ECHO-LOCATION igt 



theory is quite erroneous; although the drum and middle 

 ear are unusual in shape and details they function as in other 

 mammals, and hearing is acute and by far the most impor- 

 tant of the special senses to these animals. 



Whales, like all living creatures, are composed of a very 

 high proportion of water, and consequently water-borne 

 sound waves tend to pass through them without interruption 

 — they are in fact almost transparent to sound. Sound waves, 

 however, find the junction of air and water an almost im- 

 passable barrier; about ninety-nine per cent of sound is 

 reflected at the meeting of the two media. The air spaces 

 below the skull of the whales, connected with the middle ear 

 by the Eustachian tube, probably function, in part at least, 

 by making use of this effect; they stop the sound waves from 

 going straight through the animal by translating them into 

 air waves that can be sent on to the inner ear through the 

 action of the drum and the middle ear. 



It is significant too that in the dominant group of fresh- 

 water fishes, the Ostariophysi, which includes the Carp, 

 Roach, Tench, Bream, Cat Fishes and many others, the air- 

 filled swim-bladder is connected to the inner ear. The swim- 

 bladder is primarily a hydrostatic organ which adjusts the 

 buoyancy of fishes so that they tend neither to sink nor to 

 float and so saves the necessity for muscular effort in main- 

 taining constant depth. In many fishes part of the swim- 

 bladder is arranged so that it lies in touch with the fluid-filled 

 spaces that fill the inner ear, but in the Ostariophysi there 

 is a chain of little bones, derived from the first four joints of 

 the backbone, which connects the front end of the swim- 

 bladder with the inner ear. The similarity of these bones 

 to those of the middle ear in mammals is striking and it 

 is almost beyond question that their function is similar. 

 The water-gas interface of the swim-bladder must certainly 

 act as an interceptor for the sound waves that would other- 

 wise pass through the fish, and the vibrations produced are 

 transmitted by the little bones — the "weberian ossicles" to 

 the inner ear. Hearing in these fishes must be very much 

 more acute and discriminatory than in those which have no 

 such arrangement. Sharks and dogfish, which have no swim- 

 bladder and keep themselves from scraping on the bottom 



