Answer
As any object increases in size, the surface area grows at a slower rate than the mass does.
A simple, straight intestinal tract that works for a worm will not have enough surface area if scaled up to human size. Therefore, humans have a relatively long and "complex and extensively folded intestinal tract" so that the area will be large enough to digest food that passes through.
Work Step by Step
This is discussed on pages 236-237.
