I'm not aware of any example in which chemical pollution has directly caused extra limbs in any species. Chemicals kill tissue, not make extra limbs grow. Nuclear radiation is a little different but all things considered, I don't think nuclear pollution is a serious problem in china -- the vast majority of their power comes from coal (80%) and hydro dams (15%). Aside from Chernobyl I can't think of any cases where nuclear pollution might have caused genetic mutations in wildlife -- most of which should have been cancerous. I guess the a-bombs in WWII are examples of that too... Anyway, I digress. Pollutants caused from coal and the burning of other fossil fuels are a good example of things that kill tissue.



Parasites can cause extra limbs to grow where there should only be one but not in animals where there aren't supposed to be any any limbs to start with (I've seen LOTS of frogs with extra limbs or feet caused by parasites burrowing into developing limb buds and causing the limb to sprout in both directions around the parasite).



Ignoring the fact that this is obviously the leg of a prey item sticking out the side of this snake, the only way I can think of a leg forming on a snake is if the genetic information that codes for legs in snakes was not removed but simply deactivated (a mutation of the start codon could do this). Without a start codon the leg genes would become non-coding "junk" DNA and would be subject to random mutation without natural selection pressures. If the start codon is re-mutated then you could get a funny looking limb like that one BUT you would expect it to be at the vent where their legs used to be. I think this is how the dolphins mentioned in this thread can be explained.

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