Question about parole in the US

I just finished watching an HBO documentary called Nature of the Crime and it had a rather surprising/puzzling factoid at the end. The film is about parole and the process that a parole board goes through to decide if someone who is eligible for parole should be released.
The factoid that puzzles me is this: "34 states have parole systems, each with their own procedures". The thing I don't understand is what happens in the other 16 states? (I'm not even going to get into DC, Puerto Rico, Guam, and all of those places.) Do the other 16 states not allow parole at all?? Or do they have some other process to decide if someone has served enough time to be considered for release?
I'm having trouble believing that a state would have no process for letting someone deemed no longer a major risk to society being released. I don't know your Constitution well enough to cite a section that guarantees all state (and federal) prisoners some kind of parole (or parole-like) process for prisoners deemed deserving.
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Rhino