ClipArt ETC
"Diagrams showing some of the stages in the increasing complexity of the simple eye in Invertebrates. A, simple pigment spot in epithelium having nerve-endings associated with pigment cells (as in some medusae); B, pigment cells in a pit-like depression (as in Patella); C, with pin-hole opening and vitreous humor in cavity (as in Trochus); D, completely closed pit, with lens and cornea (as in Triton and many other Mollusks); E, pigment area elevated instead of depressed, lens of thickened cuticula (as in the Medusa, Lizzia); F, retinal cells more highly magnified. ep., epidermis; f, nerve fibre; l, lens; op, optic nerve; p, pigment cells; r, retina; v.h., vitreous humor." -Galloway, 1915

Invertebrate Simple Eye

"Diagrams showing some of the stages in the increasing complexity of the simple eye in Invertebrates.…