ESSENTIAL PROBLEM IN AMPHIBIOUS VISION 429 



above, 'terrestrial' characteristics are present along with others such as 

 the salamander-like accommodation; and most of them occur in any 

 given species of the group. If it were not almost unthinkable, we might 

 conclude, from a cursory examination of a shark eye, that the elasmo- 

 branchs must once have lived on land and^ like the whales, secondarily 

 returned to the ocean! Surely, these peculiarities of the elasmobranch eye 

 ail have explanations other than those which hold for their seeming 

 counterparts in the higher vertebrates; but we cannot be sure at present 

 that we know quite all the answers. 



The arching of the elasmobranch cornea and its distance from the lens 

 appear to go simply with the method of accommodation peculiar to the 

 group (see p. 260) . The ciliary and iridic folds are probably mechanical 

 devices for anchoring the thick zonule (whose rim covers the whole sur- 

 face of the ciliary body), and not secretory — indeed, there is reason to 

 suspect them of being absorptive. But the presence of distinct upper and 

 lower lids in so many forms, the lower lid often having an extra trans- 

 parent fold comparable to a frog's 'nictitans' (Fig. 131b, p. 382), is a 

 deep mystery. The complex is best developed in the largely bottom- 

 loving sharks (galeorhinid) which, if they were teleosts, might be expected 

 to show the simpler protective device represented by the secondary spec- 

 tacle. The great blue shark Prionace glauca, a pelagic species which is 

 most active at night (when it hunts by scent) has been observed to blink 

 the nictitans rapidly in bright light when pursuing prey or when other- 

 wise excited, as though the irritation of the light were controlled some- 

 what by the membrane. But there is no evidence that this is always, or 

 ever, its primary purpose. Indeed, Franz found that Scylliorhinus and 

 Mustelus would not use their lids to shield their eyes from the strongest 

 light, though they would struggle violently to get away from it; nor 

 would Ra]a, capable of concealing the eye by retracting it, do so in order 

 to avoid dazzlement. 



An interesting problem awaits the investigator who attempts to corre- 

 late the palpebral complex of the elasmobranchs with something else in 

 their biology. Its solution will be most welcome. 



(C) AiR-AND- Water Vision 



The Main Problem — Those vertebrates which wish to eat their cake 

 and have it too, by attempting amphibious vision, have a considerable 

 problem. If they happen to be fishes, they not only have their optical 

 difficulties in seeing in air, but must somehow get along without the 



