as to eyes:
planaria have no lens, eyeball, humours, corneas, etc...
they have specialized nerves running from the ganglia which can be (loosely) considered its brain, terminating on the surface in clusters of photoreceptive cells. These are the most primitive eyes of which I am aware. They are called "eyespots". They are, as you say, useful only as light detectors. However, even such limited sensors can differentiate intensity and, due to the fact that they are located in bilateral symmetry across the planarian's long axis and are found on one end of the animal, can allow the planarian to determine that light's directionality.
Limited, then, but advantageous nonetheless.
IIRC, one of the links on the List O'Links has detailed examples of intermediate eye developments, from the simple eyespot to the advanced structure with two of which the human normally comes equipped.
Except Moms of course who have that extra set on the back of their heads.