During evolution, the eyes of vertebrates develop different adaptations in order to fit into the different habitats. In this chapter, 27 vertebrates, both rare and common, from different animal groups were selected to illustrate the histological differences in their retinae. For example, it is interesting to compare vertebrates between groups e.g. fish and amphibians. It is also interesting to compare vertebrates that are herbivores with those that are omnivores. In addition, comparing between species is also enlightening. A cat, which is an agile and fast moving animal, has a lot more types of visual cells than other slow moving species, while bottom living Chinese soft shelled turtles have visual cells that are rudimentary. In other cases, the Japanese eels, which inhabit both ocean and freshwater streams during their life-cycle, have retinae with highly packed and numerous visual cells while the goldfishes, which live in freshwater streams and ponds, have retinae with scattered visual cells. The differences in the retinae among different groups of vertebrates demonstrate a significant increase in visual cells in the mammalian retinae.
Keywords: Visual cell layer, outer nuclear layer, outer plexiform layer, inner plexiform layer, inner nuclear layer, ganglion cell layer, rods, double cone, chief cone, accessory cone, single cone, twin cone, pigment epithelium, various animals.