Johnny Bridge, age 12, of Latta, S.C., for his question:

HOW MANY KINDS OF TURTLES ARE THERE?

All turtles lay their eggs on land, including freshwater and sea species. Usually the mother digs a hole in the ground with her back feet when she's ready to lay her eggs and then covers them with soil, sand or rotting plant matter. The number of eggs varies with the type: an African pancake tortoise lays only one egg, while a sea turtle may lay as many as 200 at a time.

Mother turtle leaves her eggs covered on the ground and never returns to them. Babies are hatched out and must dig their way to the surface of the ground, find their own food and immediately start protecting themselves.

Not too many turtles reach adulthood because many animals prey on turtle eggs and babies. Large beach birds eat sea turtle eggs or snatch the babies as they head for water. Fish also attack the youngsters as they reach the sea. Snakes, raccoons and skunks also are famous for their ability to make fast work with a batch of freshwater turtle eggs.

But if the turtle makes it through his first months, chances are good he will lead a long, pleasant life. Scientists say turtles live longer than any other backboned animal. Many live to be more than 100 years old.

There are 240 different species of turtles on earth, with about 50 of them found in North America north of Mexico.

Turtles also vary greatly in size. The leatherback turtle can be eight feet long, while the common mud turtle will be only three inches in length.

Fastest of all turtles is the sea variety. The green turtle can hit speeds of 20 miles per hour. And not all of the land turtles are slow and plodding. The softshell turtle, a freshwater variety who often goes ashore, can often outrun a man on level ground.

While most turtles spend their entire lives within a few miles of where they were hatched, the sea turtles migrate thousands of miles from their places of birth.

Most turtles have the ability to pull their legs, tails and heads into their shells for protection. The shells have two layers: an inner layer made of bony plates that is actually part of the skeleton and the outer shell that covers the turtle. The section that covers the turtle's back is called the carapace, while the shell covering the belly is called the plastron. These two sections are joined on each side of the body by a bony structure called the bridge.

There are seven main groupings of turtles: (1) mud and musk turtles, (2) pond and marsh turtles, (3) sea turtles, (4) side necked turtles, (5) snapping turtles, (6) soft shelled turtles and (7) tortoises.

Tortoises are a family of about 40 species of land turtles that live in North and South America, Africa, Asia and Europe. Three types can be found in the United States: the desert tortoise in the Southwest, the gopher tortoise in the Southeast and the Texas tortoise in the scrub forests of southern Texas.