Livebearers

In livebearing species, the anal (belly) fin of the male os modified into a mating organ (called a gonopodium in the Guppy and fellow members of the family Poeciliidae). During mating, sperm packets are introduced into the body of the female, where they fertilize the eggs. These then remain in their egg sacs  until they hatch a few weeks later and can be released by the female as fully formed fry. Poeciliid females also possess the remarkable ability of being able to store unused sperm to fertilize subsequent batches of eggs.