Chapter 2 Protozoa
Chapter 2 Protozoa
Aims and requests
To predominate the general characters of phylum Protozoa; to understand classification system
of protozoan; to grip the structure and function of the representatives; to understand the
evolutionary relationships of groups and connections with human.
Keys and difficulties
The general characters of phlylum Protozoa; the configuration, structure, and characters of
life cycle of representative animals
Methods and instruments
Multimedia, lecture and discussion, experiments
Contents:
Part 1 general characteristics of Protozoa and its classification
1. General characteristic of Protozoa
2. Classification of Protozoa
Part 2 Mastigophora
1. Euglena viridis
2．Characters of Mastigophora and its main groups
Part 3 Sarcodina
1. Amoeba proteus Pallas
2. Characters of Sarcodina and its main groups
Part 4 Sporovoa
1. Plasmodium
Part 5 Ciliata
1. Paramecium, a representative ciliate
2. Characters of Cilita
The protozoan is a complete organism in which all life activities are carried on within the limits of a single plasma
membrane. As a protoplasmic mass not divisible into cells, a protozoan may be termed "acellular" and may be said to
belong to a protoplasmic level of organization.
Fig. 2.1 Animal-like protista: A generalized evolutionary tree depicting the major events and
possible lines of descent for the protozoa
The number of named species of Protozoa lies somewhere between 15,000 and 50,000, but this figure probably
represents only a fraction of the total number of species. Some protozoologists think that there may be more protozoan
species than all other species together because each species of the higher phyla may have its own unique protozoan
Chapter 2 Protozoa
parasites, and many protozoans bear parasites themselves. There are probably more parasitic species than free-living
ones．
Because the protozoans are not made up of cells they are termed acellular animals and they represent the
protoplasmic level of organization. Many biologists place them close to the common ancestor of the many-celled forms.
Some of the flagellates are quite close to the plants and may be considered as connecting links between plants and
animals. Many flagellates are autotrophic (holophytic), that is, they contain chlorophyll and, like the plants, can
manufacture their own carbohydrates by photosynthesis. Perhaps in the early evolution of animals some of these
autotrophic flagellates may have lost their green chloroplasts to become colorless animals that must either absorb
nutrients from their environment (saprozoic) or feed upon other plant or animal matter (heterotrophic, or holozoic)．
Part 1 general characteristics of Protozoa and its classification
1. General characteristic of Protozoa
› Acellular (or one cell)，some colonial.
Fig. 2.2 A protozoan. This drawing of a stylized protozoan with a flagellum illustrates the
basic protozoan morphology
A protozoan is an organism that is made up of a mass of protoplasm that is not divided into cells and that carries on
all the life processes within its cell membrane. In structure a protozoan may be likened to a cell from a multicellutar
animal, but functionally it is a complete organism that performs all the essential life processes of an animal.
It is erroneous to think of protozoans as simple animals. They have many complicated structures, and
physiologically they are quite complex little animals. Within the cytoplasm of a protozoan there is specialization and
division of labor. The specialized structures within the cytoplasm are called organelles, and each is fitted for a specific
function similar to those of specialized organs or groups of cells of the metazoan (many-celled) animal. These
organelles may perform as skeletons, sensory systems, conducting mechanisms, contractile systems, organs of
locomotion, defense mechanisms, and so on.
Most protozoans are single, but many of them particularly among the flagellates, live in distinct colonies of from
several to hundreds of protozoan zooids. The distinctions between such protozoan colonies and metazoans made up of
from several to millions of body cells are mainly a degree of division of labor. In metazoans the cells are dependent on
each other for such functions as nutrition, movement, excretion, reproduction, and certain cells become highly
specialized for distinctive functions. In the protozoan colony, however, certain cells are specialized for reproduction, but
all the rest can perform all other body functions independently of each other.
› Mostly microscopic, although some large enough to be seen with the unaided eye.
Most Protozoa are small or microscopic, usually from 3 to 300μ long. The largest are among the Foraminifera（有
孔虫类）, some of which have shells 100 to 125 millimeter in diameter. Certain amebas may be 4 to 5 mm in diameter.
Some of them are found in colonies in which each individual carries on its functions independent of the others, although
in a few colonies there is a small amount of differentiation.
Chapter 2 Protozoa
› All symmetries represented in the group; shape variable or constant (oval, spherical, etc.)
› No germ layer present.
› No organs or tissues, but specialized organelles found; nucleus single or multiple.
› Free living, mutualism, commensalism, parasitism all represented in the group.
Symbiotic relationships
The term symbiosis（共生现象）refers to the intimate interrelationships between two organisms of different species
for the purpose of deriving energy or for some other benefit. This special relationship may be beneficial to both species
(mutualism) or beneficial to only one species but not harmful to the other (commensalism)，or the relationship may be
forced so that one receives benefit and the other furnishes all the energy and may actually be harmed (parasitism). These
relationships are not always clear cut, and are not limited to protozoans, but protozoans are represented by all three major
types of symbiosis.
Mutualism（互利）. Several species of flagellates live in the intestines of termites（白蚁）and wood-feeding roaches
（蟑螂）, where they secrete enzymes for digesting the cellulose that is thus made available to their hosts. The flagellates
cannot live outside the host; the termite or roach would starve without the flagellates. The protozoans ingest the wood
after the insects have chewed it up into small bits．When termites lose their protozoan fauna (by high oxygen exposure,
high temperatures, or prolonged starvation), they can survive only a short time even when fed an abundance of wood,
unless they are reinfested with the flagellates.
Commensalism（共生）. Commensal protozoans may live on the outside (ectocommensals) or, more commonly, on
the inside (endocommensals) of another organism. Some Vorticella（钟形虫）and suctorians（吸管类动物）are found
attached to hydroids（水螅）as ectocommensals. Endocommensals are common in the digestive tubes of higher forms.
Herbivorous mammals contain great numbers of ciliates and also a few flagellates and amebas. In ruminants（反刍动物）
such as cattle and sheep they are found in the first two stomach compartments, where they digest bacteria and foodstuffs
in the host's food. Eventually they pass into the other compartments of the stomach and intestine, where they are
destroyed and digested so that the host gets all the nutrition after all. Estimates are given of as many as 100,000 to 1
million ciliates per cubic millimeter of gut contents and a total number in a mature cow of 10 to 50 billion. Apparently
Protozoa are not essential to the cattle, which can live and grow normally when the commensals are removed.
Parasitism（寄生）．Parasitism is the most common form of symbiosis among Protozoa. Ectoparasites live on the
outside of the body and endoparasites live within the host. Most animals, especially higher ones, have one or more kinds
of protozoan parasites. Protozoans themselves, even protozoan parasites, are often parasitized by other protozoans. For
instance, the opalinid mastigophoran that lives in the frog's intestine is parasitized by a certain ameba.
Parasitic species are found among all classes of Protozoa; the class Sporozoa is entirely parasitic. Every vertebrate
species probably harbors a parasitic or endocommensal ameba. Protozoan parasites have different ways of infesting the
host. Some are transferred by contact of bodily parts (Entamoeba histolytica); by arthropod or other vectors
(Trypanosoma and Plasmodium); by placenta（胎盘） (blood parasites)；and by invasion of ovary or egg (Babesia).
Protozoan parasites may differ little in structure from free-living forms, but they undoubtedly have physiologic
adaptations. Some protozoan parasites are adapted to a wide range of hosts and parasitize many species; others are
restricted to a few species．
› Locomotion by pseudopodia, flagella, cilia, and direct cell movements; some sessile.
Amoebas move by extending part of their cell
membrane into a lobe, or pseudopodia（伪足）, that can
attach to a surface. Then, Cytoplasm streams into the
pseudopodia and pulls the organism forward. This
movement is called ameboid movement. ameboid
movement is a form of cytoplasmic streaming, the Fig. 2.3 ameboid movement
Internal flowing of a Cell's Cytoplasm.
Cilia (sing., cilium; L. "eyelashes") and flagella (sing., flagellum; L. "small whips") are elongated appendages on
Chapter 2 Protozoa
the surface of some cells by which the cells, including many unicellular organisms, propel themselves. In stationary cells,
cilia or flagella move material over the cell's surface.
Although flagella are 5 to 20 times as long as cilia and move somewhat differently, cilia and flagella have a similar
structure. Both are membrane-bound cylinders that enclose a matrix. In this matrix is an axoneme or axial filament（轴
丝）, which consists of nine pairs of microtubules arranged in a circle around two central tubules．This is called a 9＋2
pattern of microtubules. Each microtubule pair (a doublet 双联体) also has pairs of dynein (动力蛋白) arms projecting
toward a neighboring doublet and spokes （辐）extending toward the central pair of microtubules. Cilia and flagella
move as a result of the microtubule doublets sliding along one another.
In the cytoplasm at the base of each cilium or flagellum lies a short,
cylindrical basal body（基体）, also made up of microtubules and
structurally identical to the centriole. The basal body controls the growth
of microtubules in cilia or flagella. The microtubules in the basal body
form a 9＋0 pattern: nine sets of three with none in the middle.
› Some provided with a simple protective exoskeleton, but
mostly naked.
› Nutrition includes all types：autotrophic (manufacturing own
nutrients by photosynthesis)，heterotrophic (depending on other plants or
animals for food)，saprozoic (using nutrients dissolved in the surrounding
medium).
› Habitat: aquatic, terrestrial, or parasitic, with or without
locomotor organoids.
› Reproduction: asexually by fission, budding, sporulation; and
sexually by gamogenesis and conjugation
Reproduction in most Protozoa is primarily by cell division
(asexual)．It is comparable in some respects to cell division in the
multicellular animals. The protozoan, however, has certain structural
specializations (organelles)，such as flagella, cilia, contractile vacuole, Fig. 2.4 Internal structure of cilia and
and gullet, that may be divided equally or unequally to the two daughter flagella. In cross section, the arms extend
cells, so that a certain amount of differentiation or regeneration may be from each microtubule doublet toward a
necessary to make the new animal complete. Some of these organelles neighboring doublet, and spokes extend
are self-reproducing, but others are lost by resorption (dedifferentiation), toward the central paired microtubules. The
then differentiated anew in each of the daughter organisms.
dynein arms push against the adjacent
The method of reproduction varies. Some Protozoa simply undergo microtubule doublet to bring about
binary fission, budding, or sporulation. All of these are basically asexual movement.
processes. In others, however, asexual reproduction is often followed at
certain periods by some form of sexual reproduction that may or may not be necessary for the continued existence of the
organism.
Binary fission．This process, the most common among Protozoa, involves the division of the organism, both
nucleus and cytoplasm, into two essentially equal daughter organisms. Binary fission may be transverse (most ciliates) or
longitudinal (Mastigophora). The nucleus divides by mitosis . In many forms the chromosomes are similar in structure
and behavior to metazoan chromosomes; in others the chromosomes are granular and highly atypical. Chromosome
numbers appear to be constant for a species; for example, Zelleriella intermedia (Ciliata) has 24, Entamoeba histolytica
(Sarcodina)，6; Oxytricha fallax (Ciliata)， 24; and Euglena viridis (Mastigophora)，30.
Budding. Budding involves unequal cell division in which usually the parent organism retains its own identity
while forming one or more small cells that assume the parent form after they become free. In some cases the bud may be
as large as the parent. Budding may be either external (certain suctorians and ciliates) or internal (suctorians and
Chapter 2 Protozoa
sporozoans).
Multiple division (sporulation)．In multiple division the nucleus divides a number of times, followed by the
division of the cytoplasm of the organism into as many parts as there are nuclei. It is a method of rapid multiplication
and is characteristic of such parasitic forms as the Sporozoa（孢子虫类）. It is often found in protozoans with
complicated life cycles, including asexual and sexual phases.
Sexual phenomena. Sex is found in certain protozoans but is absent in others. When sex is found, it may involve
the formation of male and Female gametes (similar or unlike in appearance) that unite to form a zygote (synkaryon), or
there may be variant forms of this, such as the complete union of two mature sexual individuals that merge their
cytoplasm and nuclei together to form the zygote. By division the zygote may give rise to many individuals or to a new
colony. Other sexual phenomena have been described for Protozoa, such as autogamy（自核交配）, in which gametic
nuclei arise and fuse to form a zygote in the same organism that produces the gametes; endomixis（内融合）, which
involves nuclear reorganization without fusion of micronuclei; parthenogenesis（单性生殖）, or the development of an
organism from a gamete without fertilization; and conjugation（接合生殖）, in which there is an exchange of gametic
nuclei of micronuclear origin between paired organisms (conjugants).
2. Classification of Protozoa
Four main groups of protozoans are commonly recognized：the flagellates, the ameboid, the sporeforming, and the
ciliates. Traditionally, each of these groups has been represented as a taxonomic class under the phylum Protozoa:
Mastigophora（鞭毛纲），Sarcodina（肉足纲），Sporozoa（孢子纲）和 Ciliata（纤毛纲）. In recent years there have
been many changes in the taxonomic arrangement of this group and some authorities assign subphylum or even phylum
rank to each of the groups.
Part 2 Mastigophora（鞭毛纲）(mas-ti-gof'o-ra) (Gr. mastix, whip,十 phora, bearing)
1. Euglena viridis 绿眼虫
Habitat. The normal habitat of Euglena viridis (order Euglenida) is
freshwater streams and ponds where there is considerable vegetation.
They are sometimes so numerous as to give a distinctly greenish color to
the water. Although light is necessary for their metabolism, they are often
found at various depths below the surface of water, for they are fairly
active forms.
Structure. The spindle-shaped body of E. viridis is about 60 μ.
(0.06 mm.) long, but some species are smaller and some larger (E.
oxyuris) is 500μ long. It is covered by a pellicle（表膜）flexible enough to
permit movement. Inside the pellicle is a thin layer of clear ectoplasm
（ 外 质 ） surrounding the mass of endoplasm （ 内 质 ） . From a
flask-shaped reservoir （ 储 蓄 泡 ） in the anterior end a flagellum
extends．It originates as the union of two delicate threads, or axonemes
（轴丝）, each of which ends as a tiny granule, or blepharoplast
(kinetosome 基体），
on the floor of the reservoir that seems to be essential
for movement of the flagellum and that may function as a centriole in
cell division. A tiny fibril, or rhizoplast（根丝体）, extending from one of
the blepharoplasts to the nucleus near the center of the cell suggests that
the flagellum is under nuclear control.
A swelling of the flagellum associated with the eyespot（眼点）
suggests a possible mechanism by which the organism reacts to light
changes.
Fig. 2.5 Phytomastigophorean Anatomy:
A large water expulsion vesicle (contractile vacuole 伸缩泡)， the structure of Euglena. Note the large,
which is formed by fusion of smaller vacuoles, empties wastes and well-organized chloroplasts.
Chapter 2 Protozoa
excess water into the reservoir the anterior opening of which is an exit.
Near the reservoir is a red eyespot, or stigma（眼点）. This is a shallow cup of pigment that allows light from only
one direction to strike a light-sensitive receptor located as a swelling near the base of the flagellum. When the euglena is
moving toward the light, the receptor is illuminated; when it changes direction, the shadow of the pigment falls on the
receptor. Thus the animal, which depends upon sunlight for its photosynthesis, can orient itself toward the light. If given
the choice，it will avoid shady areas and regions of bright light.
Within the cytoplasm are oval chromatophores (chloroplasts) that bear chlorophyll and give the euglena its
greenish color. Paramylum bodies（副淀粉粒）of various shapes are masses of starch, a means of food storage.
Metabolism．The euglena derives its food mainly through autotrophic (holophytic) nutrition which makes use of
photosynthesis, a process that takes place within the chromatophores（色素体） through the action of chlorophyll（叶
绿素）. This form also makes use of saprozoic nutrition, which is the absorption of dissolved nutrients through the body
surface. It is doubtful whether the euglena ingests solid food particles through its mouth region (holozoic nutrition)
although some flagellates such as Peranema ingest other organisms. Respiration and excretion occur by diffusion
through the body wall.
Locomotion．The euglena swims freely by the movement of its flagellum, which moves in a whiplike manner or
with rotary motion, with the undulation passing from base to tip. This motion pulls the animal forward in a straight
course while the body rotates spirally. Flagellates can travel from a few tenths to 1 mm. per second, according to their
size with the speed increasing with the larger flagellates. Some flagellates use the flagella to push rather than to pull
themselves along. Euglena can also change its shape by peristalsis-like "euglenoid" movement.
Reproduction. Euglena reproduces by longitudinal binary division. The nucleus undergoes mitotic division, while
the body, beginning at the anterior end, divides lengthwise．During inactive periods and for protection the euglena
assumes a spherical shape surrounded with a gelatinous covering, thus becoming encysted. In this condition it can
withstand drought and can become active when it is in water again. Encysted euglenas usually divide so that each cyst
may contain two or more euglenas.
2．Characters of Mastigophora and its main groups
2.1 characters
Mastigophora is made up of the flagellates, or the protozoans that move by means of flagella. They are usually
considered the most primitive of the protozoan groups. This group is divided into the phytoflagellates
(Phytomastigophorea) which have chlorophyll and are thus plantlike, and the zooflagellates (Zoomastigophorea)，which
do not have chlorophyll but are either holozoic or saprozoic and thus are animal-like.
› With locomotor organelles of flagella generally. Many species with 1 to 4 or a little flagella and a few with
more.
› The forms of nutrition are autotrophic, or heterotrophic including osmotrophy and phagotrophy
› Reproduction is longitudinal binary fission, sexual reproduction is conjugation or the complete union of two
mature sexual individuals that merge their cytoplasm and nuclei together to form the zygote.
Encystment is common among protozoans, helping them withstand drought and extreme weather. There is usually a
complex series of events when free-living forms encyst. The organism becomes quiescent and many organelles (cilia,
flagella, contractile vacuole, etc）may disappear. A cyst wall is secreted over the surface so that the animal can withstand
desiccation, temperature changes, and other harsh conditions. Reproductive cycles, such as budding, fission, and
syngamy（配子配合）, may also occur in the encysted condition of some protozoans. The cysts of some protozoans may
be viable for many years.
2.2 The main groups
2.2.1 Phytomastigina
The phytoflagellates usually have one or two flagella (sometimes four) and chromatophores (also called
chromoplasts or chloroplasts)，which contain the pigments used in photosynthesis. They are mostly free living and
contain such familiar forms as Euglena, Chlamydomonas, Peranema，Volvox, the dinoflagellates（腰鞭毛虫），and
Chapter 2 Protozoa
Noctiluca, a marine form, is luminescent and produces a striking greenish light at night. Some of these flagellates are
colonial，living in characteristic groups.
Fig. 2.6 Volvox, a colonial flagellate. (a) a Volvox colony, showing asexually produced daughter
colonies. (b) an enlargement of a portion of the colony wall
Volvox (团藻) is a green hollow sphere that may reach a diameter of 0.5 to 1 mm. It is a colony of many thousands
of zooids (up to 50,000) embedded in the gelatinous surface of a jelly ball. Each cell is much like a euglena, with a
nucleus, a pair of flagella, a large chloroplast (a type of chromatophore)，and a red stigma. Adjacent cells are connected
with each other by cytoplasmic strands. At one pole (usually in front as the colony moves)，the stigmata are a little larger.
Coordinated action of the flagella causes the colony to move by rolling over and over.
Here we have the beginning of division of labor to the extent that most of the zooids are somatic cells concerned
with nutrition and locomotion, and a few germ cells located in the posterior half are responsible for reproduction.
Reproduction is asexual or sexual. In either case only certain zooids located around the equator or posterior half take
part.
Asexual reproduction in Volvox occurs by the repeated mitotic division of one of the germ cells, to form a hollow
sphere of cells, with the flagellate ends of the cells inside. The sphere then invaginates or turns itself inside out, to form a
daughter colony like the parent colony. Several daughter colonies are formed inside the parent colony before they escape
by rupture of the parent.
In sexual reproduction some of the zooids differentiate into macrogametes (ova) or microgametes (sperm). The
macrogametes are fewer and larger and are loaded with food for nourishment of the young colony. The microgametes, by
repeated division, form bundles or balls of small flagellated sperm that, when mature, leave the mother colony and swim
about to find a mature ovum. When a sperm fertilizes an egg, the zygote so formed secretes a hard, spiny, protective shell
around itself. When released by the breaking up of the parent colony, the zygote remains quiescent during the winter.
Within the shell the zygote undergoes repeated division until a small colony is produced that is released in the spring. A
number of asexual generations may follow, during the summer, before sexual reproduction occurs again.
2.2.2 Zoomastigina
The zooflagellates are the colorless flagellates (because they lack chromatophores)， with holozoic or saprozoic
nutrition, and many are parasitic; some have pseudopodia as well as flagella. An example is Proterospongia（原绵虫）, a
colony of collared zooids that has been suggested as a forerunner of the sponges, in which collared cells are typical.
Some of the worst of the protozoan parasites are zooflagellates. Many of them belong to the genus Trypanosoma（锥
虫）and live in the blood of fishes, amphibians, reptiles, birds, and mammals. Some are nonpathogenic, but those that
infect the mammals produce severe diseases.
Members of the genus Leishmania infect many vertebrates, including humans, dogs, and rodents. The life cycles
of members of the genus involve a vertebrate host (e.g., the human) and a vector (a sand fly) that transmits the parasite
between vertebrate hosts. In the vector the parasite takes on a characteristic morphological form known as the
promastigote（前鞭毛体）, and it reproduces asexually in the vector's gut. When the vector bites the vertebrate host,
promastigotes are injected into the vertebrate host. The promastigotes enter cells of the vertebrate host and change into
a form called the amastigote (无鞭毛体). The amastigote reproduces in the host's cells, and when the cell eventually
Chapter 2 Protozoa
dies the amastigotes are released and infect other cells. The symptoms and pathology associated with leishmaniasis
result from the amastigotes killing the host's cells.
There are many different "diseases" caused by Leishmania. The various types of leishmaniasis are confined
primarily, but not exclusively, to Central and South America, central Africa, and parts of southern and central Asia.
Part 3 Sarcodina（肉足纲）(sar-ko-di'na) (Gr. sarkos, flesh, +-ina, belonging to).
1. Amoeba proteus Pallas
Fig. 2.7 Ameoba proteus
Habitat. Amoeba proteus is widely distributed. It lives in slow streams and ponds of clear water, often in shallow
water on the underside of lily pads and other aquatic vegetation, or on the sides of dams, in watering troughs, and in the
sides of ledges where the water runs slowly from a brook or spring. They are rarely found free in water, for they require a
substratum on which to glide.
Structure. The shape of the ameba is irregular and continuously changing because of its power to thrust out
pseudopodia, or false feet, at any point on its body. It is colorless and about 250 to 600 μ in its greatest diameter.
Sometimes its shape is almost spherical when all its pseudopodia are withdrawn. Although it possesses no cell wall, it
has a thin delicate outer membrane called the plasmalemma（质膜）. Just beneath this is a nongranular layer, the
ectoplasm, which encloses the granular endoplasm. The endoplasm is made up of an outer, relatively stiff plasmagel
（原生质凝胶）and a more fluid inner plasmasol（原生质溶胶）, which exhibits flowing or streaming movements. In the
gel layer the crystals and granules keep a fixed distance from each other; in the sol they bump into and move over each
other.
Fig. 2.8 feeding ameba
A number of organelles are found within the endoplasm. The disk-shaped nucleus is granular and refractive to light.
Another organelle is the water expulsion vesicle (contractile vacuole)，a bubblelike body that grows to a maximum size
and then contracts to expel its fluid contents to the outside. Scattered through the endoplasm are food vacuoles which are
drops of water enclosing food particles. There are also other vacuoles, crystals, and granules of various shapes and forms.
Foreign substances such as sand and bits of debris may also be in the protoplasm, having been picked up accidentally.
Chapter 2 Protozoa
Metabolism. The ameba lives upon algae, protozoans, rotifers（轮虫）, and even other amebas. It shows some
selection in its food, for it will not ingest everything that comes its way. Food may be taken in at any part of the body
surface. When the ameba engulfs food, it thrusts out pseudopodia to enclose the food particle completely (phagocytosis
吞噬作用). Along with the food, some water in which the food is suspended may also be taken in. These food vacuoles
are carried around by the streaming movements of the endoplasm. A lysosome with enzymes fuses with each food
vacuole, and digestion proceeds within the vacuole. As digestion proceeds, the vacuoles decrease in size because of loss
of water and the passage of the digested material into the surrounding cytoplasm. Finally, indigestible material is
eliminated by passing out through the plasmalemma as the animal flows away.
The ameba is able to live for many days without food but decreases in volume during this process. The time
necessary for the digestion of a food vacuole varies with the kind of food, but is usually around 15 to 30 hours.
The ameba needs and utilizes energy like any other animal. It gets this energy by oxidation, which results in waste
products such as carbon dioxide, water, and urea. Some of these waste substances are eliminated through the body
surface, but some are discharged through the water expulsion vesicle, which also gets rid of excess water that the ameba
is continually taking in. It is thus responsible for regulating the osmotic pressure of the body. The ameba has a certain
amount of salt in its protoplasm that makes it hypertonic to the surrounding fresh water. Water will therefore enter the
ameba by osmosis through its plasmalemma. It is interesting to note that marine amebas do not have contractile vacuoles
because they are immersed in isotonic sea water (but when placed in fresh water they will form them)
Respiration occurs directly through the body surface by diffusion. Oxygen is dissolved in the water which is
everywhere in contact with the cell membrane so that the gas is easily accessible and diffuses into the ameba.
Locomotion. Locomotion takes place by the formation of temporary locomotor structures, the pseudopodia, which
are thrust out on any part of the body surface and into which the cytoplasm flows. This characteristic movement is called
ameboid movement. When a pseudopodium is beginning to form, a blunt, fingerlike projection called the hyaline cap,
composed only of ectoplasm, first appears. A little later the granular plasmasol flows into this projection as it extends
forward. Usually the ameba forms several small pseudopodia at the start of the movement; one of these gradually
becomes larger, while the others disappear.
Ameboid movement is also found elsewhere in the animal kingdom, notably in the white corpuscles of blood, in
amebocytes of sponges, etc.
Reproduction．When the ameba reaches full size, it divides into two animals by the process of binary fission.
Typical mitosis occurs with all the phases, taking about 30 minutes．During the process of division the shape of the
ameba is spherical, with a number of small pseudopodia. The nuclear membrane disappears during the metaphase and
the body elongates and separates by fission into two daughter cells. Under ordinary conditions the ameba attains a size
for division about every 3 days.
Sporulation and budding have been reported to occur in the ameba, but binary fission seems to be the only regular
method employed.
2. Characters of Sarcodina and its main groups
2.1 Characters
Locomotion by pseudopodia; no definite pellicle; free-living or parasitic; uninucleate or multinucleate; mostly
holozoic; mostly naked but a few with exoskeleton.
2.2 the main groups
2.2.1 Rhizopoda（根足亚纲）
The amebas belonging to this subclass have pseudopodia similar to those of Amoeba proteus. There are many
species of Amoeba, for example, A. verrucosa with short pseudopodia; Pelomyxa carolinensis (Chaos chaos), which is
several times as large as A. proteus; and A. radiosa with many slender pseudopodia.
There are many entozoic（内寄生的）amebas, most of which live in the intestine of man or other animals. Two
common genera are Endamoeba and Entamoeba. Endamoeba blattae is an endocommensal in the intestine of
cockroaches, and related species are found in termites.
Chapter 2 Protozoa
Entamoeba histolytica（痢疾内变形虫）is the only serious rhizopod parasite of man. It lives in the intestinal wall,
which it enters by secreting a substance that dissolves away the intestinal lining. It causes amebic dysentery and often
produces severe lesions and abscesses that may spread to other organs. Some infected persons do not show severe
symptoms but are carriers. Infection is spread by contaminated water or food containing cysts discharged in the feces.
Not all rhizopods are "naked" as are the amebas. Some have their delicate plasma membrane covered with a
protective test or shell. Arcella（表壳虫）and Diffugia（砂壳虫）are common sarcodines，which have a test of secreted
siliceous material or pseudochitin reinforced with grains of sand. They move with pseudopodia that project from
openings in the shell.
Fig. 2.9 Variations in pseudopodia. (a) Lobopodia of Ameoba contain both ectoplasm and
endoplasm and are used for locomotion and engulfing food; (b)filopoida of shelles amoeba contain
ectoplasm only and provide constant two-way steaming that delivers food particles to this
protozoan in a convey-belt fashion; (c) reticulopodia are similar to filopodia except they branch and
rejoin to form a netlike series of cell extensions. they occur in foraminferans such as Globigerina;
(d) axopodia on the surface of a heliozoan such as Actinosphaerium deliver food to the central
cytoplasm.
The foraminiferans are an ancient group of shelled rhizopods found in all oceans and a few in fresh and brackish
water. They are mostly bottom living, but a few live in open water. Their tests are of numerous types. Most tests are
many-chambered and are made of calcium carbonate, although silica, silt, and other foreign materials are sometimes
used. Slender pseudopodia extend through openings in the test then branch and run together to form a protoplasmic net
in which they ensnare their prey. Here the captured prey is digested and the digested products are carried into the interior
by the flowing protoplasm. Their life cycles are complex, for they have multiple division and alternation of generations.
Fig. 10 Foraminiferans. The shells of these
ancient protozoans, which are composed of
calcium carbonate, accumulate on sea
bottoms and contribute over time to the
formation of chalk and limestone. It is largely
the bodies of these foraminiferans that have
formed England's White Cliffs of Dover and
the limestone used to build the Egyptian
pyramids.
2.2.2 Actinopodea（辐足亚纲）. In this subclass of sarcodines the pseudopodia are slender and usually radiate out
Chapter 2 Protozoa
from the central test. Some forms have the slender pseudopods stiffened by an axoneme running down their center. These
protozoans are beautiful little animals. Included in this group are the heliozoans（太阳虫）, which are mostly freshwater
forms. They include Actinosphaerium，which is about a millimeter in diameter and can be seen with the naked eye, and
Actinophrys，only 50 μ. in diameter; neither has a test. Clathrulina secretes a latticed test.
The radiolarians（放射虫） are the oldest known group of animals．They are all marine and nearly all pelagic (live
in open water). Most of them are planktonic, though some live in deep water. Their highly specialized siliceous skeletons
are intricate in form and of great beauty. The body is divided by a central capsule that separates inner and outer zones of
cytoplasm. The central capsule, which may be spherical ovoid, or branched, is perforated to allow cytoplasmic continuity.
The skeleton is made of silica or strontium sulfate and usually has a radial arrangement of spines that extend through the
capsule from the center of the body. At the surface a shell may be fused with the spines. Around the capsule is a frothy
mass of cytoplasm from which stiff pseudopodia arise. These are sticky for catching the prey that are carried by the
streaming protoplasm to the central capsule to be digested. Radiolarians may have one or many nuclei. Their life history
is not completely known, but binary fission, budding, and sporulation have been observed in them.
Part 4 Sporovoa 孢子纲(spor-o-zo'a) (Gr. sporos, seed,＋zoon, animal).
1. Plasmodium
间日疟原虫（P. vivax）、三日疟原虫（P. malaria）
、恶性疟原虫（P. flaciparum）和卵形疟原虫（P. ovale）.
Fig. 11 The life cycle of Plasmodium
Plasmodium is probably the best known parasite that causes malaria in man. Malaria is one of the most widespread
diseases in the world. It is mainly a disease of tropical and subtropical countries but is also common in the temperate
zones. The vectors of the parasites are female mosquitoes of the genus Anopheles. Four species of Plasmodium are
known to infect man. Each produces its own peculiar clinical picture, although all malarial parasites have similar cycles
of development in the host．
Chapter 2 Protozoa
Man acquires malaria from the bite of the mosquito, which introduces the parasites from its salivary glands into the
blood in the form of sporozoites（子孢子）. It was found in 1948 that the sporozoites first enter the cells of the liver.
Here, as cryptozoites（潜隐体）, they pass through a process of multiple division (schizogony 裂体生殖). The products
of this division, merozoites（裂殖子）, then enter the red corpuscles. The period when the parasites are in the liver is
called the incubation period. During this time antimalarial drugs may have little effect upon the parasites.
When the parasites enter the red blood corpuscles (usually only one to a cell), they become amebalike trophozoites
（滋养体）. These feeding forms then develop into schizonts（裂殖体）, which have granules of black pigment. Each
schizont, by multiple fission (schizogony), divides into many daughter asexual merozoites (6 to 36 in number, according
to the species of Plasmadium); these break out of the red cells to enter other red corpuscles and repeat the asexual cycle.
In a few days the number of parasites is so great that the characteristic chills and fever occur; these symptoms are caused
mainly by the toxins released by the parasites
The time elapsing between the fever-chill stages of the cycle depends on the type of malaria. In P. vivax (benign
tertian) the chills and fever occur every 48 hours; in P. malariae (quartan), every 72 hours; in P. falciparum (malignant
tertian)，usually every 24 to 48 hours, although it may be irregular; and in P. oval, every 48 hours.
After this period of asexual reproduction, or schizogony, the merozoites become sexual forms, gametocytes（配子
母细胞）. When these gametocytes are sucked up into the stomach of the mosquito, they become microgametocytes
(mate) and macrogametocytes (female)．Zygotes formed by the union of gametes develop into ookinetes（动合子）that
penetrate into the stomach walls of the mosquito. Later the ookinetes enlarge to form oocysts（卵囊）. Each oocyst
divides in a few days into thousands of sporozoites（子孢子）that rupture the cyst and migrate to the salivary glands,
whence they are transferred to man by the bite of the mosquito.
The developmental cycle in the mosquito requires from 7 to 18 days but may be longer in cool weather. After being
inoculated by the mosquito, man usually manifests the symptoms of the disease 10 to 14 days later.
Some latent forms of malaria may persist for some years without showing clinical symptoms, probably because of
the small number of parasites in the blood. The body gradually acquires a resistance to the disease; however, this
resistance does not prevent relapses.
The elimination of mosquitoes and their breeding places by insecticides, drainage, etc. has been effective in
controlling malaria.
2. the characters
All sporozoans are endoparasites and their hosts are found in all of the animal phyla. The adult stages have no
organelles of locomotion such as pseudopodia, flagella, or cilia. Sporozoans are a heterogeneous group; the various
taxa have little in common except their parasitic habit and the fact that they bear spores during some stage of their life
cycle. Hosts are usual1y infected by means of spores enclosed in hard walls, or sometimes the transmission is by naked
young. There is sometimes an intermediate host such as mosquitoes, leeches, flies, or other vectors．
Although there are no locomotor organelles in the adult stages, the reproductive cells are often flagellated, and some
of the stages exhibit ameboid movement. Some forms have myonemes for contraction.
Nutrition is mostly saprozoic, or by absorption of liquid nutrients through the body surface.
The life cycle usually includes both an asexual and a sexual stage, although some, such as the gregarines have
only a sexual stage. In many sporozoans the complete life histories are not yet known.
Part 5 Ciliata 纤毛纲
1. Paramecium, a representative ciliate
Paramecia are usually abundant in ponds or sluggish streams containing aquatic plants and decaying organic matter.
Structure. The paramecium is often described as slipper shaped. Paramecium caudatum is from 150 to 300μ (0.15
to 0.3 mm）in length and is blunt anteriorly and somewhat pointed posteriorly. The animal has an asymmetric appearance
because of the oral groove, a depression that runs obliquely backward on the ventral side.
The pellicle（表膜）is a clear, elastic membrane divided into hexagonal areas by tiny elevated ridges and is covered
over its entire surface with cilia arranged in lengthwise rows. Just below the pellicle is the thin clear ectoplasm that
Chapter 2 Protozoa
surrounds the larger mass of granular endoplasm. Embedded in the ectoplasm just below the surface are the
spindle-shaped trichocysts（刺丝泡）filled with a semifluid substance that may be discharged for attachment and defense.
The trichocysts alternate with the bases of the cilia. The infraciliature can be seen only by special fixing and staining
methods.
Fig. 2. 12 Paramecium caudatum
The cytostome（胞口）, or mouth, at the end of the oral groove leads into a tubular cytopharynx（胞咽）, or gullet.
Along the gullet is an undulating membrane to keep food moving. Fecal material is discharged through an anal pore
(cytoproct 胞肛) posterior to the oral groove．The endoplasm contains food vacuoles containing food in various stages
of digestion. There are two water expulsion vesicles (contractile vacuoles)，each consisting of a central space
surrounded by several radiating canals that collect fluid and empty it into the central vacuole.
P. caudatum has two nuclei：a large, kidney-shaped macronucleus and a smaller micronucleus fitted into the
depression of the former. These can usually be seen only in stained specimens. The micronucleus is the reproductive
nucleus and also gives rise to the macronucleus. The macronucleus is not essential to reproduction but is essential for
normal metabolism. The number of micronuclei varies in different species. P. multimicronucleatum may have as many as
seven.
Metabolism．Paramecia are holozoic, living upon bacteria, algae, and other small organisms. They are selective in
choosing their food, for some items are taken in and others rejected. The cilia in the oral groove sweep food particles in
the water into the cytostome, whence they are carried into the cytopharynx by the undulating membrane. From the
cytopharynx the food is collected into a food vacuole that is constricted off and dropped into the endoplasm. The food
vacuoles circulate in a definite course through the protoplasm (cyclosis) while the food is being digested by enzymes
from the endoplasm. The indigestible part of the food is ejected through the anal pore. Digestion in ciliates is fairly rapid.
A Didinium can digest a whole paramecium in about 20 minutes.
Respiration takes place through the body surface by diffusion, oxygen dissolved in the surrounding water passing
in and the waste, including carbon dioxide, passing out.
The two contractile water expulsion vesicles regulate the water content of the body and help eliminate nitrogenous
waste. They lie close to the dorsal surface and drain fluid from the cytoplasm by means of radiating canals that connect
the vesicle with the endoplasmic reticulum. When the vacuole reaches a certain size, it discharges to the outside through
a pore. The two vacuoles contract alternate1y. They contract more frequently at higher temperatures and in the mature
animal. Since the contents of the paramecium are hypertonic to the surrounding fresh water, osmotic pressure would
cause water to diffuse into the cell, and thus one of the main functions of the vacuoles is to get rid of the excess water.
Chapter 2 Protozoa
Locomotion．The body of the paramecium is elastic, allowing it to bend and squeeze its way through narrow places.
Its cilia can beat either forward or backward, so that the animal can swim in either direction. The cilia beat obliquely,
thus causing the animal to rotate on its long axis. In the oral groove the cilia are longer and beat more vigorously than the
others so that the anterior end swerves aborally. As a result of these factors, the animal follows a spiral path in order to
move forward．In swimming backward the beat and path of rotation are reversed.
Behavior. When a ciliate, such as a paramecium, comes in contact with a barrier or a disturbing chemical stimulus,
it reverses its cilia, backs up a short distance, and swerves the anterior end as it pivots on its posterior end. This is called
an avoiding reaction．While it is doing this, samples of the surrounding medium are brought into the oral groove. When
the sample no longer contains the unfavorable stimulus, the animal moves forward. In this "trial-and-error" method the
animal attempts many directions until it finds one that is favorable and then makes its escape from the injurious
environment.
Paramecia do not always respond in the same manner to the same stimuli. Their physiologic states vary with
conditions．A hungry animal will react in a different way from one that is well fed. In general its behavior is conditioned
by factors that favor or hinder the normal life processes.
Automatic and fixed responses in orientation to particular stimuli are often called tropisms or taxes. Taxes refer
more specifically to movement or locomotor responses to the stimuli. If the response is movement toward the stimulus, it
is a positive response; an avoiding reaction is a negative response. With respect to the type of stimulus, a taxis or tropism
might be classified as one of the following：thermotaxis (thermotropism)，response to heat; phototaxis, response to light
rays; thigmotaxis, response to contact; chemotaxis, response to chemical substances; rheotaxis, response to currents of
air or water; galvanotaxis, response to constant electric current; or geotaxis, response to gravity.
Since no nervous system is found in protozoans (except perhaps the neuromotor system in ciliates), these responses
must be attributable to the innate irritability of protoplasm. The complex responses of higher forms are believed to have
developed from these simple mechanical responses.
Reproduction ． Paramecia reproduce only by transverse binary fission but have certain forms of nuclear
reorganization called conjugation and autogamy.
In binary fission the micronucleus divides mitotically into 2 daughter micronuclei, which move to opposite ends of
the cell．The macronucleus elongates and divides amitotically. Another cytopharynx is budded off and two new
contractile vacuoles appear. In the meantime a constriction furrow appears near the middle of the body and deepens until
the cytoplasm is completely divided. The process of binary fission requires from 1/2 to 2 hours.
Fig. 2.13 Conjugation in Paramecium. (a) random contact brings individuals of opposite types of
together. (b) meiosis results in four haploid pronuclei. (c) three pronuclei and macronucleus degenerate.
(d-f) conjugants separate. Nuclear divisions that restore nuclear characteristics of the species follows.
Chapter 2 Protozoa
Cytoplasmic divisions may accompany these events.
The process known as conjugation occurs in ciliates and a few other protozoans. This phenomenon happens only at
intervals．It is the temporary union of two individuals that mutually exchange micronuclear material. Conjugating
individuals come together and attach by their oral surfaces and a protoplasmic bridge forms between them. In thriving
cultures one may see a number of these conjugating pairs swimming about.
A series of nuclear changes now occurs. The macronucleus starts to disintegrate and finally disappears. The
micronucleus enlarges, forms a spindle, and divides by meiosis, resulting in 4 daughter nuclei, each with a haploid
number of chromosomes.
Three of the daughter micronuclei degenerate. The remaining micronucleus divides unequally into 2 pronuclei, the
smaller of which in each animal moves across the protoplasmic bridge into the other animal.
Each of these exchanged (male) pronuclei fuses with the larger (female) pronucleus of the other animal thus
restoring the normal (diploid) number of chromosomes in the micronucleus of each animal.
The two paramecia now separate, and in each the fused micronucleus, which is comparable to a zygote in higher
forms, divides by mitosis into 2, 4, and 8 micronuclei. Four of these enlarge and become macronuclei, and 3 of the other
4 disappear. Now the paramecium itself divides twice, resulting in four paramecia, each with 1 micronucleus and 1
macronucleus. After this complicated process, the animals may continue to reproduce by binary fission without the
necessity of conjugation.
Autogamy refers to a process of self-fertilization. After the disintegration of the macronucleus and the division of
the 2 micronuclei to form 8 micronuclei, 2 of the haploid gametic nuclei that result from this division enter a small bulge
(paroral cone)，fuse together, and restore the diploid number of chromosomes in the synkaryon, or zygote. The other 6
micronuclei degenerate, and the synkaryon divides twice to produce 2 macronuclei and 2 micronuclei. At the first binary
fission each daughter cell will receive 1 of the macronuclei and by division of the micronuclei also 2 micronuclei. This
process is similar to conjugation but does not involve two individuals.
2. Characters of Cilita
The ciliates are an interesting group, with a great variety of forms living in all types of freshwater and marine water.
Ciliates are the most complex and diversely specialized of all the protozoans. Most of them are free living, but some are
commensal and a few are parasitic. Most of them are solitary and motile, but some are sessile and some form colonies.
There is great diversity of shape and size. In general they are larger than most other protozoans, but they range from very
small (10 to 12 μ) up to 3 mm. long. All have cilia that beat in a coordinated rhythmic manner, though the
arrangement of the cilia may vary.
Ciliates are always multinucleate, possessing at least one macronucleus and one micronucleus, but varying
from one to many of either type. The macronuclei are apparently responsible for metabolic and developmental functions
and for maintaining all the visible traits, such as the pellicular apparatus. Macronuclei are varied in shape among the
different species. The micronuclei control the sexual and reproductive processes and have a long-range control over the
macronuclei. They divide mitotically; the macronuclei divide amitotically.
Ciliates are covered by a pellicle, which may be very thin or, in some species, form a thickened armor. The
cilia are short and usually arranged in longitudinal or diagonal rows. Like flagella, they have two central and nine
peripheral fibrils．Cilia may cover the surface of the animal or may be restricted to the oral region or to certain bands. In
some forms the cilia are fused into a sheet called an undulating membrane, or into smaller membranelles, both used to
propel food into the cytopharynx (gullet). In other forms there may be fused cilia forming stiffened tufts called cirri,
often used in locomotion by the creeping ciliates.
The motor-coordinating system for ciliary movement apparently lies in the intraciliature just beneath the pellicle.
Each cilium terminates beneath the pellicle in a basal granule called a kinetosome. From each kinetosome a
kinetodesmal fibril arises and passes along beneath the row of cilia, joining with the other fibrils of that row. The
kinetosomes and fibrils (kinetodesmata) of that row make up what is known as a kinety. All ciliates seem to have kinety
systems, even those that lack cilia at some stage.
Chapter 2 Protozoa
Many ciliates have contractile fibrils called myonemes that run in rows parallel with the rows of kinetosomes and
permit extensive contraction and alteration in shape in the animal. In Stentor waves of contraction spread over the animal
in both an anterior and a posterior direction, but electrical stimulus will cause contraction simultaneously in all areas of
the body. The electron microscope reveals two systems of fibrils, the km fibers (composed of stacks of microtubules) and
the M bands (bundles of microfilaments) lying beneath the km fibers. High-speed cinematic analysis (Newman, 1972)
indicates that the M bands may be the ones responsible for contraction.
Fig. 2.14 Some representatives of Ciliata. (a) Paramecium (b)Spirostomum (c) Stentor (d)
Vorticella (E) Euplotes (f) Eudiplodinium
Most ciliates are holozoic. Most of them possess a cytostome (mouth) that in some forms is a simple opening and in
others is connected to a gullet or ciliated groove. The mouth in some is strengthened with stiff, rodlike trichites for
swallowing larger prey; in others ciliary water currents carry microscopic food particles toward the mouth, as in the
paramecia. Didinium has a proboscis for engulfing the paramecia it feeds upon．Suctorians paralyze their prey and then
suck out their contents through tubelike sucking tentacles. In any case the food is digested within food vacuoles.
Parasitic ciliates
Most parasitic ciliates are not very harmful. Balantidium coli, parasitic in man, is an exception．It is often found in
hogs, where it usually does no harm. Man becomes infected by water and food contaminated by cysts from the hog's
feces. The parasite enters the intestinal submucosa and causes ulcers and severe and even fatal dysentery. It is not as
common in America as it is in Europe, Asia, and Africa. Other similar species are found in cattle and horses-Epidinium in
cattle, for example. Some ciliate parasites such as Nyctotherus also occur in the colon of frogs and toads. Tadpoles are
infected when they eat the feces of frogs containing the cysts.
Chapter 2 Protozoa
Suctorians
Suctorians are ciliates in which the young possess cilia and are free swimming and the adults grow a stalk for
attachment, become sessile, and lose their cilia. They have no cytostome but have protoplasmic processes that serve as
tentacles. Some of these have rounded knobs for capturing their prey-usually ciliates; some are sharp for piercing and for
sucking up the protoplasm．
One of the best places to find freshwater suctorians is in the algae that grows on the carapace of turtles. Common
genera of suctorians found there are Anarma (without stalk or test) and Squalorophrya (with stalk and test)．Other
freshwater representatives are Podophrya and Dendrosoma. Acinetopsis and Ephelota are saltwater forms.
Suctorian parasites include the Trichophrya, which is parasitic on the gills of the small-mouthed black bass and may
cause serious damage to the fish; Allantosoma, which occurs in the intestine of certain mammals；and Sphaerophrya,
which is found in Stentor.
思考题
1. 原生动物门的主要特征是什么？如何理解它是动物界里最原始、最低等的一类动物？原生动物群体与多
细胞动物有何区别？
2. 原生动物门有哪几个重要纲？划分的主要根据是什么？
3. 掌握眼虫、变形虫和草履虫的主要形态结构与机能特点，并通过它们理解和掌握鞭毛纲、肉足纲和纤毛
纲的主要特征，并初步了解这些动物在科学或实践上的价值。
4. 掌握疟原虫的主要形态结构特点及其生活史、危害和防治原则，初步了解我国在抗疟方面的主要成就。
通过疟原虫掌握孢子纲的主要特征。
5. 掌握各亚纲的简要特点，并通过各纲或亚纲中的一些重要种类初步了解各类群动物与人生的关系。
6. 初步了解原生动物的系统发展。