70 BRAIN AND BODY OF FISH 



have proved. Noise is made when for some reason or other bubbles 

 are formed, and when an animal dives into the water after its prey, 

 it leaves no air cavity to make a noise. However, when a body 

 moves so quickly through the water that it does leave a cavity 

 behind it, the water rushes into the vacuum with all the sudden- 

 ness of an explosion and is the cause of the noise made by moving 

 steamers. This effect, called cavitation, is well known to naval 

 engineers, as it has often a destructive effect on ships' propellers. 



A fish moves with little fuss and very fast because it leaves no 

 whu'ls behind it. This is simply due to its lines, as a naval architect 

 would express it. There is a fisherman's saying that a cod's head 

 and mackerel tail make for speed. The writer was the happy pos- 

 sessor of a small yacht designed on these principles, and it is a 

 ■constant source of surprise to his guests, how quietly she sails with 

 no wave or fuss with a fresh beam wind. The design of a modern 

 air liner which recently appeared in the press, recalled the form of a 

 gurnard, even to the downward slope of the prow-like head, and 

 showed that the body had been designed for the least air resistance. 



A fish has, therefore, the right shape for moving through the 

 water without noise and without unnecessary effort. Sir William 

 Bragg concludes, " that as animals that move under water can do 

 so with such little noise, it is to be expected that they cannot or do 

 not listen for sounds." However, there is other evidence that can 

 be brought forward that suggests that this conclusion is open to 

 adverse criticism. Dr. Beatty, of the Department of Scientific 

 Research and Experiment under the Admiralty, also writes on the 

 silence of the sea. He says, " the noises that at rare intervals float 

 upwards from the hydrophone are of a somewhat bizarre character ; 

 in the vicinity of an oyster-bed, a continuous rattle is heard from the 

 closing sheUs ; whales produce, by some mechanism as yet unknown, 

 a noise like the tinl^ling of a spoon on a plate ; and there is the 

 phenomenon of the drumming fishes. In the surface layers there 

 are the splashes due to leaping fish ; there is the roar of" breakers " 

 and the " brabble " of the beach along the coast, and at river 

 mouths, and the swirl and rip of off-shore conflicting currents. 



Some degree of hearing power for such sounds may be advan- 

 tageous. When herrings swim, that is, come to the surface, they 

 ■discharge bubbles from their swim-bladder and these bubbles 

 bursting must produce some sound and it is significant that herrings 

 have an accessory organ of hearing. The question of drumming 

 must now be discussed fully, and we will deal in the first place with 

 the Maigre " Sciaena aquila," an inhabitant of the INIediterranean 

 Sea.. This fish is well known to possess a voice, the production of 



