by 
Christine
ANSWERS: 1
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I'm not sure anybody really knows the story (we weren't around when it happened). At a guess (and it is a complete guess), given the shape of their shells, when they're moving underwater there would be more water resistance if they walked forward. (Notice how the left and right edges of their shells are pointed.) The ultimate answer to your question, though, is that at some point some not-quite-yet-a-crab that walked sideways ate more and reproduced more effectively than crabs that didn't. It's a bit circular, but the answer is basically -- because that's what works best for them. Neat question. +5
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