sensory neurons that generate signals in response to various types of stimulus,
such as odors; motor neurons that tell muscles to contract; all connected by "cobwebs" of
intermediate neurons. As well as forming the "signal cables", intermediate neurons also form
ganglia that act as local coordination centers. The cilia of the cnidocytes detect physical contact.
Nerves inform cnidocytes when odors from prey or attackers are detected and when
neighbouring cnidocytes fire. Most of the communications between nerve cells are via chemical
synapses, small gaps across which chemicals flow. As this process is too slow to ensure that the
muscles round the rim of a medusa's bell contract simultaneously in swimming the neurons
which control this communicate by much faster electrical signals across gap junctions.[6]
Medusae and complex swimming colonies such as siphonophores and chondrophores sense tilt
and acceleration by means of statocysts, chambers lined with hairs which detect the movements
of internal mineral grains called statoliths. If the body tilts in the wrong direction, the animal
rights itself by increasing the strength of the swimming movements on the side that is too low.
They also have ocelli ("little eyes"), which can detect the direction from which light is coming.
Box jellies have camera eyes, although these probably do not form images, and their lenses
simply produce a clearer indication of the direction from which light is coming.[5]
[edit] Feeding and excretion
Cnidarians feed in several ways: predation, absorbing dissolved organic chemicals, filtering food
particles out of the water, and obtaining nutrients from symbiotic algae within their cells. Most
obtain the majority of their food from predation but some, including the corals Hetroxenia and
Leptogorgia, depend almost completely on their endosymbionts and on absorbing dissolved
nutrients.[5] Cnidaria give their symbiotic algae carbon dioxide, some nutrients and a place in the
sun.[6]
Predatory species use their cnidocytes to poison or entangle prey, and those with venomous
nematocysts may start digestion by injecting digestive enzymes. The "smell" of fluids from
wounded prey makes the tentacles fold inwards and wipe the prey off into the mouth. In medusae
the tentacles round the edge of the bell are often short and most of the prey capture is done by
"oral arms", which are extensions of the edge of the mouth and are often frilled and sometimes
branched to increase their surface area. Medusae often trap prey or suspended food particles by
swimming upwards, spreading their tentacles and oral arms and then sinking. In species for
which suspended food particles are important, the tentacles and oral arms often have rows of
cilia whose beating creates currents that flow towards the mouth, and some produce nets of
mucus to trap particles.[5]
Once the food is in the digestive cavity, gland cells in the gastroderm release enzymes that
reduce the prey to slurry, usually within a few hours. This circulates through the digestive cavity
and, in colonial cnidarians, through the connecting tunnels, so that gastroderm cells can absorb
the nutrients. Absorption may take a few hours, and digestion within the cells may take a few
days. The circulation of nutrients is driven by water currents produced by cilia in the gastroderm
or by muscular movements or both, so that nutrients reach all parts of the digestive cavity.[6]
Nutrients reach the outer cell layer by diffusion or, for animals or zooids such as medusae which
have thick mesogleas, are transported by mobile cells in the mesoglea.[5]
Indigestible remains of prey are expelled through the mouth. The main waste product of cells'
internal processes is ammonia, which is removed by the external and internal water currents.[6]
[edit] Respiration
There are no respiratory organs, and both cell layers absorb oxygen from and expel carbon
dioxide into the surrounding water. When the water in the digestive cavity becomes stale it must
be replaced, and nutrients that have not been absorbed will be expelled with it. Some Anthozoa
have ciliated grooves on their tentacles, allowing them to pump water out of and into the
digestive cavity without opening the mouth. This improves respiration after feeding and allows
these animals, which use the cavity as a hydrostatic skeleton, to control the water pressure in the
cavity without expelling undigested food.[5]
Cnidaria that carry photosynthetic symbionts may have the opposite problem, an excess of
oxygen, which may prove toxic. The animals produce large quantities of antioxidants to
neutralize the excess oxygen.[5]
[edit] Regeneration
All cnidarians can regenerate, allowing them to recover from injury and to reproduce asexually.
Medusae have limited ability to regenerate, but polyps can do so from small pieces or even
collections of separated cells. This enables corals to recover even after apparently being
destroyed by predators.[5]
[edit] Reproduction
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Life cycle of a jellyfish:[5][6]
1–3 Larva searches for site
4–8 Polyp grows
9–11 Polyp strobilates
12–14 Medusa grows
[edit] Sexual
In the Cnidaria sexual reproduction often involves a complex life cycle with both polyp and
medusa stages. For example in Scyphozoa (jellyfish) and Cubozoa (box jellies) a larva swims
until it finds a good site, and then becomes a polyp. This grows normally but then absorbs its
tentacles and splits horizontally into a series of disks that become juvenile medusae, a process
called strobilation. The juveniles swim off and slowly grow to maturity, while the polyp regrows and may continue strobilating periodically. The adults have gonads in the gastroderm, and
these release ova and sperm into the water in the breeding season.[5][6]
Shortened forms of this life cycle are common, for example some oceanic scyphozoans omit the
polyp stage completely, and cubozoan polyps produce only one medusa. Hydrozoa have a
variety of life cycles. Some have no polyp stages and some (e.g. hydra) have no medusae. In
some species the medusae remain attached to the polyp and are responsible for sexual
reproduction; in extreme cases these reproductive zooids may not look much like medusae.
Anthozoa have no medusa stage at all and the polyps are responsible for sexual reproduction.[5]
Spawning is generally driven by environmental factors such as changes in the water temperature,
and their release is triggered by lighting conditions such as sunrise, sunset or the phase of the
moon. Many species of Cnidaria may spawn simultaneously in the same location, so that there
are too many ova and sperm for predators to eat more than a tiny percentage — one famous
example is the Great Barrier Reef, where at least 110 corals and a few non-cnidarian
invertebrates produce enough to turn the water cloudy. These mass spawnings may produce
hybrids, some of which can settle and form polyps, but it is not known how long these can
survive. In some species the ova release chemicals that attract sperm of the same species.[5]
The fertilized eggs develop into larvae by dividing until there are enough cells to form a hollow
sphere (blastula) and then a depression forms at one end (gastrulation) and eventually become
the digestive cavity. However in cnidarians the depression forms at the end further from the yolk
(at the animal pole), while in bilaterians it forms at the other end (vegetal pole).[6] The larvae,
called planulae, swim or crawl by means of cilia.[5] They are cigar-shaped but slightly broader at
the "front" end, which is the aboral, vegetal-pole end and eventually attaches to a substrate if the
species has a polyp stage.[6]
Anthozoan larvae either have large yolks or are capable of feeding on plankton, and some
already have endosymbiotic algae that help to feed them. Since the parents are immobile, these
feeding capabilities extend the larvae's range and avoid overcrowding of sites. Scyphozoan and
hydrozoan larvae have little yolk and most lack endosymbiotic algae, and therefore have to settle
quickly and metamorphose into polyps. Instead these species rely on their medusae to extend
their ranges.[6]
[edit] Asexual
All known cnidaria can reproduce asexually by various means, in addition to regenerating after
being fragmented. Hydrozoan polyps only bud, while the medusae of some hydrozoans can
divide down the middle. Scyphozoan polyps can both bud and split down the middle. In addition
to both of these methods, Anthozoa can split horizontally just above the base.[5][6]
[edit] Classification
Cnidarians were for a long time grouped with Ctenophores in the phylum Coelenterata, but
increasing awareness of their differences caused them to be placed in separate phyla. Cnidarians
are classified into four main groups: sessile Anthozoa (sea anemones, corals, sea pens);
swimming Scyphozoa (jellyfish); Cubozoa (box jellies); and Hydrozoa, a diverse group that
includes all the freshwater cnidarians as well as many marine forms, and has both sessile
members such as Hydra and colonial swimmers such as the Portuguese Man o' War. Staurozoa
have recently been recognised as a class in their own right rather than a sub-group of Scyphozoa,
and there is debate about whether Myxozoa and Polypodiozoa are cnidarians or closer to
bilaterians.
Modern cnidarians are generally classified into four classes:[5]
Number of species
Examples
Cells found in
mesoglea
Nematocysts in
exodermis
[4]
Hydrozoa
3,600
Hydra,
siphonophores
Scyphozoa
228
Jellyfish
Cubozoa
Anthozoa
42
6,100
Box
Sea anemones,
jellies
corals, sea pens
No
Yes
Yes
Yes
No
Yes
Yes
Yes
Yes, except for
Medusa phase in life
In some species Stauromedusae if they are Yes
cycle
scyphozoans
Number of medusae
Many
Many
One
produced per polyp
No
(not applicable)
Stauromedusae, small sessile cnidarians with stalks and no medusa stage, have traditionally been
classified as members of the Scyphozoa, but recent research suggests they should be regarded as
a separate class, Staurozoa.[17]
The Myxozoa, microscopic parasites, were first classified as protozoans,[18] but recently as
heavily modified cnidarians, and more closely related to Hydrozoa and Scyphozoa than to
Anthozoa.[19] However other recent research suggests that Polypodium hydriforme, a parasite
within the egg cells of sturgeon, is closely related to the Myxozoa and that both Polypodium and
the Myxozoa are intermediate between cnidarians and bilaterian animals.[20]
Some researchers classify the extinct conulariids as cnidarians, while others propose that they
form a completely separate phylum.[21]
[edit] Ecology
Coral reefs support rich ecosystems
Many cnidarians are limited to shallow waters because they depend on endosymbiotic algae for
much of their nutrients. The life cycles of most have polyp stages, which are limited to locations
that offer stable substrates. Nevertheless major cnidarian groups contain species that have
escaped these limitations. Hydrozoans have a worldwide range: some, such as Hydra, live in
freshwater; Obelia appears in the coastal waters of all the oceans; and Liriope can form large
shoals near the surface in mid-ocean. Among anthozoans, a few scleractinian corals, sea pens
and sea fans live in deep, cold waters, and some sea anemones inhabit polar seabeds while others
live near hydrothermal vents over 10 kilometres (6.2 mi) below sea-level. Reef-building corals
are limited to tropical seas between 30°N and 30°S with a maximum depth of 46 metres (151 ft),
temperatures between 20°C and 28°C, high salinity and low carbon dioxide levels.
Stauromedusae, although usually classified as jellyfish, are stalked, sessile animals that live in
cool to Arctic waters.[12] Cnidarians range in size from Hydra, 5–20 millimetres (0.20–0.79 in)
long,[22] to the Lion's mane jellyfish, which may exceed 2 metres (6.6 ft) in diameter and 75
metres (246 ft) in length.[23]
Prey of cnidarians ranges from plankton to animals several times larger than themselves.[12][24]
Some cnidarians are parasites, mainly on jellyfish but a few are major pests of fish.[12] Others
obtain most of their nourishment from endosymbiotic algae or dissolved nutrients.[5] Predators of
cnidarians include: sea slugs, which can incorporate nematocysts into their own bodies for selfdefense;[25] starfish, notably the crown of thorns starfish, which can devastate corals;[12] butterfly
fish and parrot fish, which eat corals;[26] and marine turtles, which eat jellyfish.[23] Some sea
anemones and jellyfish have a symbiotic relationship with some fish; for example clown fish live
among the tentacles of sea anemones, and each partner protects the other against predators.[12]
Coral reefs form some of the world's most productive ecosystems. Common coral reef cnidarians
include both Anthozoans (hard corals, octocorals, anemones) and Hydrozoans (fire corals, lace
corals) The endosymbiotic algae of many cnidarian species are very effective primary producers,
in other words converters of inorganic chemicals into organic ones that other organisms can use,
and their coral hosts use these organic chemicals very efficiently. In addition reefs provide
complex and varied habitats that support a wide range of other organisms.[27] Fringing reefs just
below low-tide level also have a mutually beneficial relationship with mangrove forests at high-
tide level and sea grass meadows in between: the reefs protect the mangroves and seagrass from
strong currents and waves that would damage them or erode the sediments in which they are
rooted, while the mangroves and seagrass protect the coral from large influxes of silt, fresh water
and pollutants. This additional level of variety in the environment is beneficial to many types of
coral reef animals, which for example may feed in the sea grass and use the reefs for protection
or breeding.[28]
[edit] Evolutionary history
[edit] Fossil record
The fossil coral Cladocora from Pliocene rocks in Cyprus
The earliest widely accepted animal fossils are rather modern-looking cnidarians, possibly from
around 580 million years ago, although fossils from the Doushantuo Formation can only be dated
approximately.[29] The identification of some of these as embryos of animals has been contested,
but other fossils from these rocks strongly resemble tubes and other mineralized structures made
by corals.[30] Their presence implies that the cnidarian and bilaterian lineages had already
diverged.[31] Although the Ediacaran fossil Charnia used to be classified as a jellyfish or sea
pen,[32] more recent study of growth patterns in Charnia and modern cnidarians has cast doubt on
this hypothesis,[33][34] and there are now no bona-fide cnidarian body fossils in the Ediacaran.
Few fossils of cnidarians without mineralized skeletons are known from more recent rocks,
except in lagerstätten that preserved soft-bodied animals.[35]
A few mineralized fossils that resemble corals have been found in rocks from the Cambrian
period, and corals diversified in the Early Ordovician.[35] These corals, which were wiped out in
the Permian-Triassic extinction about 251 million years ago,[35] did not dominate reef
construction since sponges and algae also played a major part.[36] During the Mesozoic era rudist
bivalves were the main reef-builders, but they were wiped out in the Cretaceous-Tertiary
extinction 65 million years ago,[37] and since then the main reef-builders have been scleractinian
corals.[35]
[edit] Family tree
Further information: Phylogeny
Metazoa
Glass sponges
Calcareous sponges
Eumetazoa
Ctenophora (comb jellies)
Planulozoa Cnidaria
Anthozoa
(sea anemones and corals)
Medusozoa
Hydrozoa
(Hydra, siphonophores, etc.)
Cubozoa
(box jellies)
Staurozoa
"Scyphozoa"
(jellyfish, excluding
Staurozoa)
Placozoa
Bilateria
Myxozoa
Other Bilateria
(more complex)
Family tree of Cnidaria and the origins of animals[2][38][39][40]
It is difficult to reconstruct the early stages in the evolutionary "family tree" of animals using
only morphology (their shapes and structures), because the large differences between Porifera
(sponges), Cnidaria plus Ctenophora (comb jellies), Placozoa and Bilateria (all the more complex
animals) make comparisons difficult. Hence reconstructions now rely largely or entirely on
molecular phylogenetics, which groups organisms according to similarities and differences in
their biochemistry, usually in their DNA or RNA.[41]
It is now generally thought that the Calcarea (sponges with calcium carbonate spicules) are more
closely related to Cnidaria, Ctenophora (comb jellies) and Bilateria (all the more complex
animals) than they are to the other groups of sponges.[38][42][43] In 1866 it was proposed that
Cnidaria and Ctenophora were more closely related to each other than to Bilateria and formed a
group called Coelenterata ("hollow guts"), because Cnidaria and Ctenophora both rely on the
flow of water in and out of a single cavity for feeding, excretion and respiration. In 1881 it was
proposed that Ctenophora and Bilateria were more closely related to each other, since they
shared features that Cnidaria lack, for example muscles in the middle layer (mesoglea in
Ctenophora, mesoderm in Bilateria). However more recent analyses indicate that these
similarities are rather vague, and the current view, based on molecular phylogenetics, is that
Cnidaria and Bilateria are more closely related to each other than either is to Ctenophora. This
grouping of Cnidaria and Bilateria has been labelled "Planulozoa" because it suggests that the
earliest Bilateria were similar to the planula larvae of Cnidaria.[2][39]
Within the Cnidaria, the Anthozoa (sea anemones and corals) are regarded as the sister-group of
the rest, which suggests that the earliest cnidarians were sessile polyps with no medusa stage.
However it is unclear how the other groups acquired the medusa stage, since Hydrozoa form
medusae by budding from the side of the polyp while the other Medusozoa do so by splitting
them off from the tip of the polyp. The traditional grouping of Scyphozoa included the
Staurozoa, but morphology and molecular phylogenetics indicate that Staurozoa are more closely
related to Cubozoa (box jellies) than to other "Scyphozoa". Similarities in the double body walls
of Staurozoa and the extinct Conulariida suggest that they are closely related. The position of
Anthozoa nearest the beginning of the cnidarian family tree also implies that Anthozoa are the
cnidarians most closely related to Bilateria, and this is supported by the fact that Anthozoa and
Bilateria share some genes that determine the main axes of the body.[2][44]
However in 2005 Katja Seipel and Volker Schmid suggested that cnidarians and ctenophores are
simplified descendants of triploblastic animals, since ctenophores and the medusa stage of some
cnidarians have striated muscle, which in bilaterians arises from the mesoderm. They did not
commit themselves on whether bilaterians evolved from early cnidarians or from the
hypothesized triploblastic ancestors of cnidarians.[7]
In molecular phylogenetics analyses from 2005 onwards, important groups of developmental
genes show the same variety in cnidarians as in chordates.[45] In fact cnidarians, and especially
anthozoans (sea anemones and corals), retain some genes that are present in bacteria, protists,
plants and fungi but not in bilaterians.[46]
The mitochondial genomes in the medusozoan cnidarians unlike that of other animals is linear
with fragmented genes.[47] The reason for this difference is unknown.
[edit] Interaction with humans
Jellyfish stings killed about 1,500 people in the 20th century,[48] and cubozoans are particularly
dangerous. On the other hand, some large jellyfish are considered a delicacy in eastern and
southern Asia. Coral reefs have long been economically important as providers of fishing
grounds, protectors of shore buildings against currents and tides, and more recently as centers of
tourism. However, they are vulnerable to over-fishing, mining for construction materials,
pollution, and damage caused by tourism.
Beaches protected from tides and storms by coral reefs are often the best places for housing in
tropical countries. Reefs are an important food source for low-technology fishing, both on the
reefs themselves and in the adjacent seas.[49] However despite their great productivity reefs are
vulnerable to over-fishing, because much of the organic carbon they produce is exhaled as
carbon dioxide by organisms at the middle levels of the food chain and never reaches the larger
species that are of interest to fishermen.[27] Tourism centered on reefs provides much of the
income of some tropical islands, attracting photographers, divers and sports fishermen. However
human activities damage reefs in several ways: mining for construction materials; pollution,
including large influxes of fresh water from storm drains; commercial fishing, including the use
of dynamite to stun fish and the capture of young fish for aquariums; and tourist damage caused
by boat anchors and the cumulative effect of walking on the reefs.[49] Coral, mainly from the
Pacific Ocean has long been used in jewellery, and demand rose sharply in the 1980s.[50]
The dangerous "sea wasp" Chironex fleckeri
Some large jellyfish species have been used in Chinese cuisine at least since 200 AD, and are
now fished in the seas around most of South East Asia. Japan is the largest single consumer of
edible jellyfish, importing at first only from China but now from all of South East Asia as prices
rose in the 1970s. This fishing industry is restricted to daylight hours and calm conditions in two
short seasons, from March to May and August to November.[51] The commercial value of
jellyfish food products depends on the skill with which they are prepared, and "Jellyfish
Masters" guard their trade secrets carefully. Jellyfish is very low in cholesterol and sugars, but
cheap preparation can introduce undesirable amounts of heavy metals.[52]
The "sea wasp" Chironex fleckeri has been described as the world's most venomous animal and
is held responsible for 67 deaths, although it is difficult to identify the animal as it is almost
transparent. Most stingings by C. fleckeri cause only mild symptoms.[53] Seven other box jellies
can cause a set of symptoms called Irukandji syndrome,[54] which takes about 30 minutes to
develop,[55] and from a few hours to two weeks to disappear.[56] Hospital treatment is usually
required, and there have been a few deaths.[54]
[edit] Notes
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
^ Classes in Medusozoa based on "The Taxonomicon - Taxon: Subphylum Medusozoa". Universal
Taxonomic Services. http://www.taxonomy.nl/Taxonomicon/TaxonTree.aspx?id=11582. Retrieved 200901-26.
^ a b c d Collins, A.G. (2002). "Phylogeny of Medusozoa and the Evolution of Cnidarian Life Cycles"
(PDF). Journal of Evolutionary Biology 15 (3): 418–432. doi:10.1046/j.1420-9101.2002.00403.x.
http://cima.uprm.edu/~n_schizas/CMOB_8676/Collins2002.pdf. Retrieved 2008-11-27.
^ Subphyla Anthozoa and Medusozoa based on "The Taxonomicon - Taxon: Phylum Cnidaria". Universal
Taxonomic Services. http://www.taxonomy.nl/Taxonomicon/TaxonTree.aspx?id=11551. Retrieved 200707-10.
^ a b Zhang, Z.-Q. (2011). "Animal biodiversity: An introduction to higher-level classification and
taxonomic richness". Zootaxa 3148: 7–12. http://mapress.com/zootaxa/2011/f/zt03148p012.pdf.
^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af Hinde, R.T., (1998). "The Cnidaria and Ctenophora". In
Anderson, D.T.,. Invertebrate Zoology. Oxford University Press. pp. 28–57. ISBN 0195513681.
^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af Ruppert, E.E., Fox, R.S., and Barnes, R.D. (2004).
Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 111–124. ISBN 0030259827.
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pp. 182–195. ISBN 0030259827.
^ Ruppert, E.E., Fox, R.S., and Barnes, R.D. (2004). Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 76–
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Proterozoic: a review". Biogeosciences 4 (2): 219–232. doi:10.5194/bg-4-219-2007.
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Invertebrate Biology 123 (1): 23–42. doi:10.1111/j.1744-7410.2004.tb00139.x.
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[edit] Further reading
[edit] Books


Arai, M.N. (1997). A Functional Biology of Scyphozoa. London: Chapman & Hall
[p. 316]. ISBN 0-412-45110-7.
Ax, P. (1999). Das System der Metazoa I. Ein Lehrbuch der phylogenetischen Systematik.
Gustav Fischer, Stuttgart-Jena: Gustav Fischer. ISBN 3-437-30803-3.
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

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Barnes, R.S.K., P. Calow, P. J. W. Olive, D. W. Golding & J. I. Spicer (2001). The
invertebrates—a synthesis. Oxford: Blackwell. 3rd edition [chapter 3.4.2, p. 54]. ISBN 0632-04761-5.
Brusca, R.C., G.J. Brusca (2003). Invertebrates. Sunderland, Mass.: Sinauer Associates.
2nd edition [chapter 8, p. 219]. ISBN 0-87893-097-3.
Dalby, A. (2003). Food in the Ancient World: from A to Z. London: Routledge.
Moore, J.(2001). An Introduction to the Invertebrates. Cambridge: Cambridge University
Press [chapter 4, p. 30]. ISBN 0-521-77914-6.
Schäfer, W. (1997). Cnidaria, Nesseltiere. In Rieger, W. (ed.) Spezielle Zoologie. Teil 1.
Einzeller und Wirbellose Tiere. Stuttgart-Jena: Gustav Fischer. Spektrum Akademischer
Verl., Heidelberg, 2004. ISBN 3-8274-1482-2.
Werner, B. 4. Stamm Cnidaria. In: V. Gruner (ed.) Lehrbuch der speziellen Zoologie.
Begr. von Kaestner. 2 Bde. Stuttgart-Jena: Gustav Fischer, Stuttgart-Jena. 1954, 1980,
1984, Spektrum Akad. Verl., Heidelberg-Berlin, 1993. 5th edition. ISBN 3-334-60474-8.
[edit] Journal articles
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D. Bridge, B. Schierwater, C. W. Cunningham, R. DeSalle R, L. W. Buss: Mitochondrial
DNA structure and the molecular phylogeny of recent cnidaria classes. in: Proceedings
of the Academy of Natural Sciences of Philadelphia. Philadelphia USA 89.1992, p. 8750.
ISSN 0097-3157
D. Bridge, C. W. Cunningham, R. DeSalle, L. W. Buss: Class-level relationships in the
phylum Cnidaria—Molecular and morphological evidence. in: Molecular biology and
evolution. Oxford University Press, Oxford 12.1995, p. 679. ISSN 0737-4038
D. G. Fautin: Reproduction of Cnidaria. in: Canadian Journal of Zoology. Ottawa Ont.
80.2002, p. 1735. (PDF, online) ISSN 0008-4301
G. O. Mackie: What's new in cnidarian biology? in: Canadian Journal of Zoology.
Ottawa Ont. 80.2002, p. 1649. (PDF, online) ISSN 0008-4301
P. Schuchert: Phylogenetic analysis of the Cnidaria. in: Zeitschrift für zoologische
Systematik und Evolutionsforschung. Paray, Hamburg-Berlin 31.1993, p. 161.
ISSN 0044-3808
G. Kass-Simon, A. A. Scappaticci Jr.: The behavioral and developmental physiology of
nematocysts. in: Canadian Journal of Zoology. Ottawa Ont. 80.2002, p. 1772. (PDF,
online) ISSN 0044-3808
J. Zrzavý (2001). "The interrelationships of metazoan parasites: a review of phylum- and
higher-level hypotheses from recent morphological and molecular phylogenetic analyses"
(PDF). Folia Parasitologica 48 (2): 81–103. PMID 11437135. Archived from the
original on 2007-10-25.
http://web.archive.org/web/20071025220832/http://www.paru.cas.cz/folia/pdf/201/Zrz.pdf. Retrieved 2009-01-26.
[edit] External links
Wikispecies has information related to: Cnidaria
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Look up Cnidaria in Wiktionary, the free dictionary.
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YouTube: Nematocysts Firing
YouTube:My Anemone Eat Meat Defensive and feeding behaviour of sea anemone
Cnidaria - Guide to the Marine Zooplankton of south eastern Australia, Tasmanian
Aquaculture & Fisheries Institute
A Cnidaria homepage maintained by University of California, Irvine
Cnidaria page at Tree of Life
Fossil Gallery: Cnidarians
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v
t
e
Eukaryota
Domain : Archaea · Bacteria · Eukaryota
Bikonta
Viridiplantae/Plantae
sensu stricto ·
Archaeplastida, or Plantae sensu lato
Rhodophyta ·
Glaucocystophyceae
AH
Haptophyta ·
Cryptophyta ·
Hacrobia,
or
non-SAR
chromalveolata
AH/SAR
Centroheliozoa
Heterokont ("S") Ochrophyta · Bigyra · Pseudofungi
Halvaria
Ciliates · Myzozoa (Apicomplexa,
SAR
Alveolata
Dinoflagellata)
Rhizaria Cercozoa · Retaria (Foraminifera, Radiolaria)
Excavata Discoba (Euglenozoa, Percolozoa) · Metamonad · Malawimonas
Unikont
a
Apusomonadida (Apusomonas, Amastigomonas) ·
Apusozoa Ancyromonadida (Ancyromonas) · Hemimastigida (Hemimastix,
Spironema, Stereonema)
Amoebozoa Lobosea · Conosa · Phalansterium · Breviata
Mesomycetozoea Dermocystida · Ichthyophonida
Filasterea
Capsaspora ·
Ministeria
Choanoflagellate Codonosigidae
Eumetazoa
(Bilateria,
Filozoa
Cnidaria,
Metazoa Ctenophora) ·
or "Animalia" Mesozoa ·
Parazoa
(Placozoa,
Porifera)
Holozoa
Opisthokont
a
Dikarya (Ascomycota, Basidiomycota) ·
Glomeromycota · Zygomycota ·
Fungi Blastocladiomycota ·
Chytridiomycota/Neocallimastigomycota
· Microsporidia
Holomycot
a
Nuclearia · Micronuclearia ·
Nucleariida
Rabdiophrys · Pinaciophora ·
e
Pompholyxophrys · Fonticula
[show]
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v
t
e
Extant phyla of kingdom Animalia by subkingdom
o
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Radiata
Ctenophora
Cnidaria
o Anthozoa
o Hydrozoa
o Scyphozoa
o Cubozoa
o Staurozoa
o Myxozoa
o Polypodiozoa
Scalidophor
a



Kinorhyncha
Loricifera
Priapulida
Nematoida


Nematoda
Nematomorpha
Cycloneuralia
Ecdysozoa
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


Panarthropo
da
Onychophora
Tardigrada
Arthropoda
Lobopodia


Platyhelminthes
Gastrotricha


Protostomi
a
Platyzo
a
Spiralia
Bilateria
Gnathif
era
Trochoz
oa
Lophopho
rata
Ambulacraria



Lophotroch
ozoa
Deuterosto
mia
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
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Chordata
o Craniata
 Vertebrata
 Myxini
Sipuncula
Nemertea
Mollusca
Annelida
Phoronida
Brachiopoda
Bryozoa (?)
Entoprocta (?)
Hemichordata
Echinodermata
Xenoturbellida
Rotifera
Acanthocepha
la
Gnathostomul
ida
Micrognathoz
oa
Cycliophora
o
o

Basal/disp
uted

Cephalochordata
Tunicata
Acoelomorpha
o Acoela
o Nemertodermatida
Chaetognatha
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