126 THE COMMON FROG. [chap, 



slightly striated portion of the skin of the body. 

 The Eustachian tube, however, still exists in the 

 frog, though it is short and wide, and the opening 

 of each is to be seen on one side of the back of 

 the mouth. 



This condition of things, however, does not exist 

 in all the members of the frog's order, still less of his 

 class. But in Pi'oteits, Siren, and M cnobrancJms there 

 is no tympanic cavity whatever, and the organ of 

 hearing is simply imbedded in the skull, and pro- 

 bably responds but to sonorous vibrations conveyed 

 to it by the denser aquatic medium, and not at all, 

 or but very imperfectly, to those of the atmosphere. 



In the ordinary efts we still meet with an Eus- 

 tachian canal, but the tympanum is absent. 



In the frog's own order, as in Pelotates and Bom- 

 binator, we may fail to find any tympanum, while 

 the Eustachian tubes are all but obliterated, being 

 reduced to the most minute dimensions. 



Another condition, however, may be presented 

 which offers a singular contrast, and is the more 

 remarkable from the widely separated geographical 

 situations of the forms which present it. In the 

 South American Pipa, as well as in the South African 

 DactyletJira, the two Eustachian tubes run together 

 and open at the back of the mouth, by a median and 

 common aperture. 



Strange to say, this is the very condition which 

 exists in birds, though most certainly it cannot be 

 taken as any sign of affinity. In the crocodile these 

 tubes have also a common median opening, but, un- 

 like birds, each tube has also its own lateral opening 



