So basically, if a population of specific organisms is set into an environment with a high content of metals, over time, some of those metals will be absorbed into the body via feeding, and the organism's cells will take advantage of the high metal content and incorporate some of those metals into their cytostructure. This will show through the manifestation of a keratin or bonelike material on the organism's body. The reason the whole body isn't covered in metal by this time is that the organism still needs plenty of bodily maneuverability, esp. on the joints and such, so the cells only incorporate the metals at vital areas.
It's just a rough draft, I'd love to hear any advice/constructive criticism you can give me.