The eye is an extremely diverse organ, varying in complexity between and within animal phyla. Here, a comparative approach is taken to describe the diversity of eye shapes in vertebrates and invertebrates. The eye morphology of various organisms has been examined. Eye function and placement on the body were also considered. Here, variation in eye shape is discussed in relation to the environment to which the organism is adapted. An organism's eye morphology is shown to function optimally for the ecological niche it occupies. Evolutionary analysis is used to account for the emergence of different eyes. Convergent evolution is used to justify the similarities between eye types observed in organisms of different species. This analysis begins with the simplest eye form, composed of single cells, found in zooplankton larvae. Such primitive forms are identified in mollusks, annelids, cnidarians, and are then compared to more advanced ocular forms containing lenses. This comparative approach provides a wide range of vertebrate and invertebrate examples, making visible the diversity of eye morphology within the animal kingdom. An inspection of modern animal phyla will reveal that eyes are as diverse as they are complex. Some organisms, like the rag worm, have pigmented cupped eyes, while others, like box jellyfish, have two lens-shaped eyes and two pairs of pigmented eyes. To account for the diversity of eye structure, we must first examine the “prototype” of the eye, the original structure on which evolution acted. The simplest organ that can be considered an eye is composed of a single photoreceptor cell and a single pigment cell, without a lens or other refractive body (Arendt, 2003). These organs are known as ocelli, and...... middle of paper ......2007).Evolution of the vertebrate eye: opsins, photoreceptors, retina and eyecup. Nat. Reverend Neurosci. 8: 960-976. Earth, MF1965. Image formation by a concave reflector in the eye of the scallop, Pecten maximus. J. Physiol. (London) 179: 138 153. Piatigorsky, Z., Kozmik. 2004. Cubozoan jellyfish: an Evo/Devo model for the eyes and other sensory systems. Int J Dev Biol 48: 719-729 Singla, CL 1974. Ocelli of hydromedusae. Cell. Fabric. Res. IJ9; 413-429. Sousounis, k., A. Ogura and PA Tsonis. 2013. Transcriptome analysis of the developing eye of Nautilus and pygmy squid provides insights into the evolution of the lens and eye. PLoS ONE 8(10): e78054 Wells, MJ 1997. Cephalopod behavior. Trends ecol evol 12(2): 82-83Wistow, GJ and J. Piatigorsky. 1988. Lens lenses: evolution and protein expression for a highly specialized tissue. Ann. Reverend Biochem. 57: 479-504.