There are four sets of eyes in a jellyfish and each set is in a corner. A structure called the rhopalium is shaped as a club and each set of eyes is clustered in this structure. In this structure known as the rhopalium there are six eyes. There are two smudges which are rather dark in color. These are located in the center. There are two more eyes which are known as camera eyes and are rather complex in nature. One eye is pointing upwards, and the other eye which is slightly larger is pointing downwards. Each eye has a retina, a lens and pigmented epithelium. The eye that is pointing downward is known as an iris which automatically shuts down if a light is too bright. It opens again in darkness. This structure is rather complicated for a jellyfish that is rather small. It is a sophisticated structure. The camera in the eyes is a specialized camera complex which hangs out in the rhopalium of each stalk.

Scientists and investigators have discovered that the lenses in the jellyfish' eyes are very good. They have investigated their optical lenses. The material of the lens differs from the core of the eye to its surface. It is a naturally spherical aberration which corrects itself. The eye at the top is almost perfectly corrected and it is able to focus on light at a sharp point which is at a distance of 3.3 radii from the center of the lens. The lens which is lower is also well corrected, and it can focus at a distance of 2.6 lens radii to 3.7 lens radii.

There is an oddity in the jellyfish eyes. They have a lens that is well corrected, and is able to focus on a distance of 3.3 lens radii, but this is located behind the retina. Jellyfish are terribly far sighted, or hyperopic. It is similar to evolving with a great pair of lenses, but it is all thrown away by mis-focusing horribly. If there is a light at a distance, it is not focused sharply on a photoreceptor that is single, but instead on the retina, and an entire diffuse region lights up. When scientists analyzed the optical models each photoreceptor did not have acuity and the light was visible at not much of a view.

Jellyfish don't have brains and so this eye is simply useless. Jellyfish will not carry out signal processing which is complex on obtaining visual information. Jellyfish are very vague and are interested in vague things. Questions such as, where's up and where's down? Is it night or day? They do not have the capacity to grasp these details and are not interested to know about them either. They consider these data as distraction and not information. Since we are not aware of their visual information about their lenses it is not possible to derive what functions it might serve. The neurons in the vertebrate brain of a visual centre which is much higher have receptive fields that are complex. The simple fact that jellyfish have four sets of eyes allows us to perceive that each set of eye must surely be specialized.
Normen FAdel
Author of the article
writer and blogger, founder of jellyfish .

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