Tom Bowen, age 13, of Marion, Ohio, for his question    

WHAT IS AN INVERTEBRATE?

An invertebrate is an animal that has no spine or backbone in his upper or dorsal side. The spine is also called the vertebral column and the bones of the spine are called vertebrae.

The scientific name for animals without backbones is invertebrata. This means "without vertebrae." Animals with backbones are called vertebrata.

Are there more animals with backbones or animals without them? Actually, the invertebrates, or the animals without backbones, outnumber the vertebrates by about 20 to one. There are more than 1 million known kinds of invertebrates and more than 50,000 species of vertebrates.

Invertebrates have been divided into several major groups called phyla. The singular form for these classifications is "phylum."

The phylum Protozoa is made up of the protozoans, which are microscopic single celled animals. Porifera contains all of the animals known as sponges while Coelenterta is the phylum that contains animals with hollow cavities, such as the hydra, jellyfish, sea anemone, coral and Portuguese man of war.

Scientists put worms into three separate phyla: Platyhelminthes, Aschelminthes and Annelida.

Platyhelminthes includes all of the worms with flattened bodies, such as the liver fluke, the tapeworm and the planarian. Aschelminthes is the phylum that includes such roundworms as the hookworm, vinegar worm and pinworm while Annelida includes all of the worms with bodies that are made of segments. In this last classification are the common earthworm and the leech.

Largest invertebrate animal phylum is Arthropoda. This classification includes animals with jointed legs and an outer skeleton of chitin, often like armor. This includes insects and crayfish.

Echinodermata is a group of invertebrates with external spines, such as the starfish, sea urchin, sand dollar, sea cucumber and sea lily.

The phylum Mollusca is made up of the mollusks, soft bodied animals that usually have limy shells. Belonging to this group are the snail, slug, clam, oyster, scallop, octopus and squid.

The phylum Chordata is made up of animals that have a rather rigid structure of bone cartilage in their backs and are considered invertebrates. The higher chordates have a spinal column divided into vertebrae. Such animals are vertebrates.

Scientists involved in biology trace the way similar structures and functions develop as they pass from animal to animal through the whole animal kingdom. They have observed 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.