The  simplest  eyes  (such  as those of
   limpets  are  patches  of  lightsensitive photoreceptor cells that can
   distinguish light from dark.

   Over time, the layer of cells fold into a cup that allows for sense of
   direction,  a  "pinhole" and later a lens and cornea allow light to be
   focused.

   Complex  eyes  of squids and octopuses and of vertebrates evolved from
   an ancestral simple cluster of photoreceptor cells.