Scientists are sure that the first sea turtle lived on the earth 150 million years ago. Nowadays there are 300 kinds of turtles in the world. The biggest one is the leatherback turtle. It is six or seven feet (about 2 meters) long, and its weight is about 600 kg. Turtles have hard shells made of 59 to 61 bones. These shells are part of their skeleton, so turtles can never leave them.