HEARING 



l8l 



Fig. 12. A Lung-Fish {Lepidosiren paradoxa from ihe swamps of South America) 

 showing tlie lateral line and the tubes connected to it ramifying on the head. 



numerous small holes, and contains many sensory cells that 

 are really touch receptors. These structures respond to 

 vibrations of very low frequency, vibrations too slow to be 

 perceptible to us as sound, and may give an effect of "touch 

 at a distance". By this means fishes probably become aware 

 of objects in the water as they approach them. J. R. Norman, 

 quoting Dr. Barton, says, "this sense enables a fish to rush 

 about in a rocky pool, swerving this way and that to avoid 

 obstructions, for the water resistance is the greater the nearer 

 such water is to a solid body." In trying to understand the 

 nature of the lateral line sense it is relevant to note that in 

 the course of evolution the ear was derived from one of the 

 sense organs of the lateral line. 



In the sharks and rays there are other sense organs in the 

 skin covering the head that may have a similar function. 

 These consist of numerous pits opening on the surface by 

 pores, but not connected to each other by canals. Their 

 internal structure is similar to that of the lateral line organ, 

 but their exact function is not known. 



