Jacobson's Organ and the Respiratory Mechanism of Amphibians. 163 



to the olfactory organ, which is accordingly used to test both the 

 external medium and the contents of the oral cavity. This condi- 

 tion, which is common to all higher vertebrates, may be called 

 diosmesis. 



5. In Single smellers, Jacobson's organ is wanting and the 

 olfactory organ has a very simple condition. In double smellers the 

 olfactory organ is complex and Jacobson's organ is present. 



6. For the developineut of a complex olfactory organ, with an 

 organ of Jacobson, the nature of the olfactory medium is of less 

 importance than the question, whether the animal is a Single or a 

 double smeller. 



Discussion. 



In the preceding pages I have shown that the simple olfactory 

 organ of Necturus is used to test only the external medium. To 

 my knowledge, this condition has not been previously recognized 

 among amphibians, but it is common enough among fishes, in whieh 

 it is associated with a more or less primitive olfactory organ. 



Seydel (1895) has advanced the view that the olfactory organ 

 of Necturus and Proteus is also a primitive one, in which all func- 

 tions, including those assigned to the organ of Jacobson, are per- 

 formed by an undifferentiated olfactory Chamber, which is freely 

 traversed by incoming and outgoing respiratory currents. Anton 

 (1911) has described a primitive organ of Jacobson in Necturus and 

 Proteus, and believes that the choanal valve directs the exspiratory 

 currents to this organ (p. 189), although he recognizes the fact that 

 the valve may also be used to close the choana (p. 196). 



According to the observations presented in this paper, if Nec- 

 turus and Proteus are primitive forms, the choanal valve has pro- 

 bably been a factor in preserving the simple olfactory structure. 

 There is, however, a considerable amount of evidence in favor of 

 the view, that Necturus and Proteus are not primitive forms but 

 permanent, more or less modified larvae of higher urodeles (Boas, 

 1881; Kingsburt, 1905; Norris, 1911; Bruner, 1913). Very im- 

 portant in this connection, is the larval character of the respiratory 

 mechanism of Necturus and Proteus, aud especially fthe fact that 

 the choanal valve of these forms is similar to the choanal valve of 

 larval Salamanders. 



But if Necturus and Proteus are permanent larvae, the choanal 

 valve of these forms offers a reasonable explanation of their peculiar 



11* 



