This lecture is a video rich examination of the various eyes found in the animal kingdom, and what they can teach us about the human eye. If you have a hard time viewing on this page, try refreshing your browser … or you can watch the video directly at Vimeo
ComparativeAnatomy.m4v (176 mb) Screen Shots from this Lecture
Earthworms have simple eye spots that can detect light. This keeps them from surfacing into daylight where they might be dry out or be eaten by predators.
The light-sensing cells on the earthworm can’t be seen with the naked eye. Even though the worm can’t truly “see” … these spots act as detectors and keep the worm out of trouble.
A simple experiment is performed. Redworms are placed into a pizza box that has been lined with damp paper towels. Half the box is put in darkness. After 15 minutes, most of the worms have found their way to the dark half of the box.
The euglena is a single-celled organism. It is a “protist” … which means it has attributes of an animal (eats food) and a plant (photosynthesis of light).
The simple light-spot of the euglena can detect light. There is a protective shade so that light can only reach the spot when it shines from the front. Once the light spot is activated, this activates the flagella … pulling the organism toward light (usually the surface of ponds).
This is an experiment to demonstrate the euglena’s ability to chase light. A culture of euglena is poured into a petri dish and covered so that light enters through a small hole. After thirty minutes, the cover is removed and the euglena is found to have accumulated in the center of the petri dish.
Here you can see that the euglena has accumulated in the center of the dish.
The planaria is a flatworm with two eye “cups” that help it avoid light. These sensors form a simple retina, but do not give good vision.
Here is a closeup of the planaria eyes. The simple bowl configuration allows them to detect the direction of light.
Planaria have been placed in this petri-dish and a flashlight is used. The planaria run away from the light and congregate on the other side of the dish.
The nautilus has a simple pinhole eye.
One of the problems with the pinhole eye is that limited light can get into the eye. This is a problem for animals that live in dim environments (like the nautilus).
Here is a closeup of the nautilus eye. It is simply a bowl of light with a hole in the front to focus images. This is the largest pinhole eye in the animal kingdom that we know of.
The sea scallop focuses light using a mirror, instead of a lens.
The scallop has a simple visual system, involving multiple eyes running along its shell edge.
You can see these eyes, as they look like little blue dots.
A closeup of the scallop eye shows that it looks like a blue sphere with a round hole in the front.
During the day, my dog’s eyes look normal. But at night, his eyes glow from the retinal reflection.
The tapetum lucidum is a mirror that sits behind the retina of dogs and cats.
The mirror bounces back, so that light passes through the retina twice. This improves night vision, but may decrease resolution acuity by scattering light.
Lobsters have unique eyes. Instead of lenses to focus on their retina, lobster and shrimp focus light using mirrors.
Here you can see that the surface of the eye actually looks like a grill of mirrors.
This reflects light onto the surface of the retina.
Eagles and other birds have striated muscle in their iris. This allows them to voluntarily control their pupil size. This gives them good aperture control to better focus. However, because of this, dilating drops aren’t as effective.
Flomax (tamsulosin) affects the smooth muscle in the urinary tract to help with difficulty peeing. Since flomax targets smooth muscle, it can have a negative effect in the eye, relaxing the iris muscle and causing difficulties during cataract surgery.
Floppy iris can occur during cataract surgery, often as a result of tamsulosin (flomax) use. After the initial keratome incision, the iris can prolapse out of the incision, making cataract surgery more challenging.
Here is a drawing of a fly with the classic compound eye.
The compound eye of an insect involves many small lenses that focus on many small retinas.
The downside of the compound eye is poor resolution. To gain the same acuity of the human eye, a compound eye would have to be very large.
Some animals, like this alligator, have slit-pupils. This configuration allows the pupil to contract very small during the daytime.
The slit pupil allows a much smaller aperature than a round pupil. This is especially important for nocturnal animals (like cats) who’s eyes are very sensitive to sunlight.
Many nocturnal animals have multifocal lenses … areas of their peripheral lens that is in focus for different wavelengths of light. By having a slit pupil, they don’t lose their multifocal vision during the day.
The anablep is known as the “four eyed fish” because it floats on the surface of water, with half its eye above water and half below.
This fish has the appearance of four eyes because it has a dual pupil, in a configuration like an hour glass.
The lens of this fish is in an oval configuration, such that half is in focus for air, and half for underwater.
The Restor implant, used in cataract surgery, has a similar configuration. Half the concentric rings are in focus for distance, and half are set for near.
Here is a Restor implant in a live patient.
The owl skull. The eye socket takes up a large component of its volume.
The owl’s eye is so large, there is no room for eye muscles in the orbit. As such, the eye is in a fixed location, and the owl has to move its entire head to look around.
The octopus has a highly evolved eye and excellent vision.
The octopus retina is laid out quite differently than the human retina. With the octopus, the photoreceptors (rods and cones) are located at the surface of the retina. The signal is then transmitted directly down the optic nerve. This configuration means that the octopus doesn’t have a blind spot like in the human eye.
The choroid is a bed of blood vessels located under the photoreceptors. Nutrition percolates up to nourish the retina, and waste product is dumped back down into the choroid.
With retinal detachments, the photoreceptors lose their choroid blood supply and atrophy quickly.
Retinal Drusen build up between the photoreceptors and the choroidal blood supply underneath. This causes the retina to atrophy in this area, leading to vision loss.