CHAPTER IX 



adaptations (continued) 

 Production of Sound, Light and Electricity 



Voice of fishes. Stridulating organs. Fishes which possess luminous 

 organs. Microscopic structure of luminous organs. Electric fishes. Structure 

 of electric organs. General remarks on evolution. 



OUR knowledge of the voice of fishes is not very complete 

 for several reasons. Although water is an excellent 

 conductor of sound, the vibrations do not pass easily 

 from the water to the air, and an investigator cannot well re- 

 main under water while he is making experiments on the sub- 

 ject. Sounds are therefore usually heard when the fish emitting 

 them is taken from the water, and then there is often some 

 doubt whether it is accidental or is produced by special mus- 

 cular action in the normal life of the fish. There are, however, 

 many cases in which there is no doubt about the voice, or the 

 special mechanism by which it is produced, and the majority of 

 these cases occur among the Teleostei. One of these of which 

 the present writer can speak from personal experience is 

 that of the sapphirine gurnard, Trigla /rir/mdo, known among 

 the North Sea fishermen as the tub or latchet. This fish emits 

 distinct sounds which may be described as a succession of short 

 grunts. The sounds are produced by the air-bladder, and the 

 mechanism was investigated by the French naturalist Moreau 

 in 1864. The air-bladder is divided by a transverse diaphragm 

 perforated by a hole in the centre; the diaphragm itself con- 

 tains radiating and circular muscle-fibres, and the bladder has 

 thick strong external muscles of the striated or voluntary kind, 

 supplied by two large nerves from the anterior part of the 

 spinal cord. The diaphragm, according to Moreau, is thrown 

 into vibration by air being forced from one compartment of the 

 bladder to the other through the aperture. Some of the other 



4°5 



