Volume of solid given base and cross sections - trouble understanding question

The following is an exercise from Rogawski's Calculus: Early Transcendentals, section 6.2:

*In Exercises 9-14, find the volume of the solid with given base and cross sections*.

9. The base is the unit circle $\displaystyle x^2 + y^2 = 1$ and the cross sections perpendicular to the $\displaystyle x$-axis are triangles whose height and base are equal.

I don't clearly understand the question. What exactly are the cross sections ("perpendicular" is ambiguous), and what do I need to integrate? (The answer key says $\displaystyle V =\frac{8}{3}$.)

--DragonLord