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It produces chemicals that weaken its exoskeleton. It then takes in water so that it begins to swell. At the same time, it starts growing a new, flexible exoskeleton under the old, weakened one. When the old exoskeleton gets weak enough, the arthropod breaks out of it. Then, the new exoskeleton hardens, and the arthropod gets rid of all the water that it took in.
What is How a crustacean molts