What is this sac like thing growing on Emarald Crab? Is it for eggs?

[Gr8Mandingo]

I aplogize if this was posted to the wrong thread

So about a month ago i seen my two emaralds having sex. Since then one has died but today i noticed a large sac growing on the surviving crab...
On closer inspection the sac looks small and has an opening ...

 

[Maritimer]

That looks as though she may have extended her tail - which is where she'd carry eggs.

Crabs' tails are normally tucked tightly against their bellies, but females will open theirs to carry eggs in their swimmerets, just as shrimp, lobsters and other crustaceans do.

~Bruce

 

[ThRoewer]

If the "sack" is that thing on the side, then it may be a parasite or tumor.
Crab females carry their eggs under their abdomen which is folded under their thorax. They are hardly to confuse with anything else.