VISUAL SYSTEM: STATE OF THE ART 29 



from the same habitat but lack a reflecting tapetum. All other factors aside, 

 teleosts without tapeta and elasmobranchs with eyeshine have the same 

 potential sensitivity. Nicol suggested that lower pigment density of sharks 

 might permit more rapid dark adaptation, but measurement of this param- 

 eter (Gruber 1967, Hamasaki et al. 1967) has shown that this is a relatively 

 slow process in elasmobranchs. The possibility of improving signal-to-noise 

 ratio at the retinal level has been discussed. One final possibility not often 

 mentioned involves camouflage. Light-absorbing pigments in the fundus con- 

 fer the familiar dead black pupil characteristic of most vertebrate eyes. This 

 "eye spot" has important behavioral consequences (Blest 1957). Some 

 organisms have evolved conspicuous nonvisual eye spots, and many have 

 evolved markings and patterns in an apparent effort to disguise their func- 

 tional eye spot. Might it be possible that elasmobranchs to some extent use 

 reflection from the fundus to hide their eye spot? Certainly the plate orien- 

 tation tends to reflect light of all optical pathways back through the lens 

 into the environment. 



The Ciliary Zone 



The ciliary zone, in the anterior segment of the eye, is an anatomically and 

 physiologically heterogeneous zone of varied embryological origins. Bounded 

 posteriorly by the ora terminalis (the termination of the sensory retina) and 

 anteriorly by the corneal endothelium, the ciliary zone is almost every- 

 where bathed with aqueous humor. Structures making up the elasmobranch 

 ciliary zone include the iris, the ciliary body, ciliary folds, and the ciliary 

 papilla on which the lens rests. Zonular fibers and suspensory ligaments that 

 hold the crystalline lens in place also occur but are thought to be condensa- 

 tions of the vitreous body (Duke-Elder 1958). The ciliary structures are 

 composed of forward extensions of the nonsensory retina in intimate con- 

 tact with the uveal (vascular) tract. Functions of the ciliary zone are many 

 and varied. For example, ciliary structures control the amount of light enter- 

 ing the eye, probably move the lens in accommodation (see Sivak elsewhere 

 in this volume), and secrete aqueous humor. These structures have lately 

 received considerable attention because anatomical, physiological, and bio- 

 chemical similarities between the ciliary apparatus of mammals and elasmo- 

 branchs suggest that the shark system may be the prototype for higher 

 organisms (Doolittle et al. 1960, Maren 1962a, Jampol and Forrest 1972). In 

 addition, elasmobranchs are among the few fish with the capacity for exten- 

 sive pupillary movements. 



Iris Anatomy— Kuchnow and Martin (19706, 1972) investigated the 

 fine structure of the iris in seven elasmobranch species. The older literature 

 presented a confusing picture of the iris, especially regarding the association 

 of neurons with contractile elements. Physiological experiments by Young 

 (1933) demonstrated that the iris sphincter is responsive directly to light. 

 This can easily be confirmed by excising a small piece of iris from a dark- 

 adapted shark and exposing it to intense light. The in vitro piece of iris will 

 vigorously contract. However, Young believed that the iris dilator was under 



