178 THE SENSES OF ANIMALS 



sending nerve impulses to the brain, though some fishes 

 mentioned below have a modification that greatly increases 

 the efficiency of hearing. 



The range of hearing in fishes is almost certainly not as 

 great as in other vertebrates; it is likely that although the 

 hearing of fishes may be acute, it is not very discriminatory 

 so that all notes sound much the same to them. It is easy to 

 show that fishes are not hard of hearing by gently tapping 

 the side of an aquarium, but the sounds to which fish respond 

 need not originate under water for fish in ponds very soon 

 learn to come to be fed in response to the ringing of a bell 

 or other sound signal given from the bank. 



Returning for a moment to snakes, we can understand 

 that they probably hear much as do fishes. It is true that 

 they live in air and that air-borne sounds are not likely to 

 be distinctly appreciated, but vibrations, especially those of 

 longer wave-length, may well be transmitted to them through 

 the ground, with which they are in closer contact than most 

 vertebrates. Although snakes have no ear-drum the middle 

 ear is not wholly degenerate, for the little rod of bone is 

 present, albeit embedded in muscular and fibrous tissue, and 

 vibrations of sufficient strength to be conducted through the 

 tissues will reach the inner ear as they do in fishes. 



Throughout the vertebrates the basic pattern of ear struc- 

 ture is the same, and consequently if we exercise due caution 

 we may legitimately draw inferences about the hearing of 

 other vertebrates from our own subjective perception of 

 sounds. In the invertebrates on the other hand such assump- 

 tions cannot be so easily made. Most of the invertebrates 

 respond to air- or water-borne vibrations, but it is impos- 

 sible for us to guess the nature of the sensations produced 

 in them by those vibrations. In many there are no organs 

 whose function we can identify as being concerned with 

 hearing; in many others which have organs consisting of a 

 small bag of sensory cells associated with one or more 

 otoliths the function generally appears to be associated 

 with balancing or orientation in space rather than with 

 hearing, although vibrations impinging upon them would be 

 expected to impart motion to the otoliths. There are some 

 invertebrates, particularly among the insects, that un- 



