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Metazoa: Information
Metazoans
Vielzellige Tiere
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Protozoa
- Mesozoa - Metazoa
Some time ago most zoologists would divide the
animals into Protozoa,
Mesozoa and Metazoa. Protozoans are primitive,
unicellular animals, and
metazoans are the more derived multicellular
animals. Mesozoans were
believed to be at the junction between these two
large groups- living
remnants linking unicellular and multicellular
animals.
This subdivision of the animals, however, does
not reflect natural
groups and today it is no longer in use.
Especially the group Protozoa
has been recognized as a polyphyletic assemblage
(i.e. containing
species that are not directly related). The term
"protozoan" is now
used only as a general name to denote
unicellular eukaryotic
organisms that do not primarily feed by
photosynthesis.
The earliest organisms on Earth were simple
cells. Their offspring are
the bacteria and archaebacteria. The origin of
eukaryotic organisms
from these bacteria and archaebacteria is still
unclear and debated.
Most biologists, however, agree that a single
event gave rise to
eukaryotic cells, and that Eukaryota, therefore,
are a monophyletic
group (i.e. all derive from a single common
ancestor).
The Eukaryota are subdivided into three large
monophyletic groups: (1)
Tetramastigota. (2) Discicristata,
Chromalveolata, Rhizaria, Planta.
(3) Amoebozoa, Opisthokonta. This subdivision
already shows that old
systematic entities like "plants" and "animals"
are para- or even
polyphyletic groups.
Tetramastigota
These are primitive eukaryotes. They do not
possess mitochondria, which
has previously been regarded as a primitive
character. However, genomic
studies revealed that mitochondrial genes are
present; now
tetramastigotes are believed to have lost their
mitochondria at an
early stage of their evolution. The
hydrogenosomes and mitosomes found
in some Tetramastigota might be remnants of
mitochondria.
The Tetramastigota comprise the following taxa:
(1) Retortamonadea. Tiny, mostly commensalic
organisms, some species
also are parasitic and pathogenic (e.g.
Chilomastix mesnili as the
cause of diarrhea in humans). No mitochondria.
(2) Diplomonadea. Two subtaxa. The primitive
species of the subtaxon
Enteromonadida are commensals in the gut
of vertebrates
including humans (e.g. Enteromonas hominis). The
derived subtaxon
Diplomonadina comprises species with a so-called
diplozoic organisation
that is basically a not fully separated 2-cell
stage. The Diplomonadina
are mostly free living or commensalic. Some
species, however, are
pathogenic in animals and humans (e.g. Giardia
intestinalis). No
mitochondria, but the so-called mitosomes might
represent reduced
mitochondria.
(3) Oxymonadea. Commensalic organisms and
symbionts, especially in the
gut of termites (e.g. Oxymonas grandis). No
mitochondria.
(4) Parabasalea. Similar to the Oxymonadea. No
mitochondria, but
similar organelles called hydrogenosomes are
present. Most species are
symbionts in
the gut of insects. Some species are pathogenic
in humans. Four
subtaxa: Trichomonadida (e.g. Trichomonas
vaginalis, pathogenic in
humans), Cristamonadida, Trichonymphida, and
Spirotrichonymphida (the
latter three are symbionst in the gut of
termites and other insects).
Discicristata,
Chromalveolata, Rhizaria,
Planta
This
large monophyletic group comprises many species
formerly classified as
"algae", "protozoa" or "plants". The four large
subgroups each comprise
several subtaxa.
(1) Discicristata
Named for the disc-like shape of the cristae in
their mitochondria. Five subtaxa: (a) The
Heterolobosa are amoebae and
slime-molds living in the soil or in limnic and
marine sediments. Some
species can be pathogenic and cause severe
illnesses in humans (e.g.
Naegleria fowleri). The Heterolobosa comprise
two subtaxa:
Schizopyrenida and Acrasea. The latter form cell
aggregates that function as quasi multicellular
fruiting bodies. (b)
The Euglenozoa (=Euglenophyta) comprise
heterotrophic forms and forms
that have
acquired the ability for photosynthesis by
incorporating other
photosynthetic organisms. Two subtaxa:
Kinetoplastida (e.g. Trypanosoma
brucei, the agent causing sleeping illness) and
Euglenida (=Euglenata)
(e.g. Euglena
viridis, the popular study object in university
courses). (c) The
Diplonemida are a small group of flagellate
organisms. Mainly marine,
in sediments or planctonic. (d) The
Hemimastigophora are very similar to the
Euglenida and might be early
offshoots of the Euglenozoan lineage. The few
known species live in the
soil. (e) The Pseudociliata superficially
recall ciliates (and previously have been
classified with them as
"Protociliata"). Ultrastructural data suggest
that the Pseudociliata
are closelely related to the Euglenozoa. The few
species are marine and
feed on other microorganisms.
(2) Chromalveolata
A very large taxon that comprises species
formerly
classified as "algae", "protozoa" or "fungi".
Two large subtaxa,
Chromista and Alveolata:
(a) Chromista. Most species incorporate a
photosynthetic symbiont.
Three subtaxa: (I) Prymnesiomonada
(=Haptomonada, =Haptophyta,
=Prymnesiophyta), mainly
marine (e.g. Chrysochromulina polylepis, the
"killer algae" producing
newspaper headlines regularly in the summer).
(II) Cryptomonada
(=Cryptophyta). Mainly
in fresh water. (III) Heterokonta. Largest
subtaxon. Comprising the following:
Proteromonadea
Bicosoecidea (=Bicocoecida)
Opalinea
Actinophryida
Chrysophyceae
(=Chrysomonadea)
including the following taxa
that are sometimes regarded as independent:
Synurophyceae
(=Mallomonadales)
Pedinellophyceae
(=Pedinellidida)
Dictyochophyceae
(=Silicoflagellida)
Parmophyceae
Sarcinochrysidophyceae
Bacillariophyceae (=Diatomophyceae,
=Diatomeae)
Xanthophyceae
(=Heteromonadea)
Eustigmatophyceae
Raphidophyceae
(=Raphidomonadea, =Chloromonadophyceae)
Phaeophyceae (=Fucophyceae)
Labyrinthulomycetes
(=Labyrinthulea)
Hyphochytriomycetes
(=Hyphochitridiomycetes)
Oomycetes (=Oomycota)
The first 4 taxa were formerly regarded as
"protozoans", whereas the
taxa 5 - 10 were regarded as "algae" and taxa 11
- 13 as "fungi". Proteromonadea are
endosymbionts in
vertebrates.
Bicosoecida are reminiscent of choanoflagellates
and can occur in
colonies; mainly marine. Opalinea are
commensalic in the gut of vertebrates.
Actinophryida were formerly
grouped with the Heliozoa, but are not related
to them. Most species
are limnic or marine, some species are
terrestrial.
Chrysophyceae (golden brown algae) live mostly
in fresh water.
Bacillariophyceae (diatomeans) feed mainly
phototrophic;
marine or limnic. Xanthophyceae (yellow
algae) live in the soil, fresh- and saltwater;
many species form cell
threads.
Eustigmatophyceae and Raphidophyceae are
unicellular organisms that
live in
freshwater or saltwater and comprise less than
20 species each.
Phaeophyceae (brown algae) are multicellular;
most species are attached
to the ground in marine habitats; only few
species live in freshwater.
Labyrinthulomycetes (web slime molds) and
Hyphochytriomycetes are
closely related to the Oomycetes. Most species
of these three groups
are multicellular organisms and many are
parasites.
(b) Alveolata. This taxon comprises 7 subtaxa (3
of which are
uncertain) that were previously
classified either as "algae" or as "protozoans".
(I) Dinoflagellata
(=Dinophyta). Most species are armored with
cellulose plates; most
species are phototrophic; limnic or marine. (II)
Perkinsozoa. Parasites
of molluscs. (III) Colpodellidae. Classification
of this family is
uncertain; Only 5 known species; parasites of
other "protozoa". (IV)
Apicomplexa. Formerly a component of the
"protozoan" taxon "Sporozoa".
Many are extracellular parasites and commensals
in the gut of a large
variety of metazoans, mainly arthropods. Others
are intra- and
extracellular parasites in different organs;
many human
pathogens e.g.: Cryptosporidium parvum (agent of
cryptosporidiosis),
Toxoplasma gondii (agent of toxoplasmosis) and
Plasmodium vivax (agent
of malaria). (V) Ciliophora. Limnic and marine
"protozoans" (ciliates)
with many well-known forms, e.g. Paramecium
caudatum, popular study
object in university zoology courses. Few
species are parasites or
commensals. (VI) Haplosporea. Classification
with the Alveolata
uncertain. Formerly classified in the
"protozoan" taxon "Ascetospora".
Parasites of various metazoans, mainly molluscs.
(VII)
Paramyxea. Parasites of polychaetes, crustaceans
and molluscs. The
classification with the Alveolata is uncertain
and is probably
incorrect. Currently the true relationships of
the Paramyxea are
unknown.
(3) Rhizaria
This taxon comprises mainly amoebae that have
been formerly classified
with the "Rhizopoda", "Heliozoa" and
"Radiolaria". These old taxa were
found to be polyphyletic. Many subgroups have
been transferred to other
groups (e.g. to the Heterokonta), but some
remaining amoeba-like forms
have been found to constitute a monophyletic
group now called Rhizaria.
8 Subtaxa. (I) Cercozoa. Some parasitic species,
others are limnic,
marine or terrestrial; some species have
previously been classified as
"slime molds" (Phytomyxa (=Plasmodiophorea));
some phototrophic species
have previously
been considered as "algae" (e.g. Paulinella
chromatophora,
Chlorarachnion spec.). (II) Foraminifera
(=Granuloreticulosa)
(foraminiferans). Most species have a chambered
shell made from
proteins, hardened organic matrix (sometimes
with iron or silicium), or
calcium carbonate. This shell preserves well and
thus these organisms
have a large fossil record. (III - VIII)
Phaeodarea, Acantharea,
Polycystinea, Centrohelida, Desmothoracida and
Taxopodida. These 6 taxa
comprise species with spicules made from silica
and other minerals,
that give them a "star-" or "sun-like"
appearance.
(4) Planta (=Plantae)
This taxon comprises the "green plants" and many
"algae". Two large
subtaxa:
(a) Biliphyta. Two subtaxa: (I) Rhodophyta (red
algae). Mostly
multicellular. Some species are intracellular
symbionts of Chromista or
parasites in other Rhodophyta; (II)
Glaucocystophyta (=Glaucophyta)
(blue green algae); unicellular, limnic.
(b) Viridiplanta (=Chlorobionta,
=Viridiplantae). Two subtaxa: (I)
Chlorophyta (green algae and green plants);
large group with
unicellular and multicellular species; limnic,
marine, or terrestrial.
A well-known example is the colonial Volvox
spec. The higher plants
(Embryophyta) also belong to this taxon. (II)
Streptophyta. These
"algae" comprise species that formerly have been
classified with the
Chlorophyta, but are now known to represent an
early offshoot of the
Viridiplanta lineage.
Amoebozoa,
Opisthokonta
This is the largest eukaryotic taxon that
comprises species that have
been classified as "protozoa", "fungi" and
"animals".
(1) Amoebozoa. This taxon comprises a large
portion of the "Rhizopoda".
Two subtaxa: (a) Lobosa. Naked and shelled
amoebae, including Amoeba
proteus, the popular study object of university
zoology courses. (b)
Conosa. This taxon contains the Archamoebae that
have previously been
regarded as primitive Eukaryota, because of the
lack of mitochondria;
however, these have been lost secondarily. The
second subtaxon of the
Conosa are the Mycetozoa comprising the majority
of the slime molds
(Eumyxa and Dictyostela) and Aconchulina
(formerly members of the
"Rhizopoda").
(2) Opisthokonta. Four subtaxa:
(a) Fungi. Comprises all true fungi
(Chytridiomycota, Zygomycota,
Ascomycota, and Basidiomycota) and the
unicellular parasitic Microspora
(=Microsporidia), that have previously been
regarded as the most
primitive Eukaryota. Many true fungi live in
symbiosis with bacteria or
"algae" and together they form a new grade of
organisation, the lichens
("Lichenes").
(b) Ichthyosporea (=Mesomycetozoa). Parasitic
species that are closely
related to the fungi. Some are pathogenic in
humans.
(c) Choanoflagellata. The choanoflagellates are
the sister group of the
Metazoa. They comprise unicellular and colonial
species.
Choanoflagellate colonies are similar to sponges
and could represent a
model for the first step towards
multicellularity.
(e) Metazoa.
Multicellular
animals; all species that occur in Germany will
be covered in this Web
repository.
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This page has
been
updated on February 15, 2008
This site is online since May 31, 2005
Copyright © 2008 by Nikola-Michael Prpic. All
rights reserved.
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