Vision

This background has all led us up to being able to understand the basis of vision. First it is

Crustacea Eyes are compound with subunits called ommatidia. Each ommatidia resolves a single point with it's own lens and neural connects to the brain.
Mollusca The molluscan eye ranges in complexity from the light-sensing apparatus of some nudibranchs to the eyes of some cephalopods which rival human eyes in their complexity.

The molluscan eye has a spherical lens. Focusing is acheived by moving this lens back and forth
Vertebrata The vertebrate eye is a complex organ capable of color vision. The lens in the mammalian eye is fized and changes shape to achieve focus.
worth noting that of the all the major phyla in the world, only 3 have developed image-resolving eyes. It is also believed that these phyla developed their vision systems independantly of one another yet all three utilize the same basic chemical principles as the basis for vision.

George Wald was the pioneer in this discovery of the chemical basis of vision. He found that Vitamin A plays a key role in vision. Vitamin A, in turn is, exactly one half of a carotene molecule with an hydrogen and a hydroxyl (OH) attached to the broken end. It's no accident that we emply this same chemical in vision that plants use for phototropism. In fact, we still rely on plants for this chemical. Animals, humans included, cannot make their own vitamin A. The adage about eating carrots for enhanced vision has it's basis in our dependance on carotene to make vitamin A and one of the effects of vitamin A deficiency is, in fact, loss of vision.

What Wald did was determine the role vitamin A plays in vision. He discovered that it was a derivative of vitamin A, called retinene which is bound to a protein belonging to a group called opsins. This opsin/retinene complex holds the key to how light can be passed from the physical realm outside the call to become some sort of signal within the body.

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References used in the Vision sections of this site.

Hartline., H.K., Wagner, H.G., Ratcliff, Floyd, Inhibition in the Eye of Limulus, Journal of General Physiology, 1956, 39:5 pp.651-673

Westerman, L.A., Barlow, R.B., Ultraviolet responses of the Limulus mediann ocellus, Biological Bulletin, 1981 161 352-353

Barlow, R.B., Ireland, C.I., Kass, L., Vision in Limulus mating behavior, Biological Bulletin, 1981 161 339-340

Powers, M.K., Barlow, R.B., Circadian changes in visual sensitivity of Limulus: behavioral evidence, Biological Bulletin, 1981 161 350-351

Hubbard, Ruth. Retinene Isomerase, Journal of General Physiology, Vol 39, No. 6 pp.935-962

Wald, G., Human Vision and the Spectrum, Science, 1945, 101, 653

Wald, G., Life and Light, Scientific American, Oct. 1959, pp 92-108

Invertebrate Photoreceptors, A Comparative Analysis, Jerome J. Wolken, Academic Press, NY, 1971

Kimbel, R.L., Poincelot, R.P., Abrahamson, E.W., Chromophore Transfer from Lipid to Protein in Bovine Rhodopsin, Biochemistry 1970 9:8 1817

Westerman, L.A, Barlow, R.B, Ultraviolet responses of the Limulus median ocellus, Biological Bulletin General Scientific Meetings. 161:3 352-353

Barlow, R.B, Ireland, L.C., Cass, L., Viision in Limulus mating behavior, Biological Bulletin General Scientific Meetings. 161:3 339-340

Sargent, William., The Year of the Crab., W.W. Norton & Company 1987