Structures in organisms (when no longer adequate for their original purpose) often become secondarily adapted for a different use.

Structures in organisms (when no longer adequate for their original purpose) often become secondarily adapted for a different use. In the course of their evolutionary history, vertebrates have utilized three different "urinary" systems. Today, however, we only use one. What has happened to the other two? Why were they no longer adequate for mammalian needs? What do you think would happen to such vestigial structures if a secondary "use" did not arise? Why?