104 PRIMITIVE ANIMALS 



in the Bony-fishes and Dipnoi it is functionless and 

 does not open to the exterior. In the passage from 

 the water to the land the Vertebrate has undergone 

 a remarkable and important change in respect to the 

 spiracular gill-cleft and the hyoid arch behind it, 

 these structures being brought into the service of 

 an entirely new organ, the sense organ of hearing 

 (Fig. 22 B). Animals living in the water have no 

 use for an organ of hearing, and in fact such an 

 organ is unknown in water animals of all kinds, the 

 only animals which can hear being the land Verte- 

 brates and certain terrestrial insects. The ear of the 

 fish, consisting of certain canals buried in the wall of 

 the skull, is not an ear at all in the sense of being 

 an auditory organ, its function being purely that of 

 balancing. The land Vertebrate, beside possessing 

 the original balancing organ of the fish, or internal 

 ear, has developed an accessory mechanism for 

 collecting and transmitting sound-waves in the air 

 to a specialized part of the internal ear, which is 

 supplied with a branch of the eighth cranial nerve 

 and is sensitive to the sound-waves. The accessory 

 mechanism developed by the land Vertebrate consists 

 in an external ear, which may be absent or rudi- 

 mentary, a tympanum or ear-drum on which the 

 sound-waves strike, and a middle ear or cavity 

 through which the sound-waves are carried to the 

 internal ear by means of a chain of small bones, the 



