Answer:
Only two backbones and thousands of ribs.
I am whole but incomplete. I have no eyes, yet I see. You can see, and see right through me. My largest part is one fourth of what I once was.
Though blind as well, can lead the blind well.
Often held but never touched. Always wet but never rusts. Often bits but seldom bit. To use it well you must have wit.
You can spin, wheel and twist, but this thing can turn without moving. What is it?
Iām grown from darkness but shine with a pale light. Very round I am and always a lady's delight.