COMPARATIVE SURVEY OF THE TWO METHODS 159 



The elasmobranchs are conspicuous among the fishes for having highly 

 mobile (though excessively slow — v.s.) pupils. The sphincter is unusual 

 and primitive in that it is never separated from the epithelium which 

 generates it, as in other vertebrates. They have no retinal photomechan- 

 ical changes — indeed, no retinal pigment except in the extreme periphery 

 to which the tapetum lucidum does not reach. Most forms are active 

 principally at night, but some like to doze, basking, at the surface. A 

 few may be found active at any hour, and a considerable number live in 

 the deep sea and are in a constant environment as regards light. The 

 light-lovers have broadly elliptical, usually vertical pupils which dilate to 

 circles in low illuminations. The more strongly nocturnal species have 

 more mobile pupils which close in bright light to narrow slits set diag- 



Fig. 66 — Eye of a shark, Squalus acanth- 



ias, showing mydriatic pupil rigor, x 1. 



Redrawn from Franz. 



a, when pupil is freshly illuminated. 



b, after illumination of long duration. 



/ o/a 



Fig. 67 — Dorsal iris-angle region of a teleost, 

 Chrysophrys aurata. After Grynfeltt. 



a- argentea; al- annular 'ligament'; cm- 

 ciliary muscle; d- dermal contribution to 

 cornea; i- iris; ip- inner portion of primary 

 cornea; n- nerve; op- outer portion of pri- 

 mary cornea; ot- ora terminalis; pc- pars 

 ciliaris retinae; pi- pars iridiaca retinae; sl- 

 suprachorioidal lymph space; so- scleral 

 ossicle; v- blood vessel. 



onally or horizontally. They are thus safe from dazzlement and defense- 

 lessness when they come up to sun themselves. The flattened, upward- 

 gazing rays are provided with an 'operculum' which can expand to fill 

 the pupil from within (Fig. 65). The electric ray or torpedo, however, 

 relies upon a horizontal slit, which a tiny operculum can divide in the 

 middle. The pupils of elasmobranchs need not necessarily hold their full 

 contraction in bright light, but are privileged to reopen again after a 

 time just as though photomechanical changes had taken place — for these 

 fishes have photomechanical changes in the chorioid which alter the sensi- 

 tivity of the eye (see Chapter 9, section D) . Some sharks (e.g., Squalus, 

 Mustelus) in fact develop a 'mydriatic pupil rigor' if kept for several 

 days in a lighted room — their pupils become widely open and refuse to 

 close down when additional light is thrown on them (Fig. 66) . 



