12 VISION 



INTRODUCTORY REMARKS 



Systematic study of the shark's visual system dates back at least 200 years. 

 Then, as now, sharks were considered interesting, even valuable, and thus 

 worthy of study. Already in 1818 Soemmering had described the tapetum 

 lucidum of sharks; the autonomic, direct action of light on the iris was 

 demonstrated in 1839 (Rochon-Duvigneaud 1943). Until the middle of the 

 20th century the major research effort was toward understanding the gross 

 and microscopic anatomy of the visual system. For example, no fewer than 

 eight studies on elasmobranch retinal histology were published between 

 1890 and 1905 (Duke-Elder 1958). 



The field of elasmobranch vision through the 1930's was dominated by 

 the views of M. L. Verrier in France and V. Franz in Germany. Both had 

 studied the visual systems of elasmobranchs for years, and both made im- 

 portant contributions (Verrier 1930, Franz 1934). Since then, several re- 

 views (Walls 1942, Rochon-Duvigneaud 1943, 1958, Duke-Elder 1958, and 

 Gilbert 1963) have summarized knowledge of the elasmobranch visual sys- 

 tem. The usual approach was to break the eye into component parts such as 

 retina, lens, cornea, etc., and discuss each, concentrating mainly on struc- 

 tural aspects (due no doubt to the paucity of physiological data). Thus these 

 articles present an overview of the visual system in sharks as it was known 15 

 years ago. 



It is not our intention to rehash this information and present again a 

 "dissected view" of the elasmobranch eye. Rather, we shall review and 

 synthesize research on the elasmobranch visual system published since the 

 last review (Gilbert 1963). The scope of this article is further limited by the 

 other contributions to vision in this volume. Thus, we have avoided review- 

 ing the areas of refractive error, optics, accommodation, and higher visual 

 centers in the central nervous system (CNS). This review is therefore limited 

 to investigations published in the past 15 years on anatomy, biochemistry, 

 physiology (including psychophysics) and natural history of photoreception 

 in the elasmobranchs. 



LATERAL EYES 

 Ocular Adnexa 



Eyelids— Little modern work has been done on the ocular adnexa of 

 elasmobranchs. Yet the palpebral complex (upper and lower lids, nictitating 

 membrane) immediately separates elasmobranchs from bony fish. While a 

 lacrimal system is unknown, elasmobranchs have well-formed eyelids that are 

 mobile in some species, i.e., Ginglymostoma and Cephaloscy Ilium, but rela- 

 tively immobile in others. In still other species the lower lid is secondarily 

 folded longitudinally into a third eyelid (Figure 1), the original lower lid 

 forming a structure similar to the nictitating membranes of amphibia, birds, 

 and mammals. However, unlike the transparent nictitating membranes of 

 terrestrial vertebrates, that of sharks is dense and opaque and its outer 



