Vertebrate vs. Invertebrate Eyes

I say "vertebrate eye" here deliberately. There's one group of non-vertebrates whose eye is nearly identical in gross appearance to that of vertebrates, and it's worth some discussion here as an aside. In the cephalopod molluscs (octopi and squids) the eye is superficially identical to the eye of vertebrates in terms of the arrangement of tunics, the cornea, and the lens. On close (i.e., microscopic) examination, one difference stands out: the cephalopod eye is not "reversed" the way the vertebrate eye is.

In vertebrates the eye forms by invagination of the optic vesicle, and the inner layer of the double walled optic cup becomes the retina. Therefore, the retina is "reversed" with the light-sensitive portion (the rods and cones) on the outside. This means that incoming light has to pass through all of the layers before it reaches the rods and cones. There appears to be no selective advantage or disadvantage to this reversed setup; it's just the way it turned out in vertebrates because of the other engineering considerations related to eye development.

In molluscs, however, the organogenesis of the eye is somewhat different. The result is that the light sensitive portion is on the inside surface, facing the incoming light. Light strikes the rods and cones before the other parts. The nerve fibers of the molluscan eye don't have to pass through the retina to enter the visual cortex of the brain, they are already on that side of it. By contrast, the neural elements of the vertebrate eye must pass through, and that's what makes the blind spot. Molluscan eyes have no blind spot.

The similarity of the vertebrate and non-vertebrate eyes is a classic example of convergent evolution. Both sets of animals, widely different in taxonomic and evolutionary terms, have independently arrived at essentially the same solution to the problem of how to convert electromagnetic radiation to neural impulses. Other than the reversal of the retina, the eyes of the two groups are so similar it's hard to tell them apart on casual examination. It's an old medical school trick to put an octopus eye on a pathology examination and ask, "What's wrong with this eye?" The correct answer, of course, is "Nothing."

The eye of arthropod invertebrates (insects, crustaceans, etc.) is vastly different from the molluscan or vertebrate eye. Its structure is nothing like these two. Nor does it form images in the same way.

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