The simplest form of sight organ and the first to evolve, eyespots are clusters of photosensitive cells which are capable of distinguishing between areas of light and areas of dark but are not able to discern shapes or to determine the direction light is coming from. The eyespot design is present in almost all of the major animal groups and is especially proliferate among the lower invertebrates, over time a number of adaptations appeared such as a transparent membrane for protection. The ability to discern the direction of a light stimulus appeared as the eyespot gradually began to take on a concave shape, no longer being present on a completely flat surface meant that instead of the entire cluster of cells being stimulated by a light source the degree of stimulation would vary depending on the position of the cell on the concave surface. As the depth of the eyespot and the concentration of photoreceptive cells increases so does the ability to determine the direction of the light stimulus being detected. A further adaptation occurred whereby the aperture of the concave pit narrowed to create a system not unlike a pinhole camera which allowed for the ability to distinguish large shapes and to determine the direction of the light source with some degree of accuracy, this pinhole design is present in some modern day organisms such as the nautilus and while lacking in resolution still represents a great advancement over the simple, flat surface eyespots.
Disclaimer: The author is not a medical doctor, optician, ophthalmologist or any other medical professional, this article has been compiled using a variety of internet reference sources and while every effort has been made to ensure accuracy this cannot be guaranteed. No preference for product or brand is inferred or intended and the contents of this article are not to be used in whole or in part to inform a decision regarding any aspect of contact lens use.
