The much abused horseshoe crabs (they are caught by the millions to be chopped up and used as fish bait) evolved quite an impressive set of eyes. They have them both on the top and on the sides of their carapaces as well on the bottom of their bodies.
While trying to photograph mating horseshoe crabs in Florida last week, I noticed how aware they were of approaching humans. Then I noticed what seemed to be eyes on their sides. This one was on the right side of a crab.
As you can tell, the lateral eye is a compound eye similar to the eyes of insects. The individual units, ommatidia, are visible. I don't know if these are the eyes the horseshoe crabs use to detect potential predators or if their other eyes also contribute to the task.
After a horseshoe crab dies, its hard carapace survives quite a long time; they are common objects on Florida beaches. The interesting thing is that the lateral eye is also immune to decay. Instead of turning into a depression or a hole on the side of the carapace, it retains even its compound structure. Here is the left lateral eye (~11 mm long) on the carapace of a long-dead horseshoe crab.
I don't know the function of the long, reddish-brown brow above the lateral eye.