404 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 5° 



for the organs of movement, the mobility of the parts was assured 

 by surrounding them with air-sacs. 



The lengthening of the neck, produced by quite a different adapta- 

 tion, made necessary an increase in the quantity of air moved during 

 respiration. This demand was met by air-currents generated through 

 a rhythmical change in the volume of the air-sacs. The connection 

 of the air-sacs with the lungs is a consequence of their phylogenetic 

 development, which is repeated in their embryological development, 

 and has no physiological significance other than that the air-sacs 

 assist in renewing the air in the trachea. 



