Michael Means, age 11, of San Diego, California, for his question:

Does a turtle have a backbone?

In a way, you might say that the turtle has two backbones, one just about the strongest in the animal kingdom. True, that one is not a pliable sort of spine that he can bend this way and that, but it is durable and can withstand a lot of severe buffeting and banging.

The shell of a turtle is a house of protective armor plating. This kind of built in equipment has its advantages. Wherever he goes he has a weather proof shelter and a safe hiding place from his hungry enemies. But, like the snail, he is stuck for life with his one and only shell. He cannot shed it or build a better one. That sturdy home happens to be firmly attached to the bones of his body. Yes, he has a backbone and all the other bones needed to make a complete skeleton.

Inside his body, the turtle has a jointed endoskeleton that works with muscle power. The human body also is supported by a Jointed endoskeleton. But the turtle also has an exoskeleton on the butsida*of his body. It is, of course, his permanently attached shell. And the outside shell is welded by ties to certain bones of the endoskeleton inside. It is formed of two differently shaped parts and in the average turtle the two parts of the shell are welded together in a single unit.

The top of the shell that humps over his back is called the carapace. It is firmly attached to the backbone of the inside skeleton. The bottom section is a flattish platform called the plastron. As a rule, the plastron is firmly attached to the turtle's inside breastbone. The carapace and the plastron are sealed along the sides with suitable openings left at each end so that he can poke out his head, his four leathery legs and his tail. In some cases, the leathery skin around the openings has extra tough scales. When the head and legs are withdrawn inside, the exits are sealed.

The world's 220 species of turtles and tortoises are classified in the animal  Order Testudinata. All of them have spines and bony endoskeletons and most of them  have the added protection of armor plated exoskeletons. A few wear outer coatings  of tough, leathery material instead of horn shells. The tortoises, of course, prefer  life on the dry land. They have sturdy feet for walking and many of them are equipped  with businesslike claws. The turtles like water and their legs are flipper type paddles made for swimming. Some turtles are at home on both land and in the water. Others live all their lives in the oceans. The females of the giant sea going turtles lumber their huge, heavy bodies ashore only when time comes to lay their batches of eggs in the wet sand.

The Testudinata Order belongs in the great class Reptila. Like all reptiles, the turtles are cold blooded vertebrate animals. But unlike their snake and lizard cousins, their backbones and endoskeletons are re enforced by tough exoskeletons.

A turtle shell is built from two layers of over lapping horny plates. However, the plates of the outside layer do not match the pattern of those on the inside layer. This kind of welding job seals up the weak spots and gives added strength to the sturdy shell. The turtles have proved that their well armed, peaceable way of life is successful. Other creatures have come and gone. The turtle clan has changed very little during the past 175 million years.