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How The Hen House Turns—The Eyes Have It

on March 22, 2016 - 5:56am
How The Hen House Turns
By Carolyn A. (Cary) Neeper, Ph. D.
The Eyes Have It

As a biology student, I was enthralled with the amazing diversity of tiny critters that swam energetically in a drop of water under the microscope. After studying embryology, I was even more amazed that any of us were ever born halfway normal. But comparative Anatomy in the Zoology Department at college topped them all. The variety of eyes that have evolved on this planet is amazing.

It's not just the eyes' variety of color and design, as pictured in a recent issue of National Geographic. It's their versatility in function. They are beautifully designed to provide a valuable service, no more and no less than what their host animal requires to make a living.

When we were in Australia, we were told to wear panty hose over our entire bodies if we went swimming with box jellyfish a certain number of meters from shore. Their stinging tentacles were deadly. We knew that these blobs of colorless jello had a simple ring of neurons for a brain, but we didn't know that they had two low-resolution eyes in each of their four dark brown rhopalia clusters. The clusters also had four light-sensitive areas—24 eyes in all. One eye points upward so the box jellyfish can find dark areas under mangrove trees that house the crustaceans it must find in order to eat.

Thus, this simple animal displays the wide variety of eyes in the animal world. The extremes are found in eagles, who need excellent focus from on high to catch supper, to earthworms, who wear simple light-sensitive patches on their slimy bodies. The moral: it's function, not locale or size that has driven eye design.

Wasps have very tiny eyes. Squid eyes are huge, much like ours, with lens and retina. Flies with compound eyes have both in each unit. Sea stars wear their eyes on the end of their arms. Eyes are arranged in rows on scallops. Other eyes can look in every direction at once, and cave fish don't need to see in their dark environment, so they are blind. It all depends on what the critter needs in order to survive.

Unfortunately, we humans need glasses as we get older. Or we need cataracts removed. We like to see clearly. A few lucky ones, like my dad, got along with grocery store glasses until his late 80s, and then his vision improved!