http://biology.clc.uc.edu/courses/bio106/protista.htm
Protista
Some members of Kingdom Protista are unicellular, others are colonial, and yet others are multicellular. Note
that in the colonial forms, all the cells are similar with similar, generalized functions, whereas in the truly
multicellular species, the “body” of the organism consists of a variety of types of cells, each type with its own
specialized function. These organisms are all eukaryotes (they have a true nucleus). They all need some kind
of a water-based environment--which can be fresh or marine water, snow, damp soil, polar bear hairs--in
which to live. All are aerobic and have mitochondria to do cellular respiration, and some have chloroplasts
and can do photosynthesis. Most of them reproduce or grow by mitosis, and some reproduce by meiosis and
fertilization. Many can form cysts in adverse conditions. Protists are a major component of plankton.
Protists are grouped into three major, unofficial categories based on means by which they obtain nutrition.
These are the Protozoa, the Algae, and the Fungus-like Protists. For some reason, botanists use the word
“Division” to mean the same taxonomic level as “Phylum”, and since, way back everything was lumped in as
either a plant or an animal, taxonomists who study Kingdom Protista (and those who study Kingdom Fungi)
also still use the word “Division” to mean “Phylum”, so for example, when “Division Rhizopoda” is listed
below, that means the same thing as saying “Phylum Rhizopoda”.
Protozoa:
These protists are animal-like, especially in their nutrition. They ingest their food by phagocytosis. Some
have mouth-like structures into which the prey is put while others use pseudopodia to move and to engulf
prey. Typical prey include bacteria and other smaller one-celled organisms.
Division Rhizopoda:
An example of a member of this Division is genus Amoeba, a fresh-water dweller. Protists in this
group are unicellular and have pseudopodia. Some secrete shells around themselves, while others do
not. None of them have flagella, cilia, or meiosis. Entamoeba histolytica is a parasitic form that
causes amoebic dysentery. These colonize the colon and feed on bacteria, causing symptoms that
range from mild diarrhea to dysentery. Typically periods of watery diarrhea, often containing blood,
may alternate with constipation, and often there is flatulence and abdominal cramping. Entamoeba can
be directly spread (anal sex), or indirectly spread (by drinking contaminated water). Fresh fruits and
vegetables may be unsafe if fertilized with human feces, watered with contaminated water, or prepared
by a person with it on his/her hands.
Division Apicomplexa:
These are all parasites and form tiny, infectious spores. All have complex life cycles. An example is
Plasmodium vivax, which causes malaria, for which certain species of mosquitoes are the secondary
host. It is also possible to become infected with Plasmodium parasites from a transfusion from an
infected person or if a drug addict shares a syringe with an infected person. One stage in this
complicated life cycle grows in the mosquito, the next stage in the newly-infected person’s liver, and
the next stage invades the person’s red blood cells, rupturing the RBCs as the parasites leave to invade
other cells. Symptoms include cyclical alternating chills, fever, and sweating which at first, can be
mistaken for flu. While usually less than 1% of the RBCs are infected, often malaria causes anemia
due to the smaller number of RBCs. Often the spleen and liver become enlarged as they try to deal
with the dying RBCs. Malaria is treated with extract from the quinine tree. Remember that people with
sickle-cell are more resistant because when a malaria parasite enters a RBC, the RBC sickles, killing
the parasite, thereby preventing it from multiplying and spreading.
Division Zoomastigophora:
This Division contains some organisms which are free-living, others which are
symbionts, and yet others which are parasites. An example of a symbiotic
member of this Division is the protozoans which live in the gut of termites and
digest cellulose in the wood the termites eat. An example of a parasitic form
would be Trypanosoma gambiense, which causes African sleeping sickness
and is spread by the bite of the tsetse fly. Symptoms include irregular fever,
general swelling of the lymph nodes, skin eruptions, and areas of painful local
swelling. Eventually CNS symptoms like tremors, headache, apathy, and convulsions appear and
become worse, leading to eventual coma and death. Early on, the parasites are found in blood and
lymph, but later only in the person’s cerebrospinal fluid.
Division Ciliophora:
An example of an organism in this Division is Paramecium. These protozoans are solitary, fresh water
organisms and use cilia to move. They have probably the most complex structure and organization of
all cells. Rather than one nucleus, they have a larger macronucleus and several smaller micronuclei.
They use a form of sexual reproduction called conjugation in which some of the micronuclei are
exchanged between the two individuals involved.
Algal Protists
These protists are photosynthetic; their nutrition is plant-like. Almost all of them have chlorophyll A,
most have chlorophyll C, but only a few have chlorophyll B. They also have a variety of carotenoids
and other pigments, and frequently they are grouped into Divisions based on similarities in pigments.
Division Dinoflagellata:
These are abundant in plankton, occasionally occurring in large numbers. They can occasionally
become so numerous that the water looks red, thus this algal bloom (meaning there are large numbers
of them, having nothing to do with flowers, which they do not have) is called Red Tide. Because
Dinoflagellates are toxic to humans, it is not safe to eat “shellfish” (clams, etc.) collected where Red
Tide is occurring (the Protists get inside the clam shell and cannot be easily removed). Dinoflagellates
are bioluminescent, that is, they are able to produce light like lightening bugs, and at night during Red
Tide, the crests of the ocean waves appear to glow in the dark.
Division Euglenophyta:
Probably the best-known example of this Division is genus Euglena. Each of these organisms has a
flagellum on its anterior end, and this is used to propel the organism. They have chloroplast and,
when in the light, do photosynthesis. If they are not in the light, they can also obtain nutrition by
phagocytosis. To help them sense light (which they then move toward), Euglena have a light-sensitive
“eyespot” or stigma near their anterior ends. This is not a true eye, in that it cannot do any image
formation, but rather it is a photoreceptor which senses the light level in the organism’s environment.
Division Chlorophyta:
These protists are also known as the “green algae.” Their chloroplasts and the pigments therein are
similar to plants (this is about the only group of algae with chlorophyll B), thus it is thought that the
green algae may be the evolutionary ancestors of plants. Various species of green algae may be found
in a variety of environments including both fresh and salt water, damp soil, the surface of snow, and
within other organisms (lichens, hydra, polar bear hair).
Chlamydomonas are unicellular and contain an eyespot (stigma), a chloroplast, two
flagella, and a nucleus.
Volvox are colonial and often contain darker green daughter colonies inside. Each cell
posesses two flagella, enabling the colony to be mobile. There is an intercellular
matrix holding the colony of cells together.
Ulva is called Sea Lettuce. This is truely multicellular, with a division of labor among
the various cells, and is macroscopic. The “body” is two cells thick, and there is a
specially-modified “holdfast” to anchor the organism to the ocean floor. Its life cycle
includes both 1n and 2n stages (see below).
Closterium is a member of the sub-group called the Desmids. Some desmids form
colonies, but Closterium is solitary. Its nucleus is in the center with a cone-shaped
chloroplast on each side. Each chloroplast contains a series of starch-storage organelles
called pyrenoids In living Closterium, each end of the cell bears a small vacuole
containing several gypsum grains which “dance” by Brownian motion.
Spirogyra are colonial, being organized into long filaments. Each cell contains a spiral
chloroplast with pyrenoids (used to store starch) and a nucleus. They have
conjugation--a type of sexual reproduction in which the contents of the male gamete
cell go over into the female cell.
Many green algae, especially the multicellular ones, have both sexual and asexual stages in their life
cycles, thus we must re-introduce the idea of Alternation of Generations we discussed along with
meiosis. When we first discussed Alternation of Generations, we looked at a very simple diagram in
which adults produced 1n gametes by meiosis, and those gametes joined by syngamy to form a new 2n
generation. In reality in algae and plants, there are a few more stages in the process, thus we now need
to re-visit this cycle. The 2n generation, which in humans is called an “adult,” in algae and plants is
called a sporophyte because it produces spores. Within specialized reproductive structures in/on the
bodies of the sporophyte, meiosis occurs to reduce the chromosome number from 2n to 1n, thus the
spores which are produced are 1n. Each spore germinates and grows into a new, independent, 1n
organism (which often looks totally different than the 2n generation). These 1n organisms are called
gametophytes because they produce the gametes (eggs and sperm), which are still 1n. An egg and
sperm unite by syngamy increasing the chromosome number from 1n to 2n, and forming a zygote
which is 2n. The zygote grows into the sporophyte, and the cycle starts over. Various of the green
algae go through this cycle as do members of the next two groups, the brown and red algae. Plants also
go through this same cycle with some interesting modifications we will discuss later.
Division Phaeophyta:
These organisms are commonly known as the “brown algae.” They are multicellular and live in
marine, temperate zone, costal areas. They all have a form of sexual reproduction with alternation of
generations. One member of this Division with which you may be familiar is Kelp, which actually can
be any of several species of seaweed in the genera Fucus and/or Laminaria. Brown algae are used in
many cultures as human food, and are good sources of iodine. We need iodine for our thyroid glands,
and if a person doesn’t enough iodine in his/her diet (most commonly in inland areas where iodine is
not added to salt), the thyroid gland enlarges in an attempt to keep making enough thyroid hormone
(which doesn’t do any good because what it’s lacking is the iodine needed to make the hormone). This
enlarged thyroid is called a goiter. Laminaria also has an interesting gynecological use. If a woman is
scheduled for some medical procedure for which the doctor needs access to the inside of her uterus,
often a day or so beforehand, rolled-up, dried pieces of Laminaria are inserted into the opening of the
woman’s cervix. As the seaweed absorbs water from her body fluids, it gently and slowly expands,
gradually stretching the cervix. Thus, by the time her surgery is scheduled, her cervix has been dilated
slowly and gently rather than the doctor having to forcibly and quickly (thus painfully) stretch the
cervix open minutes beforehand.
Division Rhodophyta:
These are called the “red algae.” They also are multicellular and marine-dwelling, but are more
typically found in tropical zones and deeper in the ocean. They also go through alternation of
generations, Many of these (such as the Nori used in sushi) are used by humans as food, and are also
good sources of iodine.
Fungus-like Protists
Division Myxomycota:
These organisms are called “slime molds.” They are fungus-like in their nutrition in that they absorb
nutrients from their environment. Their “body” structure is unusual in that the nuclei undergo mitosis,
but there is no cytokinesis--there are no individual cells with one nucleus each. Rather, the “body” is a
giant, multinucleate mass of cytoplasm. Slime molds are mobile: they move by amoeboid movement,
in other words, like a giant Amoeba with giant pseudopodia. They live in decayed wood and move
around in between the fibers, ingesting bacteria, etc. by phagocytosis. Slime molds are often brightlycolored (yellow or orange).
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Protists.ht
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The Protists
What are protists?
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They are eukaryotes because they all have a nucleus.
Most have mitochondria although some have later lost theirs (Link). Mitochondria were derived
from aerobic alpha-proteobacteria that once lived within their cells.
Many have chloroplasts with which they carry on photosynthesis. Chloroplasts were derived
from photosynthetic cyanobacteria living within their cells.
Link to a discussion of the "endosymbiosis"
theory of the origin of eukaryotes.
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Many are unicellular and all groups (with one exception) contain some unicellular members.
The name Protista means "the very first", and some of the 80-odd groups of organisms that we classify
as protists may well have had long, independent evolutionary histories stretching as far back as 2
billion years. But genome analysis added to other criteria show that others are derived from more
complex ancestors; that is, are not "primitive" at all.
Genome analysis also shows that many of the groups placed in the Protista are not at all closely related
to one another; that is, the protists do not represent a single clade.
So we consider them here as a group more for our convenience than as a reflection of close kinship,
and
a better title for this page would be "Eukaryotes that are neither Animals, Fungi, nor Plants".
The Euglenozoa
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Most are unicellular.
Many swim by means of a single flagellum.
They are not encased in a cell wall so they are flexible as well as motile.
Euglena is a typical member of the group (which numbers about 1600 species).
Because some members of the group (like Euglena) have chloroplasts, these organisms used to be
called "Euglenophytes", but in fact they are neither plants ("phytes") nor animals ("zoa"). Rather —
like the other organisms on this page — they are the living descendants of some of the very earliest
eukaryotes.
Trypanosoma brucei, the cause of African sleeping sickness in humans, is a member of the group.
The electron micrograph (by L. Tetley; courtesy of Keith Vickerman) shows T. brucei as it occurs in
the salivary gland of the tsetse fly ready to be injected into the mammalian host when the fly bites. The
specimen is 12 µm long.
How trypanosomes evade the immune response of their host.
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In Latin America, Trypanosoma cruzi, another member of the group, is the cause of Chagas disease
in humans.
Ciliates, Sporozoans, and Dinoflagellates: the Alveolates
These three phyla are grouped in a clade — the
alveolates — because
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they all have a system of saclike structures
("alveoli") on the inner surface of their plasma
membrane as well as
close homology in their gene sequences.
Ciliates
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Move by the rhythmic beating of their cilia.
Although single-celled, some are large enough to
be seen with the naked eye.
 Examples: Paramecium, Stentor, Vorticella,
Tetrahymena thermophila.
 Feed by sweeping a stream of particle-laden
water through a "mouth" and "gullet" and into a
food vacuole.
 Undigested wastes are discharged at a permanent
site.
 Fresh water ciliates cope with the continuous
influx of water from their hypotonic surroundings
by pumping it out with one or more contractile
vacuoles. Parasitic ciliates, which live in isotonic surroundings, have no contractile vacuole.
All of this rightly suggests that although they are unicellular, there is nothing rudimentary about the ciliates.
Their single cell is far more elaborate in its organization than any cell out of which multicellular organisms
are made.
Link to discussion of reproduction in the ciliates.
Sporozoans (Apicomplexa)
The members of this group share an "apical complex" of microtubules at one end of the cell (hence the name
that many prefer to the old name of sporozoans). All the members of the phylum are parasites.
The genus Plasmodium causes malaria, one of the greatest scourges of humans. Malaria has probably caused
more human deaths than any other infectious disease; even today it is estimated to kill a million people a year
in the sub-Saharan Africa.
The organism is transmitted from human to human through the bite of mosquitoes of the genus Anopheles.
The diagram shows the life cycle of Plasmodium vivax.
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The mosquito bite injects sporozoites into the human host.
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These invade the liver where they develop into merozoites.
The merozoites invade red blood cells where they reproduce.
Periodically, they all break out of the red cells together bringing on the chills and fever characteristic
of the disease.
Eventually some merozoites develop into either male or female gametocytes.
These will die unless they are sucked up by the bite of an anopheline mosquito.
Once in the stomach of the mosquito, the gametocytes form gametes: sperm and eggs.
These fuse to form zygotes.
The zygote invades the stomach wall of the mosquito forming thousands of sporozoites.
These migrate to the salivary gland, ready to be injected into a new human host.
Most forms of malaria are chronic. The organisms may coexist with their host for years (but cannot complete
their life cycle there).
How they evade the immune response of their host.
Toxoplasma gondii is another parasitic member of this group.
Plasmodium, Toxoplasma, and some of the other members of this group contain a membrane-enclosed
organelle called the apicoplast. They seem to have inherited it from a common ancestor that acquired it by
engulfing a chloroplast. Link to a discussion of this example of secondary endosymbiosis.
Dinoflagellates
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About 1000 species.
Most are unicellular.
Most use chlorophylls a and c
Unlike most eukaryotes, they
o lack histones on their chromosomes and
o have a simpler form of mitosis
They do have the eukaryotic type ("9 + 2") of flagellum (two of them in fact).
Occasionally they reproduce explosively, creating poisonous red tides that may cause extensive kills
of marine fish and make filter-feeding marine animals like clams unfit for human consumption.
Diatoms, Golden Algae, Brown Algae, and Water Molds: The
Stramenopiles
These organisms belong to a single clade, the stramenopiles (a/k/a heterokonts). The first three members
share:
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a yellow-brown pigment (which gives them their color). It is a carotenoid called fucoxanthin.
chlorophylls a and c
All four of them (plus a number of other groups not listed) share genes closely-homologous to those in both
green and red algae. This suggests that they are all descended from a heterotrophic eukaryotic ancestor that
acquired both a green alga and a red alga by a secondary endosymbiosis. (While the water molds no longer
are photosynthetic, they still retain both green and red alga genes.)
Diatoms
Diatoms are unicellular. Their cell wall or shell is made of two
overlapping halves. These are impregnated with silica and often
beautifully ornamented. The photo (courtesy of Turtox) is of
Arachnoidiscus ehrenbergi magnified some 400 times.
Diatoms are major producers in aquatic environments; that is, they are
responsible for as much as 40% of the photosynthesis that occurs in fresh
water and in the oceans. They serve as the main base of the food chains in
these habitats, supplying calories to heterotrophic protists and small
animals. These, in turn, feed larger animals.
Golden Algae (Chrysophyta)
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Most are unicellular.
Found in fresh water.
Important producers in some aquatic food chains.
In low light conditions, may lose their chlorophyll and turn heterotrophic feeding on bacteria and/or
diatoms.
Over 1000 species alive today; many more in the fossil record.
Brown Algae (Phaeophyta)
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The rockweeds and kelps. Some kelps grow as long as 30 meters.
All are multicellular although without much specialization of cell types.
Most are found in salt water.
Used for food in some coastal areas of the world and harvested in the U. S. for fertilizer and as a
source of iodine.
Water Molds (Oomycetes)
As their name suggests, water molds were once considered to be fungi. But unlike fungi, the cell wall of water
molds is made of cellulose, not chitin. Furthermore, their gene sequences are very different from those of
fungi (and most closely related to those of diatoms, golden and brown algae).
Some notable water molds:
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Some species (e.g., Saprolegnia, Achyla) are parasites of fishes and can be a serious problem in fish
hatcheries.
Downy mildews damage grapes and other crops.
Phytophthora infestans, the cause of the "late blight" of potatoes. In 1845 and again in 1846, it was
responsible for the almost total destruction of the potato crop in Ireland. This led to the great Irish
famine of 1845–1860. During this period, approximately 1 million people starved to death and many
more emigrated to the New World. By the end of the period, death and emigration had reduced the
population of Ireland from 9 million to 4 million.
Phytophthora ramorum, which is currently killing several species of oaks in California.
Red Algae
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The red algae are almost exclusively marine.
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Some are unicellular but most are multicellular.
Approximately 4000 species have been identified.
They are photosynthetic using chlorophyll a.
Their closest relatives are the green algae and land plants.
However, the structure of the membranes in their chloroplasts is quite different from that of the green
plants but resembles that found in the cyanobacteria.
Like the cyanobacteria, they use
o phycoerythrin (which makes them red) and
o phycocyanin
as antenna pigments.
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They do not have the eukaryotic "9+2" flagellum.
Some are used as food in coastal regions of Asia.
Agar, the base for culturing bacteria and other microorganisms, is extracted from a red alga.
Slime Molds (Mycetozoa)
Cellular Slime Molds
The organisms in this group have a complex life cycle during the course of which they go through unicellular,
multicellular, funguslike (form spores) and protozoanlike (amoeboid) stages.
Thousands of individual amoebalike cells aggregate into a slimy mass — each cell retaining its identity
(unlike plasmodial slime molds). The aggregating cells are attracted to each other by the cyclic AMP (cAMP)
that they release.
With the exception of one species that causes powdery scab on potatoes, these organisms are of little
economic importance. However, their combination of traits makes them of great scientific interest. Molecular
phylogenies place them in the same clade as animals (metazoa) and fungi. The link below will introduce you
to one of the most popular members of the group.
External Link
Dictyostelium discoideum: Model System in Motion
Please let me know by e-mail if you find a broken link in my pages.)
Plasmodial (Acellular) Slime Molds (Myxomycetes)
At one stage in their life cycle, these
organisms consist of a spreading,
slimy, multinucleate mass called a
plasmodium that moves slowly over
its substrate (e.g., a rotting log)
engulfing food and growing as it does
so.
Eventually, the plasmodium develops
stalks that produce and release spores.
If the spores land in a suitable
location, they germinate forming
single cells that move by both flagella
and pseudopodia. These fuse in pairs
and start forming a new plasmodium.
The left photo (courtesy of Prof. I. K.
Ross) shows the plasmodial stage of
Stemonitis just before it formed
sporangia. The right photo (courtesy of Turtox) shows the fully developed sporangia of Stemonitis.
Physarum polycephalum, another member of this group, is the subject of many laboratory studies.
External Link
PhysarumPlus — An Internet Resource for Students of Physarum polycephalum and Other Acellular Slime
Molds
Please let me know by e-mail if you find a broken link in my pages.)
Protists without typical mitochondria
There are several groups of protists that were long thought to have no mitochondria. However, most (perhaps
all) had them in the past. Today, only remnants of their ancestor's mitochondria — called mitosomes —
remain.
Some examples are:
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Microsporidia
o All are unicellular obligate intracellular parasites.
o Many are pathogenic in insects (one is even marketed commercially as a biocontrol agent).
o Some contaminate drinking water supplies and can cause gastrointestinal upsets in humans.
Microsporidia, such as Encephalitozoon cuniculi, are a common cause of diarrhea in AIDS
patients. Encephalitozoon cuniculi has a tiny genome [Link] with only 1,997 protein-encoding
genes — fewer than many bacteria (e.g., E. coli has 4,290). Obliged to live within the cells of
its host, it has lost the genes for many important functions (e.g., the citric acid cycle)
depending instead on its host.
o Fungi are their closest relatives.
Entamoeba histolytica.
o Causes amebic dysentery, the third most common parasitic disease of humans (after malaria
and schistosomiasis). Its closest relatives are the slime molds.
Giardia intestinalis (also known as Giardia lamblia)
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Frequently encountered in
public water supplies
contaminated by animal
feces. Causes diarrhea in
humans. Avoids the host
immune response by
periodically changing its
surface protein coat. [Link
to other examples of this
defense.]
Choanoflagellates
These are single-celled (e.g., Monosiga),
aquatic (both fresh water and marine)
protists that have a single flagellum
surrounded by a collar ("choano" =
collar) of microvilli. Some (e.g.,
Proterospongia) form simple colonies
during part of their life. The flagellum is
used for swimming and also beats
bacteria-containing water through the collar for feeding.
Sponges also use collar cells to filter food from the water.
Not only does this suggest a close relationship between the two groups, but other evidence indicates that
choanoflagellates are the closest protistan relatives of all animals (metazoa). Although single cells, they
express genes for several proteins that are essential to cell-cell interactions in metazoans, such as
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cadherins (attach cells to each other — Link)
tyrosine kinases (used in many examples of cell-cell signaling — Link)
What function these proteins have in the choanoflagellates is unknown.
Other Groups of Eukaryotes
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The metazoa (animals) are described in two separate pages:
o The Invertebrates and
o The Vertebrates
The fungi are discussed in a page of their own. Link to it.
Plants, including the green algae (Chlorophyta) are described in a separate page. Link to it.
External Link
A portal into the various groups of protists (and other eukaryotes) with many illustrations in color.
Please let me know by e-mail if you find a broken link in my pages.)
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