Marvin Holt, age 11, of Charlotte, Mich., for his question:

ARE THERE MORE INVERTEBRATE ANIMALS THAN VERTEBRATES?

As you study animals from the lowest one celled creatures to highest form, you will see that similar structures and activities are present but become more complex as you move from the bottom of the scale to the top. This relationship, scientists tell us, suggests that the higher types have slowly and gradually developed from the lower ones.

An invertebrate is an animal that does not have a spine or backbone in the upper part of his body. The spine is called the vertebral column and bones of the spine are called vertebrae. Invertebrata, meaning without vertebrae, is the scientific name for animals without backbones, while vertebrata or vertebrates is what they call those who have them.

There are more than 1 million known kinds of invertebrates on the earth today but only 50,000 different species of vertebrates.

There are so many different types of invertebrates, and they take so many different forms, that scientists have divided them into a number of major groups. These groups are called phyla. One single group is called a phylum.

At the bottom of the scale is the phylum Protozoa which is made up of microscopic, single celled animals. Next comes Porifera which is made up of spore bearing animals known as sponges and this is followed by the phylum Coelenterata which includes animals with hollow cavities such as coral, jellyfish and sea anemone.

Many additional invertebrate phyla follow, each moving up the scale to a more complicated animal. There are a number of groups which include different types of worms.

Echinodermata phylum is made up of a group with external spines. Included here are the starfish, sea urchin and the sand dollar.

Largest phylum is Antropoda. In this group are all of the animals that have jointed legs and an outer skeleton of chitin, often like armor. Included are crayfish and most insects.

The phylum Chordata is made up of animals that have a rather rigid structure of bone or cartilage in their backs. The lower chordates have a one piece rod of cartilage in their backs and are considered to be invertebrates. The higher chordates have a spinal column divided into vertebrae and these animals are vertebrates.

An interesting study in biology traces the way similar structures and functions develop as we pass from one animal to another through the whole animal kingdom. One can observe that all living things are constantly changing from time to time, or producing slightly different types. The theory of progressive development, or evolution, is based on these observations.