![Steps in the evolution of the eye as reflected in the range of eye complexity in living mollusk …[Credits : Encyclopædia Britannica, Inc.]](http://media-2.web.britannica.com/eb-media/43/79543-004-C3F00EE8.jpg)
Steps in the evolution of the eye as reflected in the range of eye complexity in living mollusk species (left to right): a pigment spot, as in the limpet Patella; a pigment cup, as in the slit shell mollusk Pleurotomaria; the "pinhole-lens" eye of Nautilus; a primitive lensed eye, as in the marine snail Murex; and the complex eye—with iris, crystalline lens, and retina—of octopuses and squids.
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