CEPHALOPOD SEEING: Octopuses and squids independently evolved a camera-style eye that superficially resembles the vertebrate eye, but the retina in these cephalopods lacks defects found in the vertebrate retina... Beckmannjan/Wikimedia Commons
PROTO-PEEPERS: The hagfish, which lives on the ocean floor, has rudimentary eyes buried under a patch of translucent skin that seem to function not as visual organs but as modulators of circadian rhythms... Linda Snook, National Oceanic and Atmospheric Administration (NOAA) / Cordell Bank National Marine Sanctuary (CBNMS)/Wikimedia Commons
FELINE FOCUS: Vertebrate animals, including cats (and humans), have eyes that act like cameras. The eye collects and focuses light, converting it into an electrical signal that the brain translates into images... Kate Wong
FLY'S EYE: Compound eyes are very effective for small animals, such as this fly, because they offer a wide-angle view and moderate spatial resolution in a small volume. Nikhil Verma/Flickr
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EYES OF OLD: Trilobites and other ancient members of the arthropod group of invertebrates had compound eyes. In this type of eye an array of identical imaging units, each of which constitutes a lens, beams light to a handful of light-sensitive elements called photoreceptors... Moussa Direct, Ltd./Wikimedia Commons
![Peeper Show: Evolution's Eyes [Slide Show]](https://static.scientificamerican.com/sciam/cache/file/3E27F119-6364-4D72-9B0EC7892438876C.jpg?w=590&h=393)
