Ricky Richards, age 12, of Atlanta, Georgia,for his question:

Are protozoa really animals?

No doubt you have played that old guessing game called Animal, Mineral or Vegetable. It goes back to a time when everybody supposed that all the substances in the world must be either living or non¬living. And that all the living things must be either members of the animal kingdom or plants of the vegetable kingdom. Scientists still agree that substances are either living or non living. But they now suspect that certain living things are neither plants nor animals.

The term "protozoa" was coined from Greek words meaning "first¬animals." At the time, this seemed logical. For protozoa are single living cells and the bodies of larger animals are merely complex arrangements of various single cells. Hence, it seemed logical to assume that simple protozoa type cells gave rise to the more elaborate forms of animal life that came later.

True, there is a lot of evidence to suggest that modern animals did indeed arise from simplified ancestors. But the evidence that the first animal cells were protozoa seems to be dwindling. In fact, many scientists now claim that the protozoa are really neither plants nor animals.

This brings us to what we mean by life and to the basic differences between plant life and animal life. Unlike non living chemicals, all living things absorb food energy from their surroundings    and use it to grow and multiply. Most plants contain chlorophylls to use the energy of sunlight for manufacturing their basic foods from air, water and simple chemicals. Most plants are rooted to the spot and none can move around unless shifted by such things as wind and water.

The members of the animal kingdom have no chlorophyll for using sunlight to manufacture their foods from simple raw materials in the environment. They get their energy by consuming prefabricated foods from the plant world. For example, we consume plants and meat, which comes from plant eating animals. Unlike the plants, animals can move themselves around from place to place.

Now where does all this leave the tiny single celled protozoa? The answer is, somewhere in the middle. We know of some 20,000 different protozoa, all of them too small to be seen by the human eye. In their microscopic world, they are as different from each other as robins and redwoods, rats and roses, and some favor features of the animal kingdom, others favor features of the plant kingdom.

Some scientists still classify the protozoa as a group of simplified, single celled animals. But more and more tend to class them in a third kingdom called Protista, along with a vast assortment of other small plant type and animal type organisms.

So, if we wish to be stylish in scientific matters, we must give up the old notion that every living thing must be either an animal or a vegetable. It can be a one celled organism called a protist    which is neither plant nor animal, but resembles both. In the modern world of science, a protozoan is a protist and so is a bacterium.