162 H. L. Brunei 



through the gill clefts. Respiratory air is taken through the opeu 

 mouth and does not enter the nasal cavity under normal conditions. 

 As the respiratory habits of Proteus are similar to those of Necturus, 

 we may assume that the olfactory habits are also the same. 



Summary. 



The more important facts of the foregoing account may be 

 summarized as follows: 



1. In all amphibians which I have examined in a living condi- 

 tion, the usual medium of smell is the same that is used in bucco- 

 pharyngeal (or branchial) respiration. This medium is renewed by 

 regulär oscillations of the floor of the mouth. Under ordinary con- 

 ditions, the intermittent pulmonary respiration has little or no value 

 for olfactory purposes. 



2. The olfactory medium may be either air or water. In adult 

 amphibians the olfactory organ reaches a higher stage of develop- 

 ment in air smellers than in water smellers. There is, however, 

 no marked difference between the olfactory organ of the higher 

 water smellers {Amphiuma and Cryptobranchus) and that of the 

 lower air smellers, such as Amblystoma. Jacobson's organ is pre- 

 sent in all of these forms. 



3. Different mechanisms are used to control the olfactory (re- 

 spiratory) medium in different groups, and even in different stages 

 of the same individual. These mechanisms may be reduced to two 

 types: a) In the first the respiratory medium passes freely inward 

 through the nasal cavity to the mouth, but its return to the nasal 

 cavity from the mouth is prevented by a mechanical valve at the 

 choana. b) In the second type the respiratory mechanism is wholly 

 under muscular control and the olfactory medium passes freely in 

 and out through the nasal cavity. 



4. Corresponding to these two types of the respiratory mechanism, 

 we can distinguish a) Monosmatic forms (single smellers), includiug 

 Necturus and the larvae of Amblystoma and Rana, in which the 

 olfactory organ is used to test only the external medium. For this 

 condition I propose the name monosmesis. b) Diosmatic forms 

 (double smellers), includiug Siren, Cryptobranchus, Amphiuma, larvae 

 of lungless Salamanders, and the adult stage of higher amphibians. 

 In these forms, ingoing and outgoing currents bear odorous matter 



