Lobsters have feathery appendages called swimmerets, or pleopods, underneath their tail. These swimmerets help a lobster swim and are also where a female lobster carries her eggs. Swimmerets also can clue you in to the sex of a lobster. The first pair of swimmerets (the pair closest to the head) are soft on a female, but hard and bony on a male.
Also, the female has a rectangular shield between her second pair of walking legs, which she uses to store sperm after mating with a male. This is where the male inserts those hard swimmerets during mating, releasing sperm that the female stores and later uses to fertilize her eggs.