The reproductive (sexual) organs of hermit crabs are located near and just below the animal's heart and open to the outside at the base of the last pair of walking legs in the male. In the female, they are located at the base of the middle pair of walking legs. Female hermit crabs usually lay their eggs shortly after copulating (mating) but they can also store sperm (male sexual cells) for many months. The eggs are fertilised as they are laid by passing through the chamber holding the sperm. The eggs are carried and hatched in a mass attached to the abdomen inside the shell. The number of eggs is usually large, but depends on the animal's size.