The Hitchhiker's Guide to the Galaxy has this to say about Zaphod Beeblebrox's extra arm:
Unlike most extra limbs which have come into fashion recently, it was not grown in a laboratory, but on a person. Simono Raxaforian had had nothing to do with his nineteen arms, until the ex-president of the galaxy offered him quite a considerable amount of Altarian Dollars for one of them.
Of course, once Zaphod Beeblebrox had demonstrated how painfully simple the whole operation was, all of the richest people in the galaxy began queuing up for one of these so-called 'natural arms'. Simono Raxaforian has subsequently become an even richer man than the people who paid him the money in the first place, but, unfortunately, he is now mostly armless.