A classification of anatomical variants of tapeta lucida defines four types: # Retinal<unk> , as seen in teleosts, crocodiles, marsupials and fruit bats. The<unk>  is within the retinal pigment epithelium; in the other three types the<unk>  is within the choroid behind the retina. # Choroidal guanine<unk> , as seen in cartilaginous fish. The<unk>  is a palisade of cells containing stacks of flat hexagonal crystals of guanine. # Choroidal<unk>  cellulosum, as seen in carnivores, rodents and cetacea. The<unk>  consists of layers of cells containing organized, highly refractive crystals. These crystals are diverse in shape and makeup. # Choroidal<unk>  fibrosum, as seen in cows, sheep, goats and horses. The<unk>  is an array of extracellular fibers. The functional differences between these four types of tapeta lucida are not known. This classification does not include tapeta lucida in birds. Kiwis, stone-curlews, the boat-billed heron, the flightless kakapo and many nightjars, owls, and other night birds such as the swallow-tailed gull also possess a<unk> . This classification also does not include the extraordinary focusing mirror in the eye of the brownsnout spookfish. Like humans, some animals lack a<unk>  and they usually are diurnal. These include haplorhine primates, squirrels, some birds, red kangaroo, and pigs. Strepsirrhine primates are mostly nocturnal and, with the exception of several diurnal Eulemur species, have a<unk> . When a<unk>  is present, its location on the eyeball varies with the placement of the eyeball in the head, such that in all cases the<unk>  enhances night vision in the center of the animal's field of view. Apart from its eyeshine, the<unk>  itself has a color. It is often described as iridescent. In tigers it is greenish. In ruminants it may be golden green with a blue periphery, or whitish or pale blue with a lavender periphery. In dogs it may be whitish with a blue periphery. The color in reindeer changes seasonally, allowing the animals to better avoid predators in low-light winter at the price of blurrier vision.
Tapetum lucidum
Corona radiata 
Stratum basale
Glia limitans
Stratum lucidum
Neuromatrix
Stratum spinosum
Habenular nuclei