Sandra Krause, age 11, of Omaha, Neb., for her question:
How do one‑celled animals reproduce?
There are countless varieties of these one‑celled creatures sharing our world. They are as different from one another as are whales and mice, birds and tigers. The largest of these mites is just big enough for our eyes to see as a pale, hazy dot. Most of them measure several thousands to the inch.
Together, the one‑celled creatures make up the huge Protozoa group of animals. Protozoa means the first animals and protozoa are classed in phylum I of the animal kingdom. There are so many of them and they come in so many varieties that some zoologists suggest giving the little follows a whole animal kingdom to themselves. These experts would class all one‑celled animals in phylum I and all the other animals in various subdivisions of phylum II.
The tiny mites are everywhere. They live in salt water, fresh water, and in the soil. Some live in the bodies of larger animals. Each variety of protozoon has its own method of life, of eating and of moving around. For all of them must eat and breathe, most of them move about and all of them reproduce ‑ or hand on life.
There are, however, no parents in the world of protozoa, no grandparents and no old folks. For a protozoon multiplies by dividing. The amoeba is a one‑celled blob of jelly. He is neither male nor female. When he is feeling fat and well fed, he simply divides into two halves. Each half goes on its way to become a full‑grown amoeba.
The volvox is a colony of one‑celled animals. They huddle together in a hollow ball. Some of the individual cells decide to divide, subdivide and maybe subdivide again. This core of divided cells settles in the center of the volvox sphere. When the sphere opens it is ready to form a new colony.
Malaria is caused by a parasite protozoon. It lives part of its life in a mosquito and part in some warm‑blooded animal. For a time it multiplies by dividing. Eventually, two types of cells develop, male and female. These parent cells become spores, or seedlets. But this type of reproduction is not usual in the world of protozoa.
The body of a protozoon is a blob of jelly-like protoplasm. Somewhere in this belly is a more solid spot known as the nucleus. This nucleus seems to control the life forces of the cell. When it is removed, the little cell can keep growing and even take food. But it cannot digest and it perishes when its stored food is used up.
The nucleus is the vital factor in the reproduction of the protozoon. For it is the nucleus which first divides into two identical parts. Some protozoa have a large nucleus and a smaller one. In this case, both divide. The now nuclei then move to opposite ends of the cell. The blob of jelly develops a waist which gets slimmer and finally breaks. The original cell has become identical twins.
No protozoon gets old and dies. When times are good it simply becomes two or more protozoa. In a way we might say that these humble mites have partially discovered the secret of immortality.