228 Shark and Company 



Sharks have been seen zigzagging through the water in an apparently 

 aimless pattern. They were probably homing in on a scent, veering to 

 the right if the right nostril detected a stronger scent, and vice versa. 

 Dr. George Parker of Harvard once demonstrated this by plugging first 

 one, then the other nostril of a shark in a tank. When the left nostril was 

 plugged, the shark swam clockwise, seemingly relying on the messages 

 transmitted by its right nostril; it swam counter-clockwise when the 

 right nostril was plugged. 



The Selachian looks out on its watery world through eyes that, to 

 man at least, may appear sinister.^ Some sharks stare balefuUy; others 

 "wink" weirdly, with a nictitating membrane that moves up instead of 

 down, as eyelids do. Some bottom-dwelling species, such as rays, have 

 a fold of skin that acts as an awning to protect the eye from light coming 

 from above. 



The eye of the shark varies from the enormous eye of some deep-sea 

 species to the comparatively tiny eye of the huge Whale shark. Many 

 nocturnal sharks have rudimentary eyes, and Electric rays of at least 

 one genus (Typhlonarke) are blind. Some South African sharks (Hap- 

 loblepharus edwardsi and Holohalaelurus regani), caught mostly at 

 night, are called Skaamoong, or "Shy Eye," because, when one is taken 

 from the water, it folds its tail over its head, as if to shield eyes sensitive 

 to light. 



Behind the retina of the eyes of at least some sharks are light-reflect- 

 ing tissues similar to those that make a cat's eye glow ghostlike in the 

 beam of a headlight on a dark country road. These natural mirrors in- 

 tensify the feeble underwater light. If the shark is in water made dazzling 

 by bright sun, a kind of curtain of non-reflecting cells drops over the 

 mirror-like tissue. The iris muscle of the eye will continue to expand or 

 contract in shadow or light— even when it is removed from the head. 

 These experiments have indicated that the muscle responds directly to 

 light falling on it and does not act through a nervous impulse from the 

 brain. Such a primitive arrangement is another example of the Selachian's 

 ability to have evolved to a simple level and then stayed there. 



For many years, the theory has persisted that sharks do not have 

 sharp eyesight. Some 50 years ago, Dr. Parker reported that Smooth 

 dogfish (Mustelus canis) in experimental tanks rarely responded to an 

 object that was held more than a foot away from their eyes. Primarily 



2 The authors have found occasional references to eye colors of various species of 

 sharks in the literature— but, strangely, there is so little reported on the subject that it 

 has been eliminated as a topic in the present work. The few occasional references do 

 indicate that they range in appearance from the dull baleful eyes of the Tiger shark 

 to browns and blues in other species which might tax the resources of The Word 

 Finder to describe. 



