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<center><a href="https://www.pexels.com/photo/blue-human-eye-816734/">Picture Source - Pexels</a></center>

Eyes is a remarkable structure for any biological creature on this planet. This organ will lead to vision when a specific kind of photoreceptors available in it, detects light. The concept seems simple, but the quality of those vision would differ between species. Flatworm, for example, have the capability to identify the origin of a light sources only. They can't see like human do because there are just a few photoreceptors available in their eyes. To make the difference crystal clear, human eyes contain approximately 100 million of photoreceptors which would be able to resolve one minute of arc making it 4000 times efficient in getting a good resolution compared to flatworm eyes. 

There are a few examples which could illustrate how different is the photo-detecting organ between animals on the planet:

- The eye of a fish which lives in the deep sea area has a different spherical design which restricted its view upward. The only possible reason is, the direction of light came from the above (surface) hence making this structure pointing upward to absorb as much light as possible, but some fishes which span the same area have another kind of eye structure which imposed double vision, for example, the eyes of a fish called Bathylchnops. This fish has an extra lens and retina which attached to its eyes. It's thought that the chief eyes, was pointing upwards to detect the source of light while the other eye is pointing downwards to look for any bioluminescent creature which emits light.

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<center><a href="https://commons.wikimedia.org/wiki/File:Expl7095_(9737933344).jpg">Ipnops By NOAA Photo Library - Wikimedia</a></center>

- In the ocean floor, where light cannot penetrate, any creatures which inhabit this particular area would have reduced eyes or even absent of it. There is no light so having an eye would be pointless. Ipnops, a fish which lives on the ocean floor have no eye, but they still have retina which covers the top, front of its head. There is literally no other eye structure except for retina, and this particular "eye" has no known function. 

- Predators usually have forward-pointing eyes which provide a binocular triangulation and allows it to judge distance making it efficient in hunting for prey. Prey such as mouse or rabbit have eyes, opposite to each other. It gives them the luxury to see almost everything on their surrounding. This is thought to be some kind of defence mechanism against predation so that they can detect and outrun a predator.

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<center><a href="https://pxhere.com/en/photo/472070">Picture Source - Pxhere</a></center>


- Animals which are nocturnal have spherical lenses which occupied the eye cavity to allow a maximum amount of light being absorbed. They usually equipped with an abundant amount of specific photoreceptors called the rod cell which is used mainly to identify size, shape and level of brightness but not colour. They also exhibit a slit-shaped pupil (compared to a diurnal creature which has a round pupil) which allow it to close rapidly when a bright light is shone on them. Nocturnal animals have a structure which resembles a reflector located behind their retina to capture any photons that they're missed the first time it passed through the retina. This structure is called as "tapetum lucidum" which allow a better vision at night. A diurnal creature, on the other hand, has lots of photoreceptors which is called cone cells which are great for perceiving colour and fine details.

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<center><a href="https://pxhere.com/en/photo/1385034">Picture Source - Pxhere</a></center>

- Any animals which make a quick transition from land to water like a seal and otter or air to water like a seagull have unique shaped corneas. There is a membrane which is transparently located just in front of the eyes to keep fluids outside from the liquid inside the eyes. The original function of the cornea in this animal is enhanced the focusing power of the eye, but due to the presence of fluid on both sides, their optical focus is greatly diminished. A seal which has a flat cornea would have to be dependent on the ability of its spherical lens to produce an image. Diving birds, on the other hand, have to adapt to the loss of optical power by squeezing its own lens in the bony ring surrounding the iris making its focal length shorten hence improving its vision. 

I think that's enough example. You should be clear by now that, some animals on this planet exhibit a different eyes structure to match its own nature/lifestyle. Some are important for them to hunt, some are important to run, and some are important to survive. As this will be a pretty long article, I've decided to divide it into three parts for you to digest and eventually understand the information that I'm about to give. The goal of the first part of this article is to show you some example which proves that different animal, in fact, has a different photo-receiving system. Stay tuned for the next part where we would discuss:

- Two primary eyes type
- The structure and function of the photoreceptors

Thank you for your continuous support.

# References and further reading material

- Michael Land (1999, July 26). Encyclopaedia Britannica. Photoreception. Retrieved March 6, 2018, from https://www.britannica.com/science/photoreception
- Wikipedia. Photoreceptor cell. Retrieved March 6, 2018, from https://en.wikipedia.org/wiki/Photoreceptor_cell

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