244 



CHEMICAL SENSES 



Figure 9 Blinker trail of lemon shark, continuing record shown in Figure 8. Shark circles 

 repeatedly in corner of pen where strongest current enters, despite the fact that the 

 stimulus comes from opposite corner and moves along olfactory corridor labeled S. 

 Record covers approximately 1 min of time. 



It was not the aim of these studies to establish absolute thresholds for the 

 effectiveness of the chemical stimuli tested. To a considerable extent, judg- 

 ments of "thresholds" necessarily depend on the refinements of criteria in 

 judging EEG effects or behavioral changes. However, the results with the 

 most effective stimuli indicate that the range of thresholds for those com- 

 pounds lies somewhere between 10~ 6 and 10~ 9 molar concentrations, under 

 the test conditions. 



Summary of EEG /Behavioral Results 



The main reason for the survey of various categories of pure chemicals was 

 to ascertain whether the chemoreceptors were responding to a narrow range 

 of ionic or molecular structures, which might make it possible to charac- 

 terize the actual receptor sites involved. The experimental system was simpli- 

 fied somewhat by narrowing the receptors to olfactory receptors in the 

 nasal sac, for all the responses noted were eliminated when a local anesthetic 

 (procaine) was introduced into the nasal sac before testing; removal of both 

 barbels from experimental nurse sharks, however, did not diminish their re- 



