202 A HISTORY OF FISHES 



The lateral line system has generally been regarded as the 

 seat of a sense akin to "feeling," but it would perhaps be more 

 accurate to describe this sense as combining the qualities of 

 hearing and touch. An American investigator has conducted 

 a series of experiments on living fishes to discover exactly what 

 part these organs play in their daily lives, and his results, 

 although by no means conclusive, are of considerable interest. 

 His method consisted in cutting the nerves supplying the lateral 

 line, and then comparing the behaviour of these injured fishes 

 with normal individuals. He has shown quite definitely that 

 the sense organs are not stimulated by heat, light, food, elec- 

 tricity, salinity, or foulness of the water, oxygen, or carbon 

 dioxide, water pressure or water currents. Nor are they 

 sensitive to sound vibrations as are the ears. The only kind of 

 stimuli that have any eflfect on the normal fish but none on an 

 injured specimen are vibrations of low frequency (about six 

 per second) produced by pulling an aquarium slightly to one 

 side and then letting it go again. As a result of his experiments 

 he concluded that "waves on the surface of the water 

 produced by air-currents, and the disturbances made by bodies 

 falling into the water, produce vibrations in the deeper water 

 that stimulate the lateral line organs." It seems certain that this 

 sense of movements in the water is peculiar to fish, or at least 

 to aquatic animals, and probably enables them to perceive 

 the movements of other fishes or of their prey. Further, these 

 sense organs may serve to notify the fish of its approach to a 

 rock or the bank of a stream, the difference in the pressure on 

 the scales covering the mucous canals being conveyed to the 

 brain by the nerves. As Dr. Barton has written: "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, and one cannot 

 but admire the marvellous muscular response, the extraordinary 

 rapidity of co-ordination of the body of the fish, to the varying 

 stimulation on the lateral-line sense on one or other side of 

 its body." 



That fishes are highly sensitive to electric currents has long 

 been known, and this knowledge has been turned to practical 

 advantage in recent years by the invention of what are known 

 as radio fish-screens, used principally in America. They are 

 designed to keep food-fishes within bounds, and to prevent 

 them from straying into irrigation canals, ditches, mill-races, 

 and other water-courses. The earlier screens were not always 



