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.
