OLFACTORY REACTIONS IN AMPHIBIANS 647 



tended. Whether stimulation is to take place in air or water, 

 it is extremelj^ difficult to work with solutions or gases abso- 

 lutely standardized. The results of the experiments as con- 

 ducted show that odors when in relation with food are not 

 sufficiently deterrent in action to compel the toad to refuse such 

 food. Odors of natural surroundings may stimulate the toads to 

 certain reactions. At present there is no evidence that odors of 

 soil or water are effective in any degree on the olfactory organ of 

 the toad. 



2. Tadpoles 



In contrast with the tadpoles of the frog, the toad tadpole 

 may be claimed. to possess an olfactory sense, and possibly to a 

 much greater degree than might be supposed. Anatomically 

 considered the two species appear similar. Differences make 

 themselves evident in a closer study of certain parts. The nasal 

 openings in the toad tadpole are relatively larger than in the 

 frog tadpole. The water stream into the nasal passage of the 

 toad tadpole is therefore of greater magnitude than in the frog 

 tadpole. 



Contrary to Exner (78), as quoted by Gaupp ('04), the nasal 

 openings in both species serve for the incurrent water stream. 

 If to this stream are added other substances: dilute solutions of 

 methylene blue or particles of carmine, the toad tadpoles are 

 very quick to respond to the stimulus. Such substances added 

 to the water current flowing into the nasal chamber of the tadpoles 

 of the frog (Rana virescens and R. catesbiana) produce similar 

 reactions. Toad tadpoles react at the immediate entrance of 

 the first substances into the nostril; tadpoles of the frog will 

 permit the stream to flow into the nose for a long period, reacting 

 much more slowly to the stimulus. Probably there is a me- 

 chanical stimulus from the carmine which sets free the response. 



It is more than probable that toad tadpoles recognize certain 

 foods and their odors. AVhen given the choice as between decay- 

 ing animal matter and decaying plant substances the former is 

 preferred. 



