Bob Shell wrote: >Many animals have a reflective greenish layer behind the >retina. This is called the tapetum. It is a light amplifier, >because in reflecting the light back it sends it to the >photosensitive cells again. It doesn't quite double the sensitivity, >but increases it greatly. Animals that are strictly diurnal lack >this extra layer and produce "redeye" in flash photos. Night animals >produce strong reflections like this raccoon.
Yep! Its full name is the "Tapetum Licidum". One of the few nocturnal animals that *doesn't* have it is the Tarsier. Tarsiers make up for their lack of a tapetum lucidum by having enormous eyes (each one as large as the tarsier's brain). Another odd vision-related bit of trivia concerning tarsiers: All males have dichromatic (two-color) vision, but some have blue and green cone cells in their eyes and some have blue and red cone cells. Some (but not all) female tarsiers have trichromatic color vision (like humans with red, green and blue cones). But there are also some females that have the same dichromatic color system(s) as the males. You can work through how it all happens by looking at genes that are carried on the X and Y chromosomes but it's a bit too involved to relate here. There's a fascinating discussion of it in Richard Dawkins' book "The Ancestor's Tale".
