Trilobites could be said to be the first complex animals to evolve real eyes on our planet Earth.
Maybe.
We just don’t know.
Abruptly, in rocks that date back about 560,000,000 years, trilobite shells began to appear in ancient sea bottom muds and silts with no evidence as to how they had reached such an highly evolved state . By "highly evolved” I mean that trilobites appeared full-formed in the Cambrian fossil record wearing eyeglasses. Very cool eyeglasses too; wrap arounds, ghetto grannies, thick-lens nerdy types,...almost every species of trilobites had their own particular style of eyeglasses except the species of trilobites which were blind.

Calcite! The stuff of caves, great carved statues, ocean bottoms, the white chalk cliffs of Dover.
Calcite, a crystalized form of CaCO, calcium carbonate of which the more transparent form is Iceland Spar. The trilobites manufactured their light-collecting lenses from this inert compound to allow them to detect and react to the phenomena of light; a biological revolution.
In the book TRILOBITE: Eyewitness to Evolution Richard Fortey quotes Shakespeare’s The Tempest...

Full fathom five thy father lies;
Of his bones are coral made;
Those are pearls that were his eyes;
Nothing of him that doth fade,
But doth suffer a sea change
Into something rich and strange.

“If to voyage back to the time of the trilobite is a historical sea-change then there is nothing stranger than the calcareous eyes of the trilobite. And pearls are chemically the same as the trilobite unblinking lenses, being yet another manifestation of calcium carbonate, although peals are exquisite reflectors of light rather than transmitters of it. The weirdness of Shakespeare’s line results from his suggestions of pearly opacity, the hints of a corpse transformed; dead, yet seeing. The trilobite saw the submarine world with eyes tessellated into a mosaic of calcified lenses; unlike the dead seafarer, his stony eyes read the world through the medium of the living rock.”

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Pretty cool book.

horse shoe crabs, --wonderful other worldly creatures that they are, also have lens(and eyes) from from hard clear parts of of their shell..

they have no openings in the top part of their shell for the lenses, they 'light sensing' cells of their very primitive eyes (they have compound eyes) just nestle up against the clearish/translucent parts of the upper shell.

(you're inland, right? i don't know how far south horse shoe crabs are found--even if you were on the coast. they are still common in sheltered areas here in NY area. LI sound, well the bays and wet lands on Long island and CT, RI, and MA still harbor a large population.)

Wonderful, Of Troy, do you happen to know if the horseshoe crab's light-gathering lenses are crystilized in the same manner as the trilobites,i.e. with a tri-axial alignments which helps the sensor cells determine the direction of the incoming light? Interestingly, the horseshoe crab is considered a modern relative of trilobites. Not a direct descendent but maybe a second cousin back in the good old days of the Ordovician, back before the big kill off at the end of the Paleozoic. According to Richard Fortey, female trilobites probably had their egg sacs in their heads as do the horseshoe crabs of today. Richard reports that he was served Limulus eggs (the only eatable part of the horseshoe crab) in Thailand. He said that inside the head section of the crab were big yolky eggs. The horseshoe crab eggs tasted rancid and intense. He thought that his beloved trilobite's eggs would have tasted sweeter,