Turtles are born with a shell, although in the first days after hatching their shell is usually a lot softer because their bones are not calcified all the way yet. Once fully grown however, turtles and/or tortoises have hard, protective shells that are made up of 59 to 61 bones that are covered by plates called scutes.
Scutes are made of keratin, the primary substance that is found in hair, nails and hooves of other animals, but the turtle’s shell is part of its skeleton. Similar to bones, these plates grow along with them as they continue to age.
There is a great difference in the protection that a turtle’s shell provides depending on the breed. Some turtles such as the American and Asian box turtles are equipped with a hinge on the lower shell that allows them to go inside of their shells and then close up the front and rear openings.
The American mud and musk turtles also have hinges, but theirs do not completely close up. Aquatic turtles such as the cooters, sliders, map and painted turtles have large protective shells, but do not possess hinges. Their flesh is exposed, front and rear, and even when the turtles are completely tucked in.
Snapping turtles are not protected very well by their shells. The lower shell in particular is tiny and does not even begin to cover the animal. These creatures are shy and naturally not inclined to look for trouble, however if threatened they will put up a fierce fight and can cause some damage.
Softshell turtles have leathery shells with a type of thick skin rather than the hard scutes that other turtles have. They mostly stay hidden in water most of the time and use camouflage for protection against predators. They are also very quick to bite if necessary as a way to protect themselves.
A turtle’s outer shell is similar to that of a human’s fingernails, as it is made up of a thin layer of keratin. However contrary to what some believe, underneath the layer, there is a layer of bony plates (scutes) that give the shell its shape. The ribs and backbone (vertebrate) are a part of this shell too. So removing this shell would in turn remove part of the skeleton of the turtle. What this means is that a turtle cannot take off its shell.