In evolutionary carcinization, multiple groups within decapod crustaceans (shrimps, lobsters and crabs themselves) developed a crablike form with a flat and rounded carapace, and an abdomen folded ...

In this crab’s case, those appendages are its 10 ... into the female as their abdomens press together. The female’s abdomen, also called the apron, is where she carries the fertilized eggs.