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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]
[4]
Number of species
Examples
Cells found in
mesoglea
Nematocysts in
exodermis
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]
1.
^
Marque
The Wikibook Dichotomous Key has a page on the topic of
Cnidaria
Wikimedia Commons has media related to: Cnidaria
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
The Hydrozoa Directory
Hexacorallians of the World
[hide]



v
t
e
Eukaryota
Domain : Archaea · Bacteria · Eukaryota
Bikonta
Viridiplantae/Plantae
sensu stricto ·
Archaeplastida, or Plantae sensu lato
Rhodophyta ·
Glaucocystophyceae
AH
Haptophyta ·
Hacrobia, or non-SAR chromalveolata Cryptophyta ·
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
Apusomonadida (Apusomonas, Amastigomonas) ·
Apusozoa Ancyromonadida (Ancyromonas) · Hemimastigida (Hemimastix,
Spironema, Stereonema)
Amoebozoa Lobosea · Conosa · Phalansterium · Breviata
Mesomycetozoea Dermocystida · Ichthyophonida
Filasterea
Capsaspora ·
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Ascaris
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Ascaris
Adult female
Scientific classification
Kingdom:
Animalia
Phylum:
Nematoda
Class:
Secernentea
Order:
Ascaridida
Family:
Ascarididae
Genus:
Ascaris
Linnaeus, 1758
Species
Ascaris lumbricoides
Ascaris suum
Ascaris is a genus of parasitic nematode worms known as the "giant intestinal roundworms".
One species, A. suum, typically infects pigs, while another, A. lumbricoides, affects human
populations, typically in sub-tropical and tropical areas with poor sanitation. A. lumbricoides is
the largest intestinal roundworm and is the most common helminth infection of humans
worldwide, an infection known as ascariasis. Infestation can cause morbidity, and sometimes
death, by compromising nutritional status, affecting cognitive processes, inducing tissue
reactions, such as granuloma, and provoking intestinal obstruction or rectal prolapse.
Contents
[hide]
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1 Morphology
2 Symptoms
3 Examination
4 Pathology
o 4.1 Lung phase
o 4.2 Intestinal phase
o 4.3 Management
5 Defense Mechanism
6 Treatment
7 References
8 See also
[edit] Morphology
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

Adult: cylindrical shape, creamy white or pinkish in color.
Male: average 15–31 cm and is more slender than female.
Female: average 20–35 cm in length.
[edit] Symptoms
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Bloody sputum
Cough
Low-grade fever
Vomiting worms
Passing of worm in stool
Gallstone formation
Liver abscesses
Pancreatitis
Pulmonary eosinophilia
[edit] Examination
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Abdominal X-ray
Complete blood count
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Stool ova and parasite exam
[edit] Pathology
[edit] Lung phase
A.lumbricoides is known as Ascaris pneumonitis. In the lung it causes hemorrhage,
inflammation, and bacterial infection. It also causes allergy in areas with seasonal transmission.
This typically occurs at 6–15 days after initial exposure.
[edit] Intestinal phase
The intestinal phase causes malnourishment, intestinal blockage, verminous intoxication.
A.lumbricoides will move around in the body in response to chemotherapy or fever. Typically
occurs at 6 to 8 weeks after initial exposure.
[edit] Management
Early diagnosis can be performed by examination of stool for the worm eggs. The spread or
infection of A.lumbricoides can be controlled by proper disposal of faeces and proper washing of
food. Control of helminthiasis is based on drug treatment, improved sanitation and health
education.
[edit] Defense Mechanism
As part of the parasite defense strategy, Ascaris roundworms secrete a series of inhibitors to
target digestive and immune-related host proteases, which include pepsin, trypsin,
chymotrypsin/elastase, cathepsins, and metallocarboxypeptidases (MCPs). Ascaris inhibits
MCPs by releasing a enzyme known as Ascaris carboxypeptidase inhibitor (ACI). This enzyme
binds to the active site of MCP and blocks the cleavage of its own proteins by the host MCP
(Sanglas et al., 2008)
[edit] Treatment
Infections with A.lumbricoides are easily treated with a number of anthelmintic drugs:
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pyrantel pamoate given as a single dose of 10 mg/kg
levamisole given as a single dose of 2.5 mg/kg
mebendazole given as a single dose of 500 mg
albendazole given as a single dose of 400 mg.sup
The drugs' main target is the absorbing cells of the worm. The drugs prevent the worm from
absorbing sugar in the intestine which is essential for its survival. This process leads to depletion
of energy in worm and its eventual death within few days. The dead worm is then excreted from
the gut in the stool. Albendazole is not well absorbed by the intestines and a high fat food or
meal should be consumed with each dose.
Many parasitic disease specialists are seeing increased initial incidence and recurrence of
roundworm in the U.S. and are thereby increasingly recommending follow up courses of
medication to treat internal eggs which have not yet hatched, in addition to the initial treatment
period as above. This consists of sporadic treatment with albendazole or similar for a period of
three days each month for up to five months after the initial treatment period.
More severe cases, blockage of intestine or pancreatic ducts require surgical removal of worms.
Some, including parasitologist Dr. Hulda Clark have advocated a diet high in Jalepeno peppers,
citing the low incidence of Ascaris infection in Mexicanos.
See ascariasis for more information.
[edit] References
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Sanglas, Laura; Aviles, Francesc X.; Huber, Robert; Gomis-Ruth, F. Xavior; Arolas, Joan L. 2008.
Mammalian metallopeptidase inhibition at the defense barrier of Ascaris parasite. University of
Barcelona, Spain.
http://health.dir.groups.yahoo.com/group/DrClark/message/19422?var=1
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Esophagus of an Ascaris worm.
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Ascaris Cross Section 40X
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Ascaris Cross Section 40X
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antivaisa Cross Section 400X
Animalia
Phylum Nematoda
Ascaris - dissected female
Return
Search
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The Phylum Nematoda
Etymology:- From the Greek Nema for Thread and Eidos for form.
Characteristics of Nematoda:1)Bilaterally symmetrical, and vermiform.
2)Body has more than two cell layers, tissues and organs.
3)Body cavity is a pseudocoel, body fluid under high pressure.
4)Body possesses a through gut with a subterminal anus.
5)Body covered in a complex cuticle.
6)Has a nervous system with pharyngeal nerve ring.
7)Has no circulatory system (no blood system)
8)Reproduction normally sexual and gonochoristic.
9)Feed on just about everything.
10)Live just about everywhere, many species are endoparasites.
Nematodes are the most speciose phylum after the arthropods, they occur in nearly every habitat
including as parasites in all sorts of plants and animals, (they don't like dry places however). One
species is known that can live in old vinegar (Turbatrix aceti)and another that as only been found
in German beer mats. Though only about 80 000 species have been described some scientists
estimate there may be as many as a million species all told. They can occur in very dense
numbers in the soil and rotting vegetation, as many as 90 000 have been found in a single rotting
apple, while millions occur in the top 3cm (1 inch) of a square metre of good quality soil. While
there are a huge number of free living Nematodes there are also a large number of parasitic
species, many of which cause diseases to man and other animals as well as to plants, nearly
every living organism has been found to be parasitised by one species of nematode or another.
Most nematodes are reasonably small, they range in size from 100 micrometres in length (1/10th
of a mm or 1/250th of an in) to the female Giant Nematode Dioctophyme renale which may be
up to 1 metre, or 3 ft long.
Free living nematodes are long thin worms with transparent and typically curled bodies, parasitic
species have a variety of less streamline shapes relating to their degenerate parasitic life styles,
one unifying characteristic that makes the phylum unique is the lack of cilia or flagella, even the
sperm of nematodes are amoeboid. Nematodes as parasites have been known for a long time and
the earliest recorded literary mention of them is an Egyptian papyrus from 1500 BC, they are
also mentioned by the ancient Greeks Aristotle and Hippocratis the father of scientific medicine.
Ecology
Nematodes live in a vast variety of habitats, ecologically they can be divided into free living
forms and parasitic forms. Free living forms have a simple life cycle involving 4 juvenile instars
on the path from egg to adult. Parasitic species have developed a wide range of variations on this
basic theme. The variations involve whether there is a secondary host and the amount of time
spent in one or either hosts. There is also considerable variability in the way that they move from
one host species to another. thus while many species lay eggs that pass out of the primary host
with the faeces where they are eaten by the secondary host which then gets eaten in turn by the
primary host after the Nematodes have developed. Because it is not always totally reliable that
the secondary host will be eaten just as the Nematode larvae have developed into the infective
stage many species have the ability to encyst themselves in the muscle or cuticle of their
secondary hosts.
Some species use another animal to transport them from one host to another thus Wuchereria
bancrofti releases minute live young called 'microfilaria' into the primary hosts blood stream
rather than eggs into the digestive tract. These microfilaria get ingested by mosquitoes when they
feed on an infected person. Inside the mosquito they live in the mosquitoes gut where they
develop until the Larva 3 stage wait for the mosquito to bite another host whereupon they enter
the host via the mosquitoes proboscis sheath and the wound it makes in the hosts skin.
Nematode Life Cycles
Colours signify:- Blue = Free Living, Red = Primary Host, Green = Secondary Host
Free Living
Eggs
Larva 1 Larva 2 Larva 3 Larva 4 Adults
Ancylostoma
Eggs
Larva 1 Larva 2 Larva 3 Larva 4 Adults
Ascaris
Eggs
Larva 1 Larva 2 Larva 3 Larva 4 Adults
Enterobius
Eggs
Larva 1 Larva 2 Larva 3 Larva 4 Adults
Oxyspirura
Eggs
Larva 1 Larva 2 Larva 3 Larva 4 Adults
Microfilariae
Larva 1 Larva 2 Larva 3 Larva 4 Adults
Eggs
Larva 1 Larva 2 Larva 3 Larva 4 Adults
Stephanofilaria
Parafilaria
Nematodes in Mankind
Human beings, along with all other living things are host to numerous Nematode parasites. The most
common of these is Ascaris lumbricoides with an estimated 700 million people effected globally, this
Nematode is not normally fatal and in low numbers may have very little effect on adults, however in
heavy doses it can be quite debilitating, especially for children. The Nematodes infecting mankind
include several species of filarial worms, the most important of these are Wuchereria bancrofti and
Brugia malayi which are very similar and cause lymphatic filariasis, Onchocerca volvulus which causes
River Blindness and Loa loa which causes Loiasis. Other species are Dranunculus medinensis known as
Guinea Worm, Trichinella spiralis causing Trichinosis, Necator americanus and Ancylostoma duodenale
causing Hookworm, Enterobius vermicularis causing Pinworms and Trichuris trichuria causing Whipworm
or Trichuriasis.
Anatomy
Basically a Nematode is a long hollow tube within which is another tube, the alimentary canal
and the reproductive organs. Nematodes are round in cross section, this is because unlike the
other worms that below them in the phyla table they maintain their body fluids under great
pressure (on average internal pressure in a nematode equals 70mm of mercury or 1.49 PSI, with
a maximum recorded value of 125mm of mercury or 2.41 PSI). To contain this high pressure
nematodes have an extremely tough, yet elastic and flexible cuticle. This cuticle consists of up to
9 layers of proteinaceous fibres, with 3 layers being easily discerned, these are called, from the
outside in, the cortex, the matrix layer and the fibre layer. Despite its complexity the Nematode
cuticle is permeable to both water and gases, so respiration occurs through it. Beneath the cuticle
is a hypodermis and a layer of longitudinal muscle. The combination of the flexure of these
muscles with the high pressure of the system produces a characteristic whip-like wriggle that
Nematodes use to swim. Scientifically this is called undulatory propulsion with sinusoidal waves
passing back along the body.
At the anterior (head) end there is a mouth which has 3 lips behind which predatory species
possess a few teeth, this leads to a pharynx which is triangular in cross section. Because of the
high pressure within the body unsupported organs such as the intestines tend to collapse in much
the same way that an uninflated bicycle tube tends to become oval or flat in cross section when
laid flat on the table. The pharynx of Nematodes is an efficient pump and forces food into the
intestines, there is a one way valve between the intestines and the pharynx. The pharynx can,
when this valve is closed, be used to suck liquid food into the mouth. Digestion is rapid and
faeces are expelled under pressure. This pressure is so great that the parasitic nematode Ascaris
lumbricoides which is about 12cm to 18cm long (5 to 7 inches) may shoot its faeces 60cm or 2
feet into the air.
Nematodes, especially free living forms generally have a reasonably well developed nervous
system. This is comprised of a circum-pharyngeal nerve ring made up from 4 nerve ganglia from
which 6 longitudinal nerves extend down through the body to the various parts of the gut and the
reproductive organs. There are also 6 shorter nerves which extend forwards from the circumpharyngeal ganglia towards the mouth. Nematodes have no circulatory or respiratory organs and
the excretion of metabolic waste is via two simple ducts or tubules which have no nephridia or
flame cells.
Nematodes are copiously reproductive and most of their body cavity, which is a pseudocoelom is
filled with paired sets of reproductive organs, either ovaries or testes. Males and females
copulate and the male introduces sperm to the females vagina with the help of 2 stiff horny
spicules that are a part of his cloaca. Fertilisation is internal and females lay eggs over a
prolonged time period, thus a female Ascaris lumbricoides may lay her eggs at the rate of
200,000 per day and have had a total 27 million eggs within her at the start of her reproductive
career. Young nematodes hatch from these eggs and go through 4 moults before they become
adults.
Classification of the Nematoda
Phylum
Class
Order
Nematoda
Adenophorea
Enoplida
Isolaimida
Mononchida
Dorylaimida
Trichocephalida
Mermithida
Muspiceida
Araeolaimida
Chromadorida
Desmoscolecida
Desmodorida
Monhysterida
Secernentea
Rhabdita
Strongylida
Ascaridida
Spirurida
Camallanida
Diplogasterida
Tylenchida
Aphelenchida
Below are two lists of organisms and the Nematodes that infect them, this is only a minute
example covering only two groups of organisms Commercial Plants and Domestic Animals, in
the plant list many of the species listed for one plant group also infest other plant groups.
Some Nematode Parasites of Domestic Animals
Animal Group
Nematode Species
Rodents
Angiostrongylus cantonensis
Nippostrongylus brasiliensis
Syphacia obvelata
Capillaria hepatica
Cattle
Dictyocaulus viviparus
Oesophagostomum radiatum
Onchocerca gutterosa
Horses
Strongylus edentatus
Parascaris equorum
Oxyuris equi
Pigs
Stephanurus dentatus
Ascaris suum
Sheep
Haemonchus contortus
Ostertagia ostertagi
Dogs
Driofillaria spp.
Dioctophyma renale
Chickens
Syngamus trachea
Ascaridia galli
Heterakis gallinarum
Some Nematode Parasites of Commercial Plants
Plant Group
Nematode Species
Chrysanthemums
Aphelenchoides ritzemabosi
Buddleja
Aphelenchoides ritzemabosi
Scabious
Aphelenchoides blastophthorus
Lillies
Aphelenchoides fragaria
Ferns
Aphelenchoides fragaria
Cacti
Heterodera cacti
Wallflowers
Heterodera cruciferae
Figs
Heterodera fici
Hops
Heterodera humuli
Roses
Xiphinema spp.
Narcissus
Pratylenchus penetrans
Potatoes
Ditylenchus destructor
Pot Plants
Meloidogyne javanica
Herbaceous Plants
Ditylenchus dipsaci
Many plants
Longidorus elongatus
Many Plants
Longidorus Macrosoma
Hookworm
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Not to be confused with Pinworm or Ringworm.
Necator americanus and
Ancylostoma duodenale
Scientific classification
Kingdom:
Animalia
Phylum:
Nematoda
Class:
Secernentea
Order:
Strongiloidae
Family:
Ancylostomatidae
Genus:
Necator/Ancylostoma
Species
N. americanus
A. duodenale
The hookworm is a parasitic nematode that lives in the small intestine of its host, which may be
a mammal such as a dog, cat, or human. Two species of hookworms commonly infect humans,
Ancylostoma duodenale and Necator americanus. A. duodenale predominates in the Middle East,
North Africa, India and (formerly) in southern Europe, while N. americanus predominates in the
Americas, Sub-Saharan Africa, Southeast Asia, China, and Indonesia. Hookworms are thought to
infect more than 600 million people worldwide. The A. braziliense and A. tubaeforme species
infect cats, while A. caninum infects dogs. Uncinaria stenocephala infects both dogs and cats.
Hookworms are much smaller than the larger roundworm Ascaris lumbricoides, and the
complications of tissue migration and mechanical obstruction so frequently observed with
roundworm infestation are less frequent in hookworm infestation. The most significant risk of
hookworm infection is anemia, secondary to loss of iron (and protein) in the gut. The worms
suck blood voraciously and damage the mucosa. However, the blood loss in the stools is not
visibly apparent.
Ancylostomiasis, also known by several other names, is the disease caused when A. duodenale
hookworms, present in large numbers, produce an iron deficiency anemia by sucking blood from
the host's intestinal walls.
Hookworm is a leading cause of maternal and child morbidity in the developing countries of the
tropics and subtropics. In susceptible children hookworms cause intellectual, cognitive and
growth retardation, intrauterine growth retardation, prematurity, and low birth weight among
newborns born to infected mothers. In developed countries, hookworm infection is rarely fatal,
but anemia can be significant in a heavily infected individual.
Contents
[hide]
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1 Signs and symptoms
2 Pathophysiology
o 2.1 Morphology
o 2.2 Pathology
o 2.3 Life cycle
3 Diagnosis
4 Prevention
5 Management
6 Epidemiology
7 History
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8 Research
o 8.1 Anemia in pregnancy
o 8.2 Malaria co-infection
o 8.3 Hygiene hypothesis
o 8.4 Vaccines
o 8.5 Hookworm in therapy
9 Quick facts
10 See also
11 Notes
12 References
13 External links
[edit] Signs and symptoms
There are no specific symptoms or signs of hookworm infection. As mentioned above, they give
rise to a combination of intestinal inflammation and progressive iron/protein-deficiency anaemia.
Larval invasion of the skin might give rise to intense, local itching, usually on the foot or lower
leg, which can be followed by lesions that look like insect bites, can blister ("ground itch"), and
last for a week or more. Animal hookworm larvae on penetrating humans may produce a
creeping eruption called cutaneous larva migrans. The larvae migrate in tortuous tunnels in
between stratum germinativum and stratum corneum of the skin, causing serpigenous vesicular
lesions. With advancing movement of the larvae, the rear portions of the lesions become dry and
crusty. The lesions are typically intensely pruritic. Coughing, chest pain, wheezing, and fever
will sometimes be experienced by people who have been exposed to very large numbers of
larvae. Epigastric pains, indigestion, nausea, vomiting, constipation, and diarrhea can occur early
or in later stages as well, although gastrointestinal symptoms tend to improve with time. Signs of
advanced severe infection are those of anemia and protein deficiency, including emaciation,
cardiac failure and abdominal distension with ascites.
[edit] Pathophysiology
[edit] Morphology
A. duodenale worms are grayish white or pinkish with the head slightly bent in relation to the
rest of the body. This bend forms a definitive hook shape at the anterior end for which
hookworms are named. They possess well developed mouths with two pairs of teeth. While
males measure approximately one centimeter by 0.5 millimeter, the females are often longer and
stouter. Additionally, males can be distinguished from females based on the presence of a
prominent posterior copulatory bursa.[1]
N. americanus is very similar in morphology to A. duodenale. N. americanus is generally smaller
than A. duodenale with males usually 5 to 9 mm long and females about 1 cm long. Whereas A.
duodenale possess two pairs of teeth, N. americanus possesses a pair of cutting plates in the
buccal capsule. Additionally, the hook shape is much more defined in Necator than in
Ancylostoma.[1]
[edit] Pathology
Hookworm infection is generally considered to be asymptomatic, but as Norman Stoll described
in 1962, hookworm is an extremely dangerous infection because its damage is “silent and
insidious.”[2] There are general symptoms that an individual may experience soon after infection.
Ground-itch, which is an allergic reaction at the site of parasitic penetration and entry, is
common in patients infected with N. americanus.[1] Additionally, cough and pneumonitis may
result as the larvae begin to break into the alveoli and travel up the trachea. Then once the larvae
reach the small intestine of the host and begin to mature, the infected individual will suffer from
diarrhea and other gastrointestinal discomfort.[1] However, the “silent and insidious” symptoms
referred to by Stoll are related to chronic, heavy-intensity hookworm infections. Major morbidity
associated with hookworm is caused by intestinal blood loss, iron deficiency anemia, and protein
malnutrition.[3] They result mainly from adult hookworms in the small intestine ingesting blood,
rupturing erythrocytes, and degrading hemoglobin in the host.[4] This long-term blood loss can
manifest itself physically through facial and peripheral edema; eosinophilia and pica caused by
iron deficiency anemia are also experienced by some hookworm-infected patients.[1] Recently,
more attention has been given to other important outcomes of hookworm infection that play a
large role in public health. It is now widely accepted that children who suffer from chronic
hookworm infection can suffer from growth retardation as well as intellectual and cognitive
impairments.[4] Additionally, recent research has focused on the potential of adverse maternalfetal outcomes when the mother is infected with hookworm during pregnancy.
The disease was linked to nematode worms (Ankylostoma duodenalis) from one-third to half an
inch long in the intestine chiefly through the labours of Theodor Bilharz and Griesinger in Egypt
(1854).
The symptoms can be linked to inflammation in the gut stimulated by feeding hookworms, such
as nausea, abdominal pain and intermittent diarrhea, and to progressive anemia in prolonged
disease: capricious appetite, pica (or dirt-eating), obstinate constipation followed by diarrhea,
palpitations, thready pulse, coldness of the skin, pallor of the mucous membranes, fatigue and
weakness, shortness of breath and in cases running a fatal course, dysentery, hemorrhages and
edema.
Blood tests in early infection often show a rise in numbers of eosinophils, a type of white blood
cell that is preferentially stimulated by worm infections in tissues (large numbers of eosinophils
are also present in the local inflammatory response). Falling blood hemoglobin levels will be
seen in cases of prolonged infection with anemia.
In contrast to most intestinal helminthiases, where the heaviest parasitic loads tend to occur in
children, hookworm prevalence and intensity can be higher among adult males. The explanation
for this is that hookworm infection tends to be occupational, so that plantation workers,
coalminers and other groups maintain a high prevalence of infection among themselves by
contaminating their work environment. However, in most endemic areas, adult women are the
most severely affected by anemia, mainly because they have much higher physiological needs
for iron (menstruation, repeated pregnancy), but also because customarily they have access to
much poorer food than the men.
An interesting consequence of this in the case of Ancylostoma duodenale infection is
translactational transmission of infection: the skin-invasive larvae of this species do not all
immediately pass through the lungs and on into the gut, but spread around the body via the
circulation, to become dormant inside muscle fibers. In a pregnant woman, after childbirth some
or all of these larvae are stimulated to re-enter the circulation (presumably by sudden hormonal
changes), then to pass into the mammary glands, so that the newborn baby can receive a large
dose of infective larvae through its mother's milk. This accounts for otherwise inexplicable cases
of very heavy, even fatal, hookworm infections in children a month or so of age, in places such
as China, India and northern Australia.
An identical phenomenon is much more commonly seen with Ancylostoma caninum infections in
dogs, where the newborn pups can even die of hemorrhaging from their intestines caused by
massive numbers of feeding hookworms. This also reflects the close evolutionary link between
the human and canine parasites, which probably have a common ancestor dating back to when
humans and dogs first started living closely together.
[edit] Life cycle
Hookworm life cycle
See the image for the biological life cycle of the hookworms where it thrives in warm earth
where temperatures are over 18°C. They exist primarily in sandy or loamy soil and cannot live in
clay or muck. Rainfall averages must be more than 1000 mm (40 inches) a year. Only if these
conditions exist can the eggs hatch. Infective larvae of Necator americanus can survive at higher
temperatures, whereas those of Ancylostoma duodenale are better adapted to cooler climates.
Generally, they live for only a few weeks at most under natural conditions, and die almost
immediately on exposure to direct sunlight or desiccation.
Infection of the host is by the larvae, not the eggs. While A. duodenale can be ingested, the usual
method of infection is through the skin; this is commonly caused by walking barefoot through
areas contaminated with fecal matter. The larvae are able to penetrate the skin of the foot, and
once inside the body, they migrate through the vascular system to the lungs, and from there up
the trachea, and are swallowed. They then pass down the esophagus and enter the digestive
system, finishing their journey in the intestine, where the larvae mature into adult worms.[4][5]
Once in the host gut, Necator tends to cause a prolonged infection, generally 1–5 years (many
die within a year or two of infecting), though some adult worms have been recorded to live for
15 years or more. On the other hand, Ancylostoma adults are short lived, surviving on average
for only about 6 months. However, infection can be prolonged because dormant larvae can be
"recruited" sequentially from tissue "stores" (see Pathology, above) over many years, to replace
expired adult worms. This can give rise to seasonal fluctuations in infection prevalence and
intensity (apart from normal seasonal variations in transmission).
Civilian Public Service workers built and installed 2065 outhouses for hookworm eradication in
Mississippi and Florida from 1943 to 1947.
They mate inside the host, females laying up to 30,000 eggs per day and some 18 to 54 million
eggs during their lifetime, which pass out in feces. Because it takes 5–7 weeks for adult worms
to mature, mate and produce eggs, in the early stages of very heavy infection, acute symptoms
might occur without any eggs being detected in the patient's feces. This can make diagnosis very
difficult.
Summary of biological life cycle
N. americanus and A. duodenale eggs can be found in warm, moist soil where they will
eventually hatch into first stage larvae, or L1. L1, the feeding non-infective rhabditoform stage,
will feed on soil microbes and eventually molt into second stage larvae, L2. L2, which is also in
the rhabditoform stage, will feed for approximately 7 days and then molt into the third stage
larvae, or L3. L3 is the filariform stage of the parasite, that is, the non-feeding infective form of
the larvae. The L3 larvae are extremely motile and will seek higher ground to increase their
chances of penetrating the skin of a human host. The L3 larvae can survive up to 2 weeks
without finding a host. While N. americanus larvae only infect through penetration of skin, A.
duodenale can infect both through penetration as well as orally. After the L3 larvae have
successfully entered the host, the larvae then travel through the subcutaneous venules and
lymphatic vessels of the human host. Eventually, the L3 larvae enter the lungs through the
pulmonary capillaries and break out into the alveoli. They will then travel up the trachea to be
coughed and swallowed by the host. After being swallowed, the L3 larvae are then found in the
small intestine where they molt into the L4, or adult worm stage. The entire process from skin
penetration to adult development takes about 5–9 weeks. The female adult worms will release
eggs (N. Americanus about 9,000-10,000 eggs/day and A. duodenale 25,000-30,000 eggs/day)
which are passed in the feces of the human host. These eggs will hatch in the environment within
several days and the cycle with start anew.[3][4][6]
Incubation period
The incubation period can vary between a few weeks to many months and is largely dependent
on the number of Hookworm parasites an individual is infected with.[7]
[edit] Diagnosis
Hookworm egg
Diagnosis depends on finding characteristic worm eggs on microscopic examination of the
stools, although this is not possible in early infection. The eggs are oval or elliptical, measuring
60 µm by 40 µm, colourless, not bile stained and with a thin transparent hyaline shell membrane.
When released by the worm in the intestine, the egg contains an unsegmented ovum. During its
passage down the intestine, the ovum develops and thus the eggs passed in feces have a
segmented ovum, usually with 4 to 8 blastomeres. As the eggs of both Ancylostoma and Necator
(and most other hookworm species) are indistinguishable, to identify the genus, they must be
cultured in the lab to allow larvae to hatch out. If the fecal sample is left for a day or more under
tropical conditions, the larvae will have hatched out, so eggs might no longer be evident. In such
a case, it is essential to distinguish hookworms from Strongyloides larvae, as infection with the
latter has more serious implications and requires different management. The larvae of the two
hookworm species can also be distinguished microscopically, although this would not be done
routinely, but usually for research purposes. Adult worms are rarely seen (except via endoscopy,
surgery or autopsy), but if found, would allow definitive identification of the species.
Classification can be performed based on the length of the buccal cavity, the space between the
oral opening and the esophagus: hookworm rhabditoform larvae have long buccal cavities
whereas Strongyloides rhabditoform larvae have short buccal cavities.[1]
Recent research has focused on the development of DNA-based tools for diagnosis of infection,
specific identification of hookworm, and analysis of genetic variability within hookworm
populations.[8] Because hookworm eggs are often indistinguishable from other parasitic eggs,
PCR assays could serve as a molecular approach for accurate diagnosis of hookworm in the
feces.[8][9]
[edit] Prevention
The infective larvae develop and survive in an environment of damp dirt, particularly sandy and
loamy soil. They cannot survive in clay or muck. The main lines of precaution are those dictated
by sanitary science:




Do not defecate in places other than latrines, toilets etc.
Do not use human excrement or raw sewage or untreated 'night soil' as manure/fertilizer in
agriculture
Do not walk barefoot in known infected areas
Deworm pet dogs — canine and feline hookworms rarely develop to adulthood in humans
(Ancylostoma caninum, the common dog hookworm, occasionally develops into an adult to
cause eosinophilic enteritis in people), but their invasive larvae can cause an itchy rash called
cutaneous larva migrans.
Moxidectin has been released in the United States as part of Advantage Multi (imidacloprid +
moxidectin) Topical Solution for dogs and cats. It utilizes moxidectin for control and prevention
of roundworms, hookworms, heartworms, and whipworms.
With an estimated 740 million individuals infected, hookworm is a major public health concern
in our world today. While hookworm infection may not directly lead to mortality, its effects on
morbidity demand immediate attention. When considering disability-adjusted-life-years
(DALYs), neglected tropical diseases, including hookworm, rank among diarrheal diseases,
ischemic heart disease, malaria, and tuberculosis as one of the most important health problems of
the developing world.
It has been estimated that as many as 22.1 million DALYs have been lost due to hookworm.
Recently, there has been increasing interest to address the public health concerns associated with
hookworm. For example, the Bill & Melinda Gates Foundation recently donated US$34 million
to fight Neglected Tropical Diseases including hookworm infection.[10] Former US President
Clinton also announced a mega-commitment at the Clinton Global Initiative (CGI) 2008 Annual
Meeting to de-worm 10 million children.[11]
Most of these public health concerns have focused on children who are infected with hookworm.
This focus on children is largely due to the large body of evidence that has demonstrated strong
associations between hookworm infection and impaired learning, increased absences from
school, and decreased future economic productivity.[4] In 2001, the 54th World Health Assembly
passed a resolution demanding member states to attain a minimum target of regular deworming
of at least 75% of all at-risk school children by the year 2010.[12] A 2008 World Health
Organization publication reported on these efforts to treat at-risk school children. Some of the
interesting statistics were as follows: 1) only 9 out of 130 endemic countries were able to reach
the 75% target goal; and 2) less than 77 million school-aged children (of the total 878 million at
risk) were reached which means that only 8.78% of at-risk children are being treated for
hookworm infection.[13] While there is progress being made, these numbers also remind us of
how much work is still to be done.
School-based mass deworming programs have been the most popular strategy to address the
issue of hookworm infection in children. School-based programs are extremely cost effective as
schools already have an available, extensive, and sustained infrastructure with a skilled
workforce that has a close relationship with the community.[12] With little training from a local
health system, teachers can easily administer the drugs which often cost less than $0.50 per child
per year.[14]
Recently, many people have begun to question if the school-based programs are necessarily the
most effective approach. An important concern with school-based programs is that they often do
not reach children who do not attend school, thus ignoring a large amount of at-risk children. A
2008 study by Massa et al. continued the debate regarding school-based programs. They
examined the effects of community-directed treatments versus school-based treatments in the
Tanga Region of Tanzania. A major conclusion was that the mean infection intensity of
hookworm was significantly lower in the villages employing the community-directed treatment
approach than the school-based approach. The community-directed treatment model used in this
specific study allowed villagers to take control of the child’s treatment by having villagers select
their own community drug distributors to administer the antihelminthic drugs. Additionally,
villagers organized and implemented their own methods for distributing the drugs to all
children.[15] The positive results associated with this new model highlight the need for large-scale
community involvement in deworming campaigns.
Many mass deworming programs also combine their efforts with a public health education.
These health education programs often stress important preventative techniques such as: always
wearing shoes, washing your hands before eating, and staying away from water/areas
contaminated by human feces. But while these may seem like simple tasks, they raise important
public health challenges. The fact is that most infected populations are from poverty-stricken
areas with very poor sanitation. Thus, it is most likely that at-risk children cannot afford shoes to
wear, do not have access to clean water to wash their hands, and live in environments with no
proper sanitation infrastructure. Health education, therefore, must address preventive measures in
ways that are both feasible and sustainable in the context of resource-limited settings.
Evaluation of numerous public health interventions have generally shown that improvement in
each individual component ordinarily attributed to poverty (for example, sanitation, health
education, footwear, and underlying nutrition status) often have minimal impact on transmission.
For example, one study found that the introduction of latrines into a resource-limited community
only reduced the prevalence of hookworm by four percent.[16] Another study in Salvador, Brazil
found that improved drainage and sewerage had minimal impact of the prevalence and no impact
at all on the intensity of hookworm.[17] This seems to suggest that environmental control alone
has minimal effect on the transmission of hookworm. It is imperative, therefore, that more
research be performed to understand the efficacy and sustainability of integrated programs that
combine numerous preventive methods including education, sanitation, and treatment.
[edit] Management
The most common treatment for hookworm are benzimidazoles, specifically albendazole and
mebendazole. BZAs kill adult worms by binding to the nematode’s β-tubulin and subsequently
inhibiting microtubule polymerization within the parasite.[3] In certain circumstances, levamisole
and pyrantel pamoate may be used.[4] The 2008 study by Keiser and Utzinger, Efficacy of
Current Drugs Against Soil-Transmitted Helminth Infections: Systematic Review and Metaanalysis, examined the relative efficacies of different drug treatments. They found that the
efficacy of single-dose treatments for Hookworm infections were as follows: 72% for
albendazole, 15% for mebendazole, and 31% for pyrantel pamoate.[18] This substantiates prior
claims that albendazole is much more effective than mebendazole for Hookworm infections.
Also of note is that the World Health Organization does recommend anthelmintic treatment in
pregnant women after the first trimester.[3] It is also recommended that if the patient also suffers
from anemia that ferrous sulfate (200 mg) be administered three times daily at the same time as
anthelmintic treatment; this should be continued until hemoglobin values return to normal which
could take up to 3 months.[1]
Other important issues related to the treatment of hookworm are reinfection and drug resistance.
It has been shown that reinfection after treatment can be extremely high. Some studies even
show that 80% of pretreatment hookworm infection rates can be seen in treated communities
within 30–36 months.[3] While reinfection may occur, it is still recommended that regular
treatments be conducted as it will minimize the occurrence of chronic outcomes. There are also
increasing concerns about the issue of drug resistance. Drug resistance has appeared in front-line
anthelmintics used for livestock nematodes. Generally human nematodes are less likely to
develop resistance due to longer reproducing times, less frequent treatment, and more targeted
treatment. Nonetheless, the global community must be careful to maintain the effectiveness of
current anthelmintic as no new anthelmintic drugs are in the late-stage development.[3]
The hookworm can be treated with local cryotherapy when it is still in the skin.
Albendazole is effective both in the intestinal stage and during the stage the parasite is still
migrating under the skin.
In case of anaemia, iron supplementation can cause relief symptoms of iron deficiency anemia.
However, as red blood cell levels are restored, shortage of other essentials such as folic acid or
vitamin B12 may develop, so these might
This is the mouth part of the hookworm Ancyclostoma
duodenale. The hookworm uses sharp, curved cutting
plates to attach to the lining of the intestine where it
sucks blood for nourishment. (Image courtesy of the
Centers for Disease Control and Prevention.)
Copyright 2007 The New York Times Company
common in the moist tropics and subtropics. It affects about 1 billion people worldwide. In
developing nations, the disease leads to the death of many children by increasing their risk for
infections that their bodies would normally fight off.
There is very little risk of getting the disease in the United States because of advances in
sanitation and waste control. The important factor in getting the disease is walking where people
who have hookworm have made feces.
The larvae (immature form of the worm) get into the skin. The larvae move to the lungs via the
bloodstream and enter the airways. The worms are about 1/2 inch long.
After traveling up the windpipe, the larva are swallowed. After the larvae are swallowed, they
infect the small intestine. They develop into adult worms and live there for 1 or more years.
Adult worms and larvae are released in the feces.
Back to TopSymptoms
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
Abdominal discomfort
Blood in the stool
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

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Bloody sputum
Cough
Diarrhea
Fatigue
Fever
Gas
Itchy rash
Loss of appetite
Nausea, vomiting
Pale skin
Most people have no symptoms once the worms enter the intestines.
Back to TopExams and Tests
Tests that can help diagnose the infection include:


Complete blood count (CBC) with differential
Stool ova and parasites exam
This disease may also affect the results of a D-xylose absorption test.
Back to TopTreatment
The goals of treatment are to:



Cure the infection
Treat complications of anemia
Improve nutrition
Parasite-killing medications such as albendazole, mebendazole, or pyrantel pamoate are usually
prescribed. Ivermectin, used for other worm infections, does not work for hookworm infections.
Symptoms and complications of anemia are treated as they arise. The doctor will likely
recommend increasing the amount of protein in your diet.
Back to To
Trichinella spiralis
Trichinella spiralis adults and their larvae are found within one host. Therefore a single host individual
serves as both definitive and intermediate host and the larvae are transmitted between hosts by predators
and carrion feeders. The adult lives for a short time in the intestinal epithelium of a wide variety of
mammals. The female is ovoviviparous, releasing first-stage larvae (L1) into the circulatory system. The
L1 are transported throughout the body, where they grow and enter the cells of striated muscle. The L1 of
Trichinella spiralis is the only first-stage larva of all nematodes, including animal parasites, in which the
sexes can be differentiated. The L1 within the host muscle cell is the infective stage. This also is unusual
since it is usually the third-stage larvae (L3) that are infective to definitive hosts. When ingested by a new
host, the L1 undergoes 4 rapid moults (within 28 hours) to the adult stage. Females mate and begin
producing larvae within a few days.
As mentioned a single host serves as both definitive and intermediate host. The life cycle is completed
when an intermediate host is ingested by another defitive host. In nature the life cycle is maintasined in
most northern terrestrial habitats, and even in some marine mammals. Humans can become infected and
severe illness or death can result from heavy infections. Humans are a dead end host as under normal
circumstances the larvae which accumulate in a human do not have the opportunity to infect another
host. In your slide box you have adult males and females and there are slides with muscle tissue with L1
larvae on display.
You should be able to distinguish males from females. Structures to identify on the adults include the
muscular esophagus, stichosome and intestine. On the females note the uterus, vulva and anus. On the
males note the ejaculatory duct and the copulatory appendage at the posterior end.
Trichinella spiralis larva in nurse cell
Trichinella spiralis male posterior
Trichinella spiralis male
Trichinella spiralis female posterior
Trichinella spiralis female anterior
Trichinella spiralis vulva
Main Index
Previous
pOutlook
Trichinella spiralis
Trichinella spiralis adults and their larvae are found within one host. Therefore a single host individual
serves as both definitive and intermediate host and the larvae are transmitted between hosts by predators
and carrion feeders. The adult lives for a short time in the intestinal epithelium of a wide variety of
mammals. The female is ovoviviparous, releasing first-stage larvae (L1) into the circulatory system. The
L1 are transported throughout the body, where they grow and enter the cells of striated muscle. The L1 of
Trichinella spiralis is the only first-stage larva of all nematodes, including animal parasites, in which the
sexes can be differentiated. The L1 within the host muscle cell is the infective stage. This also is unusual
since it is usually the third-stage larvae (L3) that are infective to definitive hosts. When ingested by a new
host, the L1 undergoes 4 rapid moults (within 28 hours) to the adult stage. Females mate and begin
producing larvae within a few days.
As mentioned a single host serves as both definitive and intermediate host. The life cycle is completed
when an intermediate host is ingested by another defitive host. In nature the life cycle is maintasined in
most northern terrestrial habitats, and even in some marine mammals. Humans can become infected and
severe illness or death can result from heavy infections. Humans are a dead end host as under normal
circumstances the larvae which accumulate in a human do not have the opportunity to infect another
host. In your slide box you have adult males and females and there are slides with muscle tissue with L1
larvae on display.
You should be able to distinguish males from females. Structures to identify on the adults include the
muscular esophagus, stichosome and intestine. On the females note the uterus, vulva and anus. On the
males note the ejaculatory duct and the copulatory appendage at the posterior end.
Trichinella spiralis larva in nurse cell
Trichinella spiralis male posterior
Trichinella spiralis male
Trichinella spiralis female posterior
Trichinella spiralis female anterior
Trichinella spiralis vulva
Main Index
Previous
Trichinella spiralis
Trichinella spiralis adults and their larvae are found within one host. Therefore a single host individual
serves as both definitive and intermediate host and the larvae are transmitted between hosts by predators
and carrion feeders. The adult lives for a short time in the intestinal epithelium of a wide variety of
mammals. The female is ovoviviparous, releasing first-stage larvae (L1) into the circulatory system. The
L1 are transported throughout the body, where they grow and enter the cells of striated muscle. The L1 of
Trichinella spiralis is the only first-stage larva of all nematodes, including animal parasites, in which the
sexes can be differentiated. The L1 within the host muscle cell is the infective stage. This also is unusual
since it is usually the third-stage larvae (L3) that are infective to definitive hosts. When ingested by a new
host, the L1 undergoes 4 rapid moults (within 28 hours) to the adult stage. Females mate and begin
producing larvae within a few days.
As mentioned a single host serves as both definitive and intermediate host. The life cycle is completed
when an intermediate host is ingested by another defitive host. In nature the life cycle is maintasined in
most northern terrestrial habitats, and even in some marine mammals. Humans can become infected and
severe illness or death can result from heavy infections. Humans are a dead end host as under normal
circumstances the larvae which accumulate in a human do not have the opportunity to infect another
host. In your slide box you have adult males and females and there are slides with muscle tissue with L1
larvae on display.
You should be able to distinguish males from females. Structures to identify on the adults include the
muscular esophagus, stichosome and intestine. On the females note the uterus, vulva and anus. On the
males note the ejaculatory duct and the copulatory appendage at the posterior end.
Trichinella spiralis larva in nurse cell
Trichinella spiralis male posterior
Trichinella spiralis male
Trichinella spiralis female posterior
Trichinella spiralis female anterior
Trichinella spiralis vulva
Main Index
Previous