Bonnie Lutton, age 11, of Muncie, Indiana, for her question:

What kind of creature is a protozoan?

Ordinary folk call him a one celled animal or a single celled animal. Biologists refer to the miraculous midget as a protozoan. Scientists need meaningful scientific terms to name their subjects s o that their colleagues who speak other languages know what they mean.

There was a time when biologists spelled this word "protozoon." The "zoo" syllable is borrowed from an older world meaning "life" or "living." We meet it in such words as "zoo," a place for the display of living animals, and in "zoology"    the branch of biology that copes with the study of living animals. The "proto" syllable of our scien¬tific term is coined from an older word meaning "first" or "original." The first or original life forms tended to be simpler than the more complex forms that developed from them and appeared later. In the sciences, the "proto" syllable may be stretched to mean the simplest form of something.

Protozoon is a rather clumsy word, and since scientists strive to grasp things at their simplest levels, they tend to shy away from clumsiness. In this, they are merely copying nature. In nature, the tiniest items have to work perfectly or perish. The rule is a neat and well balanced pattern with no excess parts to foul up the smooth operation. Designs of this sort are always beautiful to behold. So it was natural for scientists to change their clumsy word "protozoon" to the more musical "protozoan."

A protozoan can be any single celled animal who is a member of the vast animal Phylum Protozoa. The number of these miraculous midgets that share our world is beyond the imagination. Almost all of them are too small for our eyes to see. Most varieties enjoy aquatic lives in streams and ponds or in the sea. Most protozoa are harmless creatures, though some live as parasites in larger plants and animals. A few species infect plants, animals and even people with diseases.

Each of the countless species of protozoan is different, though all of them share basic family features. Each depends upon one and only one small cell to carry on all the processes of life. That one cell must be able to find food, devour and digest it. It must be able to absorb oxygen or some other fuel from its surroundings and use it to operate the chemical processes that keep the little cell alive. These chemical processes create waste materials and the cell must have its own garbage disposal system to eliminate them. Every protozoan is a miraculous unit of animal life in which all these wondrous systems and processes operate economically on miniature scale.

Life, of course, must multiply or it would soon be wiped out by hardships and unexpected events in the world of nature. Even the tiny protozoan must cope with the problem of multiplication. But the bitsy creature hands on life: in its own astonishing way. At the right time, the tiny living cell simply splits itself into a pair of twins. It produces no children and never ages. The humble protozoan has the secret of immortality    for barring accidents, he can go on living indefinitely.