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Arthropods grow in a very different way than you and I do. Blue crabs, like all arthropods, shed their exoskeletons to grow. For crabs, that means shedding their shells and growing a new one, a process called molting. Watch a blue crab molting: http://shapeoflife.org/video/behavior/arthropods-blue-crab-molting
Watch some other arthropods molting:
http://www.youtube.com/watch?v=2CONWvddogc
