[Octopoda ]
The octopus is an extraordinary animal in many ways, but the eyes of this creature are especially incredible. When you consider that they, and their relatives the squid and cuttlefish (collectively known as Cephalopods), are related to mollusks such as clams and snails, this structure is even more amazing.
Looking at the eyes in the photo above you can see they are very similar to mammalian eyes. Although in an octopus the slit-like pupils are horizontal rather than round (like ours) or vertical (like cats). And the way these eyes work, and indeed the parts that make up the eye, are also similar to ours. This is a wonderful example of convergent evolution.
Convergent evolution occurs when two unrelated groups of animals develop similar structures independent of each other. One example is the wings of bats and those of birds. But the similarities between the eyes of mammals and cephalopods are so close it would be like if bats had independently developed feathers and hollow bones. You may wonder how we know the octopus and mammalian eye is truly convergent evolution and not just common traits inherited from common ancestors. This is because the vertebrate/invertebrate lines diverged around 500 million years ago, before either branch had eyes at all.
Photo Via Wikipedia (http://en.wikipedia.org/wiki/File:Evolution_eye.svg)
