Cylindrical Shells (Little on Disks too)

I'm having trouble figuring out what exactly the shell radius and shell height is. The problem is as follows:

The region between y=x^3 and y=x is rotated around the x-axis. Find the volume of the generated solid using a. Disks and b. Cylindrical Shells

First of all, using disks I've been getting (8*pi)/21 which is double the real answer (4*pi)/21. I think it's because my limits were from -1 to 1 and I think they were supposed to be 0 to 1 although I'm not sure why.

And as for b, well, like I said above, the shell radius and height confuse me.