Animals with bad design.

But it wouldn't need to do this, since the anatomy involved is not fundamentally different from our hands and feet. Both our nails and cats claws attach to the distal phalanges, the flexing of fingers and claws are both carried out with muscles further up the hand pulling on tendons, and these tendons both have sheaths (just like all tendons in the body.
The main difference I can see is that the tendons in cats attach further up the distal phalanges, allowing them to be pulled back further. There is also some modification of the intermediate phalanges to accommodate this. Here's a nice pic of the structure, can you see any fundamental differences: As for nails themselves they continuously grow because, under normal circumstances they are susceptible to wear and tear. However as the retractable claw developed, the claws themselves became protected from being worn down, allowing them to maintain their sharpness for catching prey. Even the Cheetah, with partially retractable claws, uses them to catch prey.Edited by Malcolm, : No reason given.