The timecourse and extent of changes in pupil area in response to light are reviewed in all classes of vertebrate and cephalopods. Although the speed and extent of these responses vary, most species, except the majority of teleost fish, show
extensive changes in pupil area related to light exposure. The neuromuscular
pathways underlying light-evoked pupil constriction are described and found to be
relatively conserved, although the precise autonomic mechanisms differ somewhat
between species. In mammals, illumination of only one eye is known to cause
constriction in the unilluminated pupil. Such consensual responses occur widely in
other animals too, and their function and relation to decussation of the visual
pathway is considered. Intrinsic photosensitivity of the iris muscles has long been
known in amphibia, but is in fact widespread in other animals. The functions of changes in pupil area are considered. In the majority of species, changes in pupil
area serve to balance the conflicting demands of high spatial acuity and increased sensitivity in different light levels. In the few teleosts in which pupil movements occur they do not serve a visual function but play a role in camouflaging the eye of bottomdwelling species. The occurrence and functions of the light-independent changes in pupil size displayed by many animals are also considered. Finally, the significance of the variations in pupil shape, ranging from circular to various orientations of slits,
ovals, and other shapes, is discussed.