520 THE GANNET 



Dr. Gadow points out that it is necessary, according 

 to their connections, to distinguish between pulmonary 

 air-sacs, of which there are ordinarily five pairs, and which 

 make their appearance in a bird before it is hatched, and 

 the naso-pharyngeal air-sacs which are restricted to the 

 head, occasionally extending to form a pouch. " The outside 

 of the sacs," he continues, " frequently possesses a covering 

 of involuntary or of voluntary muscles ; for instance, in 

 Vultures, Gannets, and Flamingos a thin fan-shaped muscle 

 extends from the furcula over the interclavicular air-sac. 

 Through contraction of these muscles the cells can be 

 emptied of air." 



In the Gannet we are more particularly concerned with 

 the subcutaneous air-sacs, although no structural difference 

 exists between them and the internal air-sacs, both being 

 ahke, connected with, and supplied from, the lungs. 

 There are several genera of birds in the world besides 

 Gannets Avhich have subcutaneous air-sacs, while all 

 species, even those which have become flightless, remarks 

 Mr. Pycraft, have internal ones. 



With regard to such birds as the Stork, the Fla- 

 mingo, the Adjutant, the Hornbill, the Crested Screamer,* 



* An example of the Screamer, Chauna chavaria, which died in the 

 Gardens of the Zoological Society, gave an opportunity for a good 

 description of the air-sacs by Mr. Beddard ("Proc. Zool. Soc," 1886, 

 p. 178). This bird has the power of soaring at great altitudes where the 

 density of the atmosphere would be much less than on the earth's surface. 



