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Annelids1 and Mollusks2 may be dissimilar to each other in physical appearance, but as far as eyes are concerned, they couldn't be more similar. Both possess a cup-like eye, which has been described by many evolutionary biologists as the next evolutionary step in eye formation toward the eyes seen today in cephalopods, sharks, and vertebrates.



The Theoretical Formation of the Cup-like Eye

Referring back to the random population model from the bacteria portion of the site, we now take a group of multi-cellular organisms with a primitive eyespot for an eye. As shown by experimentation, any type of bending of the surface of the organism to form a concave surface where the light-sensing organ is located will cause an advantage for that organism over the other organisms as far as sight is concerned. With a bend in the photosensitive area of the organism, visual perception gains the ability to see more directionally (as seen in Figure 2) (Land, 6). It allows the organism to get a clearer picture due to the ability of a cup to block light coming in from the sides (as shown in Figure 1). The evolution itself would not be immediate, but rather, quite gradual, starting with the smallest bend, and resulting in a photoreceptor looking much like a cup or cauldron (Nilsson, 300). Each time such a bend in the receptor takes place, the organism's vision becomes a little better in resolution, distance perception, and in the prevention of light interference from obtuse sources. This can continue on until there is only a pinhole left for light to be received through, but that shall be addressed later in the presentation (Patton).


Vision Through the Eye of an Annelid or Mollusk

Vision through a cup-like eye would be blurry, but not nearly as blurry as looking through the eye of a bacterium. There may be some color information, although it would be limited judging on the numbers and kids of receptors in the organism. These eyes may not display the best of pictorial information, but they do show motion much better than the more primitive, bacteria eyespot.




1Annelid: any of a phylum (Annelida) of usually elongated segmented coelomate invertebrates (as earthworms, various marine worms, and leeches)
(as according to: Merriam-Webster Online)

2Mollusk: any of a large phylum (Mollusca) of invertebrate animals (as snails, clams, or squids) with a soft unsegmented body usually enclosed in a calcareous shell; broadly
(as according to: Merriam-Webster Online)