Whence the Shadows? 231 



study of the shark's sensory system. Wisby's subjects are Nurse sharks 

 (Ginglymostoma cirratu?n), and his observations are carried on not in a 

 tank— but in a drainpipe. 



The drainpipe, 16 feet long and 3 feet in diameter, was chosen so 

 that distracting sounds and sights could be blocked out. One end of the 

 pipe is buried in a box of water-soaked sand, which absorbs sound. The 

 pipe rests horizontally on springs that further absorb sounds from the 

 outside. When the shark is strapped on a kind of sled and suspended in 

 the water-filled pipe, it is thus isolated from any stimuli except those 

 which Wisby introduces. 



The shark is next conditioned to associate a sound with an electrical 

 shock. When it detects a sound in its drainpipe prison, the shark's heart 

 skips a beat— as it does when it gets an electrical shock. The telltale 

 heart-skip, which proves that the shark hears a given sound, is regis- 

 tered by a "lie detector." This is simply an electrode implanted near 

 the shark's heart and connected to recording devices in the laboratory. 

 From these recordings of shark reactions. Dr. Wisby believes, scientists 

 may eventually be able to determine what types of sound attract— and 

 repel— sharks. 



The sense of hearing alone does not fully explain the shark's de- 

 tection of and reaction to low-frequency water vibrations— caused, for 

 instance, by the struggles of a hooked fish. Certain fish, such as Croakers, 

 make clearly audible sounds. But the struggles of a fish on a hook are not 

 audible; they are vibrations undetectable by what we normally call hear- 

 ing- 

 Skin-divers, whose observations are adding vast lore to marine sci- 

 ence, report that schools of fish do not always take Alight when sharks 

 appear. Why are these fish apparently unconcerned about the presence 

 of predatory sharks? One explanation, as yet unproved, is that they can 

 somehow detect, possibly through varying vibration patterns, the differ- 

 ence between a "hunting" and a "non-hunting" shark. 



Such low-frequency vibrations, however, are apparently picked up 

 by a mysterious sense, peculiar to fish and well represented in sharks. 

 The organ that copes with this sense is apparently the lateral line, a net- 

 work of nerve tunnels which run the length of the shark's body and 

 fan out on its head and jaw. Reaching up vertically from the tunnels 

 are shafts that end as large pores of the skin. The lateral line might be 

 compared to a subway line, the shafts corresponding to the passages that 

 lead from the subway to the stations on the surface. 



The importance to the shark of the lateral line has been dramatized 

 by experiments in which sharks, rendered deaf and blind, still responded 

 to wave motions, such as those produced when a stone is thrown into 

 the water. When the nerves linking the lateral line to the brain were 



