164 H. L. Bruner 



olfactory conditions. Such an explanation is the following: In the 

 typical lung-bearing larvae of higher urodeles the choanal valve 

 exists side by side with a complex olfactory organ, which is desig- 

 ned particularly for adult use under conditions which perarit the 

 olfactory medium to pass freely in and out through the nasal cavity. 

 This choanal valve persists until the time of the metamorphosis, 

 and since it prevents the outgoing respiratory currents from entering 

 the nasal cavity, it is evident that the complex olfactory organ can 

 not perform all of its functions during larval life. If such a larval 

 condition should become a permanent one, we should certainly ex- 

 pect degeneration of the olfactory organ from disuse. 



Under similar conditions, apparently, the olfactory organ of 

 Necturus and Proteus has degenerated and reached its present con- 

 dition. This view is entirely consistent with the occurrence of a 

 rudimentary Jacobson's organ. It explains the difference between 

 the olfactory organ of the ordinary larva and that of the Proteidae, 

 and thus removes a serious objection to the view that the latter 

 are permanent larvae. 



In the case of Siren, the facts are in perfect harmony with the 

 above-mentioned theory, although the history must have been very 

 difterent. If Siren is a modified permanent larva, the degeneration 

 of the olfactory organ has apparently been prevented by the modi- 

 fication of the choanal mechanism, so that the olfactory medium may 

 pass from the mouth into the nasal cavity. In this manner Siren 

 became a double smeller. 



The foregoing views are in agreement with Seydel's theory, 

 that the organ of Jacobson is used primarily to test the Contents 

 of the oral cavity. If Necturus and Proteus are permanent larvae, 

 he loss of Jacobson's organ is simply part of a general process of 

 degeneration which has aifected the entire olfactory organ. In Siren 

 the same conditions which have preserved the olfactory organ as 

 a whole have also prevented the loss of the organ of Jacobson. 



Butler College, Indianopolis, Indiana. 



