260 J. S. NICHOLAS 



must be screened for accurate observation in experimental work. 

 The screening has the added advantage of reducing the amount 

 of light in the aquarium and rendering the animals more quiet 

 than they are under a light of higher intensity. The animals 

 have a shght tendency to be more restless than usual under 

 conditions of hunger. 



RESPIRATORY MECHANISM 



Bruner ('14 a) has described the mechanism of the olfactory 

 sense organ in relation to the respiratory mechanism in a com- 

 parative study of a number of Amphibia. He states that in 

 Amblystoma larvae the olfactory process is similar to that of 

 frog tadpoles. "Respiratory water is taken through the nos- 

 trils and is the only medium of smell." He finds that the passage 

 from the mouth to the nasal cavity is occluded by a fold of mucous 

 membrane, the choanal valve, which prevents either air or water 

 from passing out in this direction. These choanal valves per- 

 sist until metamorphosis. He distinguishes in general two dis- 

 tinct types of respiratory mechanism: in one the respiratory 

 medium is allowed to pass from the nasal cavity to the buccal 

 cavity, the passage of return being occluded by the choanal 

 valves; in the other the respiratory mechanism is completely 

 under muscular control and the respiratory medium can" pass 

 freely in and out of the nasal cavity. According to the type of 

 respiratory mechanism present, he classifies the forms as "monos- 

 matic, single smellers, in which the olfactory organ is used to test 

 only the external medium; and diosmatic forms, double smellers, 

 including Siren, Cryptobranchus, Amphiuma, larvae of lungless 

 salamanders and the adult stage of higher amphibians." In 

 the diosmatic forms, the olfactory organ is used in testing the 

 content of the buccal cavity as well as that of the external 

 medium. 



Bruner ('14 b) mentions that Jacobson's organ is present in 

 Amblystoma adults and that, from the existence of this organ, 

 this form should be classed as a diosmatic form. 



As a preliminary to the experiments on the olfactory sense, it 

 was deemed expedient to make a study of the water currents 



