Protozoan groups represent early phylogenetic
splits from all multicellular
(metazoan) animals. The common
ancestor of protozoan groups and
metazoans was unicellular. Some protozoa,
particularly among colonial and
multicellular flagellates (Figure 11-11),
show various degrees of cell aggregation
and some differentiation that may
parallel the body plans of early metazoa.
Current evidence indicates that
metazoans are the sister group of
choanoflagellate protozoa, but available
data do not strongly support this
Traditionally, Sarcodina and Mastigophora
were considered separate
classes in phylum Protozoa. Observations
that some flagellates could
form pseudopodia, that some species
of amebas had flagellated stages, and
that a supposed ameba was really a
flagellate without a flagellum, all
seemed to support the concept of a
phylum Sarcomastigophora. However,
analyses of sequences of bases in
genes, particularly the gene encoding
the small subunit of ribosomal RNA
, have provided strong evidence
that neither Sarcodina nor Mastigophora are monophyletic groups. For example,
unicellular green algae (members
of class Phytomastigophorea, subphylum
Mastigophora in the classification
to follow) and higher plants are more
closely related to animals than they
are to other protozoan groups. Ameboid
forms arose independently many
times, and there is no justification
for placing them all in a subphylum
In fact, molecular evidence has almost
completely revised our concepts
of protozoan phylogeny. Sequence
analyses suggest that the earliest
branch from the eukaryote ancestor
led to a group called Euglenozoa
(including Euglena and its relatives, Trypanosoma and its relatives, and
others), followed by divergence of
slime molds. There was then a rapid
radiation producing many algal and
protozoan groups, in addition to fungi,
plants, and animals. Two large
algal/protozoan groups are now called stramenopiles and alveolates. Other
major groups contain, for example, red
algae and green algae and plants.
The stramenopile grouping includes
brown algae, yellow algae,
diatoms, and many others. Alveolates
are dinoflagellates (many of which
are phototrophic), apicomplexans
(coccidians, Plasmodium spp., etc.),
and ciliates. The nearest common
ancestor of dinoflagellates and apicomplexans
apparently acquired photosynthetic
plastids by a secondary
endosymbiotic event. (Secondary
endosymbiosis is an endosymbiotic
acquisition of an eukaryote by
another eukaryote, rather than the
combination of two prokaryotes, as in
the origin of eukaryotes)
The endosymbiosis in alveolates
apparently occurred after divergence
of ciliates, and photosynthetic ability
was lost subsequently by ancestral
apicomplexans, which are now all
parasitic. It is believed that stramenopiles
acquired their photosynthetic
plastids by another secondary
endosymbiosis, independent of other
major autotrophic assemblages such
as red and green algae.
We have described some of the
wide range of adaptations of protozoan
groups in the preceding
pages. Ameboid forms range from
bottom-dwelling, naked species to
planktonic forms such as the foraminiferans
and radiolarians with
beautiful, intricate tests. There are
many symbiotic species of amebas.
Flagellated forms likewise show adaptations
for a similarly wide range of
habitats, with the added variation of
photosynthetic ability in many species
Within a single-cell body plan, the
division of labor and specialization of
organelles are carried farthest in the
ciliates. These have become the most
complex of all protozoa. Specializations
for intracellular parasitism have
been adopted by Apicomplexa and
Four main protozoan groups were recognized
traditionally: flagellates, amebas,
spore formers, and ciliates. Biologists
then realized that the phylum “Protozoa”
was not valid and was composed of several
more-or-less unrelated phyla. They
further believed that there was sufficient
evidence of a relationship between flagellates
and amebas to support a phylum
Sarcomastigophora. These concepts are
reflected in the following classification.
However, molecular data do not support
this arrangement, and when new classification
of protozoan groups becomes formalized,
we will incorporate it. Groups
for which there is evidence of polyphyly
and/or paraphyly are marked with an
Phylum Sarcomastigophora* (sar´komas-
ti-gof´o-ra) (Gr. sarkos, flesh, + mastix, whip, + phora, bearing).
Flagella, pseudopodia, or both types of
locomotory organelles; usually with only
one type of nucleus; typically no spore
formation; sexuality, when present,
Subphylum Mastigophora* (mas-tigof´o-ra) (Gr. mastix, whip, + phora,
bearing). One or more flagella
typically present in adult stages;
autotrophic or heterotrophic or both;
reproduction usually asexual by
Class Phytomastigophorea* (fi´to-mas-ti-go-for´e-a) (Gr. phyton, plant,+ mastix, whip,
+ phora, bearing). Plantlike flagellates, usually bearing
bodies; chromoplasts with
chlorophyll are chloroplasts),
which contain chlorophyll.
Euglena, Volvox, Ceratium,
Class Zoomastigophorea* (zo'o-mas-ti-go-for´e-a) (Gr.
zoon, animal,+ mastix, whip,
+ phora, bearing). Flagellates
without chromoplasts; one to
many flagella; ameboid forms
with or without flagella in
some groups; species predominantly
symbiotic. Examples: Trichomonas, Trichonympha,
Subphylum Opalinata (o´pa-lina
´ta) (N.F. opaline, like opal in
appearance,+ata, group suffix).
Body covered with longitudinal rows
of cilium-like organelles; parasitic;
cytostome (cell mouth) lacking;
two to many nuclei of one type.
Examples: Opalina, Protoopalina.
Subphylum Sarcodina* (sar-ko-di´na)
(Gr. sarkos, flesh,+ ina, belonging to).
Pseudopodia typically present; flagella
present in developmental stages of
some; free living or parasitic.
Superclass Rhizopoda* (ri-zop´oda)
(Gr. rhiza, root, + pous, podos, foot). Locomotion by lobopodia,
filopodia, or reticulopodia, or by
cytoplasmic flow without production
of discrete pseudopodia.
Composed of eight classes, some of
which are listed here.
Class Lobosea* (lo-bo´se-a)
(Gr. lobos, lobe). Pseudopodia
lobose or filiform but produced
from broader lobe; usually
uninucleate; no fruiting bodies.
Examples: Amoeba, Entamoeba,
Chaos, Arcella, Difflugia.
Class Eumycetozoea* (yu´miset-
o-zo´e-a) (Gr. eu, good,
true, + mykes, fungus, +zoon,
animal). Ameboid feeding
stage, flagellated stage present
or absent; produce aerial fruiting
bodies with one to thousands
of spores. Examples: Dictyostelium, Physarum.
Class Filosea (fi-los´e-a) (L.
filum, thread). Hyaline, filiform
pseudopodia, often branching,
sometimes rejoining; no spores
or flagellated stages known.
Class Granuloreticulosea (gran´yu-lo-re-tik´yu-los´e-a)
(L. granulum, dim. of granum,
grain, + reticulum, dim. of
rete, net). Delicate, finely granular
or hyaline reticulopodia or,
rarely, finely pointed, granular
but nonrejoining pseudopodia.
Examples: Allogromia, Fusulina,
Textularia, Elphidium, Globigerina, other foraminiferans.
Superclass Actinopoda (ak´tinop
´o-da) (Gr. aktis, aktinos, ray,
+ pous, podos, foot). Often spherical,
usually planktonic; pseudopodia
in form of axopodia, with
microtubular supporting structure.
Class Acantharea (a´kanthar
´e-a) (Gr. akantha, spine or
thorn). Strontium sulfate skeleton
composed of 20 or more
radiating spines more or less
joined in cell center; marine,
usually planktonic. Examples: Acanthometra, Lithoptera.
Class Polycystinea (pol´e-sistin
´e-a) (Gr. polys, many, + kystis,
bladder). Siliceous skeleton
in most species, usually of solid
elements, consisting of one or
more latticed shells with or
without radial spines, or of
spicules; capsular membrane
usually of grossly polygonal
plates with many more than
three pores; marine, planktonic.
Class Phaeodarea (fe´o-dar´-
e-a) (Gr. phaios, dusky, + daria, suffix). Skeleton of mixed silica
and organic matter, consisting of
usually hollow spines and shells;
very thick capsular membrane
with three pores; marine, planktonic.
Class Heliozoea (he´le-o-zo´-
e-a) (Gr. helios, sun,+ zoon,
animal). Without central capsule;
skeletal structures, if present,
siliceous or organic; axopodia
radiating on all sides; most
species freshwater. Examples: Clathrulina, Actinophrys,
Phylum Labyrinthomorpha (la´birinth-
o-morf´a) (Gr. labyrinth, maze,
labyrinth, + morph, form; + a, suffix).
Small group living on algae; mostly
marine or estuarine. Example: Labyrinthula.
Phylum Apicomplexa (a´pi-complex
´a) (L. apex, tip or summit, + complex, twisted around, + a, suffix).
Characteristic set of organelles (apical
complex) associated with anterior end
present in some developmental stages;
cilia and flagella absent except for flagellated
microgametes in some groups;
cysts often present; all species parasitic.
Class Sporozoa (spor´o-zo´e-a)
(Gr. sporos, seed, + zoon, animal).
Spores or oocysts typically
present that contain infective
sporozoites; flagella present
only in microgametes of some
groups; pseudopods ordinarily
absent, if present they are used
for feeding, not locomotion;
one or two host life cycles.
Examples: Monocystis, Gregarina,
Toxoplasma, Babesia, Pneumocystis. [Note: taxonomic position
of Pneumocystis not
known with certainty.]
Phylum Microspora (mi-cros´por-a)
(Gr. micro, small,+ sporos, seed).
Parasites of invertebrates, especially
arthropods, and lower vertebrates.
Phylum Ascetospora (as-e-tos´por-a)
(Gr. asketos, curiously wrought,+ sporos,
seed). Small group that is parasitic in
invertebrates and a few vertebrates.
Phylum Ciliophora (sil-i-of´or-a) (L. cilium, eyelash, + Gr. phora, bearing).
Cilia or ciliary organelles in at least one
stage of life cycle; two types of nuclei,
with rare exception; binary fission across
rows of cilia, budding and multiple
fission also occur; sexuality involving
conjugation, autogamy, and cytogamy;
nutrition heterotrophic; contractile vacuole
typically present; most species free
living, but many commensal, some
parasitic. (This is a very large group,
now divided by the Society of Protozoologists
classification into three classes and
numerous orders and suborders. The
classes are separated on the basis of
technical characteristics of the ciliary patterns,
especially around the cytostome,
the development of the cytostome, and
other characteristics.) Examples: Paramecium,
Colpoda, Tetrahymena, Balantidium,
Stentor, Blepharisma, Epidinium,
Euplotes, Vorticella, Carchesium,
Trichodina, Podophrya, Ephelota.