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Protozoans in General

Protozoa are single-celled eukaryotes (organisms whose cells have nuclei) that commonly show characteristics usually associated with animals, most notably mobility and an inability to manufacture their own food (like plants do). The word protozoan comes from Greek, and means first animal.

Some classification schemes place protozoans in the kingdom Protista together with the plant-like algae and fungus-like water molds and slime molds, but most modern schemes now place them in their own separate kingdom. Although this website places protozoans in their own kingdom, we have also placed them with other animals because the vast majority of them possess very animal-like characteristics.

Structure

The amoeba is the one of the simplest protozoans. The single cell that makes up its body carries on all the necessary life processes by itself. The cell eats, breathes, and responds to its surroundings. There are no special organs for any of the amoeba's activities. To move, the amoeba thrusts a part of the cell in one direction and draws the rest of the cell after it. Other protozoans are more complicated in structure. Some of them, called ciliates, have tiny hairlike projections that help them move about. The paramecium has a definite groove on one side that serves as a mouth. Some protozoans have a bright red spot called the eyespot which may be sensitive to light. There are even some protozoans that contain chlorophyll, the green substance found in plants that enables them to make their own food.

structure of a typical Paramecium
structure of a typical Paramecium

Reproduction

Some protozoans reproduce by splitting in two (fissioning), with each half of the original cell becoming a separate animal. In other protozoans, the parent cell suddenly swells in one direction. The swollen part breaks off and forms a new parent (budding). A third form of reproduction occurs among certain parasitic protozoa, such as the tiny animal that causes malari. These protozoans reproduce by dividing into many cells called spores. In all these forms of reproduction, the cell's nucleus is divided among the "children."

Kinds of Protozoans

Flagellates have one or more long hairlike projections from their bodies called flagella which whip about rapidly to move the animals through the water. Flagellates are usually oval in shape, and many have chlorophyll in their bodies. The green euglena is shaped much like a submarine and is common in fresh water. The volvox is a green ball of flagellated cells that live together. This ball moves about when the flagella of the invidivual members are whipped in the water. The trypanosomes, which cause African sleeping sickness, are also flagellates.

Giardia lamblia, a typical flagellate
Giardia lamblia, a typical flagellate

The Ciliates are the most complex protozoans. All of them have, at one time or another, fine hairlike projections known as cilia. These cilia help the ciliates move about to capture food. The stentors, which are shaped something like a horn or trumpet, rank among the largest of all the protozoans. Another kind of ciliate, the vorticella, looks something like a funnel with a long tube. The vorticella creates a little whirlpool around the top of the funnel to draw in its food.

Stentor roeseli, a typical ciliate
Stentor roeseli, a typical ciliate

The Sarcodina group includes various amoeba and similar animals. Many of these protozoans live in the bodies of human beings and other animals. Some cause disease and others do not. Radiolarians have a tiny skeleton that is made of silica. Afater a radiolarian dies, this skeleton sinks to the ocean floor. The foraminifers have shells made of chalklike material. Some of the ancient foraminifers were almost as large as a quarter. Geologists seeking oil study foraminifer fossil shells in rocks found below the suface. All members of this group move by thrusting out a part of the cell and then drawing the rest of the cell after it.

Sporozoa is a special group of protozoans that reproduce by means of spores. All live as parasites, and many are capable of causing serious diseases in humans. The malarial parasite is the best known of the Sporozoa.

the organism that causes malaria is a sporozoan
the organism that causes malaria is a sporozoan

SOURCE
The World Book Encyclopedia Chicago: World Book-Childcraft International, 1979

SEE ALSO
Amoeba

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This page was last updated on 06/06/2017.