A protozoan [prO'tO zO'un] is a one-celled animal that is a member of the subkingdom Protozoa. Protozoans are the simplest kind of animals, and are therefore the lowest group in the animal kingdom. All other animals consist of many cells, and belong to the subkingdoms Parazoa and Metazoa. The term protozoan comes from Greek, and means first animal.
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.
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
There are four basic divisions among the protozoans, with the basic difference between each being the ways they move about.
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.
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 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.
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The World Book Encyclopedia. Chicago: World
Book-Childcraft International, 1979.
SKC Films Library
Science.--Zoology.--Invertebrates.--Subkingdom Protozoa.
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