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Most turtles are instantly recognizable by their protective shell. This bony structure is different than shells used by animals such as hermit crabs, who can leave their shells and switch to new ones; it's attached to the turtle's spine and lower ribcage as a permanent suit of armor.
The main benefit of wearing a shell is that is helps protect turtles from predators. Made from bone covered by hard plates called scutes, the shell makes it difficult for many predators, such as raccoons and otters, to get a bite of tasty turtle meat. Many land turtles can draw their legs, tail and head into their shells, leaving only a hard, oddly shaped shell in reach of predators.
Turtles sport different shell shapes depending on their preferred habitat and other defenses. Some large land turtles, called tortoises, move too slowly to avoid predators, so their only protection is their shells. These shells tend to have a domed top section, called the carapace. The rounded shape makes it harder for predators to get a good grip or mouth hold, allowing the turtles to hide in their shells and wait for the predator to give up. Sea turtles, on the other hand, have shallower, more streamlined shells to help them swim faster through the water.
While some turtles, such as the snapping turtle, have other defenses, such as a strong, knobbed tail and powerful beak, others rely almost completely on their shells for defense. Land turtles such as mud turtles have hinged shells. They can draw their extremities completely inside their shells and close the shells, bringing the bottom of the shells up to touch the top for a solid sheet of protective covering.
The weight of the shells varies based on what kind of protection the turtles need. Sea turtles tend to have lightweight shells that allow them to swim quickly away from predators; the shells are there as protection when necessary, but they aren't heavy enough to bog down the turtles. Land turtles tend to have heavier shells, which weigh them down and cause them to walk extremely slowly. Although the shells restrict their movements, they offer stronger protection than the shells of sea turtles.
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