Identify story where prisoners are sent to a planet where tiny creatures attach to their bodies and cause them to grow organs and other "spare body parts" for transplanting.
I'm pretty sure this is the short story A Planet Named Shayol by Cordwainer Smith.
It begins with a new prisoner, Mercer, being prepared for the prison planet Shayol by having his skin toughened in a medical centre on a satellite. One of the doctors tells him what is in store for him.
"There is an attendant. He is a man, but not a human being. He is a homunculus fashioned out of cattle material. He is intelligent and very conscientious. You specimens are turned loose on the surface of Shayol. The dromozoa are a special life-form there. When they settle in your body, B'dikkat - that's the attendant - carves them out with an anaesthetic and sends them up here We freeze the tissue cultures, and they are compatible with almost any kind of oxygen-based life. Half the surgical repair you see in the whole universe comes out of buds that we ship from here."
When Mercer first looks out on the surface, he sees
...an enormous human foot, the height of a six-storey bulding.
"But It can't be a real foot," said Mercer.
"It is," said B'dikkat. "That's Go-Captain Alvarez, the man who found this planet. After six hundred years he's still in fine shape. Of course he's mostly dromozootic by now, but I think there is some human consciousness inside him."
As an aside, one of the anthologies containing this story (Spectrum 4), features it as cover art.