154 A HISTORY OF FISHES 



to bear in mind the following facts. Firstly, the complicated 

 apparatus often present for concentrating the beam of light 

 and for regulating its intensity, together with the abundant 

 nerve supply to each organ, suggests that the emission of light 

 can be controlled by the fish. Secondly, the position of the 

 main organs on the sides and belly of the fish, and the presence 

 of special organs in the neighbourhood of the eyes and jaws, 

 provides evidence that they may be used to light up the sur- 

 rounding water in front of and beneath the fish. Thirdly, and 

 this seems to be a fact of some importance, the number and 

 arrangement of the photophores exhibits considerable variation 

 in the diflferent genera and species, but, with the exception of 

 small differences, remains constant in any particular species. 

 Indeed, in the Lantern-fishes [Myctophidae) the number and 

 pattern of the photophores provides the most important 

 character for distinguishing the different species. Finally, 

 there is some evidence that the colour of the emitted light 

 may vary in different fishes. It seems probable, therefore, 

 that the luminous organs may fulfil the same function among 

 the dwellers in darkness or semi-darkness as do the spots and 

 stripes of pigment in many Httoral fishes, and that one of the 

 important uses of these structures is to act as recognition marks, 

 enabling their possessor to pick out another individual of its 

 own kind, and thus assisting the members of a shoal to keep 

 in touch with each other. 



There is a widespread and popular belief that fishes possess 

 no voices. This is quite untrue, for, although incapable of 

 vocal efforts comparable to those of mammals and birds, a 

 number of Bony Fishes produce sounds of one sort or another, 

 and some forms are provided with special sound-producing 

 organs. These may be associated with the air-bladder, fin- 

 spines, vertebrae, and so on, and provide another example of 

 the assurnption of new duties by organs originally designed for a 

 totally different function. Owing to the impossibility of an 

 investigator remaining under water for any length of time, it 

 is very difficult to obtain definite data on the production of 

 sound, and, at the same time, it is by no means easy to dis- 

 criminate between sounds actually due to the action of vocal 

 organs and those of a purely accidental or abnormal nature. 

 The simplest type of sound, for example, is that produced 

 merely by the expulsion of air from the air-bladder through 

 the pneumatic duct, and the grunting or gurghng noises made 

 by fishes as they are taken from the water, including the so-called 



