hermit crab: boy or girl?

Question
I had a hermit crab for a while but I can't tell if it is a boy or girl how can you tell???

Answer
On the underside of the crab, on the underside of the third set of walking legs there will be a small dot right in the middle of the segments under the body. The dots are called gonopores. If you find them, it is a girl, no dots and it's a boy. The best way to look for the gonopores is to wait until your crab is climbing the glass and use a flashlight to peek underneath.