Reighard. — Senses and Learning in Fishes 145 



to slow vibrations. The same slowing or stoppage of 

 breathing took place when a stream of water was played 

 on the lateral line. 



Meantime Hofer (1908), had investigated the lateral- 

 line organs of five species of European fresh-water fish. 

 He did not find them sensitive to vibrations, i.e., back 

 and forth movements of the water, however slow, so that, 

 in this particular he does not agree with Parker. He 

 found the lateral line organs in these fish sensitive only 

 to continuous pressure of the water in one direction, that 

 is to currents. They enable the fish to detect currents 

 and to regulate the position of its body and its move- 

 ments with reference to them. This view has been con- 

 firmed by Steinmann (1914), who found that the fish 

 upon which he worked tended to turn so as to head 

 against a stream of wafer played against the lateral line. 

 Hofer believes that fish detect by the lateral line organs 

 the presence of unseen solid bodies such as stones, and 

 thus avoid colliding with them. As a fish swims for- 

 ward its body ordinarily thrusts aside the water. As it 

 approaches a solid there is resistance to the displacement 

 of the water so that a back-pressure is produced which 

 affects the lateral-line organs. In this way a moving 

 fish is made aware of its approach to a solid body and 

 is able to avoid it even in darkness. 



2. The Sense of Hearing in Fishes. 



Vibrations more rapid than fifteen per second affect 

 our ears and are perceived as sounds. Our ears consist 

 of three principal parts, the external ear, the middle ear 

 and the internal ear. The structures of the external and 

 middle ears are not sensitive to sound, but serve merely 

 to collect the sound waves and conduct them to the in- 

 ternal ear, which alone is sensitive to them. 



In fishes no trace of any part of an ear is usually visi- 

 ble on the outer surface of the head and this gives rise 

 to the impression that fishes lack the ear and are there- 

 fore deaf. But although the external and middle ears 



