Leslie Johnson, age 9, of Salina, Kansas, for the question:

How many different animals are on this earth?

We share our beauteous world with more than a million different animals. We know that there are at least this many, because scientists have studied them and named them. But there are others waiting to be introduced. Every year, a few more are discovered and the list of known animals grows longer. Most of these strangers are insects and other small creatures that nobody had noticed. Once in a while zoologists discover a larger animal, living secretly in some forgotten corner of the world.

It would take years to count the earth's one million different animals. And who could remember all their names? Scientists know how to cope with big problems of this kind and many years ago they worked out a plan to class all the known animals in groups. Naturally, each different animal is given his own special name. But experts who study them also want to know what each one is like, how each one lives and where it lives. And they are very much interested in animals that are somewhat alike    and that may be related to each other.

The animals are named and classed by zoology scientists. They use a neat system that tells other experts quite a lot about each animal. For example, they class more than 50,000 animals as vertebrates. Every vertebrate animal has a backbone, plus other bones inside his body. Cats and dogs, pigs and porcupines, frogs and fishes, birds and scaly reptiles are vertebrates. These animals with bony skeletons are very well made    they can get around and cope with life. But fishes are not at all like lions and experts need more clues to sort out the different vertebrates.

So the zoologists divide them into different classes. The cats and about 5,000 other furry animals are in the class of mammals. There are about 9,000 members of the feathery bird class and about 6,000 in the scaly reptile class. The frogs and 3,000 of their relatives belong in the class of amphibians. Most of the vertebrates, strange to say, live in the water. They are the 30,000 members of the fish class.

We tend to think that most of the world's different animals have backbones. Perhaps we notice them more than animals that have no bones inside their bodies. These are called the invertebrates    and they far outnumber the vertebrates. Most of them are small, or fairly small, creatures. Those that are somewhat alike are placed in huge groups called phyla. The boneless earthworm and his relatives belong in one phylum, the snails and clams and their relatives belong in another phylum. There is a phylum for the jellyfishes and their kinfolk and another for the starfish types. The biggest phylum of all belongs to the lobsters and centipedes, the spiders and the teeming insects.

All the animals in this biggest phylum have tough jackets instead of bones. Their stiff coats have jointed bands to move their legs and feet. Zoologists call them the arthropods, which means the ones with jointed feet. The different arthropod types are separated into classes. The biggest class belongs to the insects. We know that there are at least 800,000 of them. This means that there are far, far more different insects than all the other kinds of animals in the whole world.