So inevitable was the development of vision among motile creatures that it has developed along two different evolutionary pathways: the brain in vertebrates and the epidermis in invertebrates. That’s right, the skin. Among the more primitive jellyfishes, the eyes are raised patches of cells, called eyespots. These eyes cannot form images but can detect the direction light is coming from. The surface of the entire jellyfish appears to be photosensitive, sensing light even when the eyespots are covered.

While the vision of the jellyfish is limited, the octopus has eyes that can form images close up and at a distance, changing the focus of its lens by a combination of muscles and internal eye pressure. The octopus adjusts its pupil size for bright or dim light and can detect the direction scattered underwater light rays are traveling. Octopus vision is limited to the blue-green spectrum of light, which corresponds to the underwater environment, and octopus vision is less refined than that of vertebrates, allowing the creature to recognize filled shapes but not outlines. The octopus is an intelligent creature, capable of learning and detecting patterns, and so it is a favorite subject for research.

The invertebrate eye reaches its apotheosis in certain arthropods, and the Jumping Spider is a prime example. While Cave Spiders are completely blind and spiders who stay home on their webs have limited acuity, the hunting spiders have exceptional vision. The tiny Jumping Spider, only about a quarter of an inch in diameter, has four pairs of eyes, each with a specialized function. The large central forward facing pair has sharp vision within its limited field. The Jumping Spider has no lens to accommodate objects near and far, but the other eyes mitigate this problem to some extent. Another pair of forward facing eyes judges distance, and both pairs on either side of the head detect motion, with one of the side pair also having wide angle vision, giving the Jumping Spider the ability to see at almost 360°. If the Jumping Spider needs to see an object clearly, it jumps within range. Jumping Spiders have excellent color vision, seeing into the ultraviolet range. The hunting spiders communicate in courtship through visual cues, and another hunting spider, the Wolf Spider, waves his legs in code for the female, in a kind of spider semaphore.




If the invertebrate has a brain, the eyes do communicate with that brain, but the eyes of invertebrates are different from vertebrates in structure. Reflecting evolution from the epidermal tissues, invertebrate eyes connect to different parts of the brain, and more visual processing is done in the eye itself. Vertebrate eyes, as extensions of the brain, process more visual information in the brain. Invertebrates truly see the world in different way than humans.


Source

Sinclair, Sandra. How Animals See: Other Visions of Our World. New York: Facts on File, 1985.


Postscript

In the course of my research I discovered that “Jellyfish Eyes” is the name of a 2013 film by Takashi Murakami currently being screened at indie art theatres around the USA. I chose the title of this post before realizing this, as I was truly interested in actual jellyfish eyes. I may be the first to point out that the eyes on these “jellyfish” creatures are in no way anatomically correct. Alas, few sci-fi artists care about such things. Here is the trailer.