The shell of a turtle is made up of 60 different bones all connected together. Most land tortoises have high domed carapaces that offer protection from the snapping jaws of terrestrial predators. Aquatic turtles tend to have flatter more aerodynamically shaped shells. An exception to the dome-shaped tortoise shell is the pancake tortoise of East Africa that will wedge itself between narrow rocks when threatened and then inflates itself with air making extraction nearly impossible. Most turtle species have five toes on each limb with a few exceptions including the American Box Turtle of the carolina species that only has four toes and in some cases only three.
Turtles have good eyesight and an excellent sense of smell.
Hearing and sense of touch are both good and even the shell contains nerve endings. Some aquatic turtles can absorb oxygen through their skin on their neck and cloacal areas allowing them to remain submerged underwater for extended periods of time and enabling them to hibernate underwater. Turtles are one of the oldest and most primitive groups of reptiles and have outlived many other species. One can only wonder if their unique shell is responsible for their success. References Ackerman, L. The Biology, Husbandry and Healthcare of Reptiles Volume 2.