440 On the Production of Sounds by certain Fishes. [Apr. 26, 



a larger opening is made in it, the sound -will considerably diminish 

 in strength. If one takes out the swim-bladder, the note will become 

 very weak, but may still be heard ; it is then produced only by the 

 vibrations of the springs. By ordinary observations I have not been 

 able to prove that those bars or cross-walls (transverse septa) which 

 project into the lumen of the chief compartment of the swim-bladder, 

 or its external diverticula, assist in the production of sound ; but if 

 one compares this with what I state later on in Pseudaroides, I believe 

 it would prove very doubtful, on account of their incomplete partition 

 walls, that the diverticula of the swim-bladder even to a great degree 

 serve to strengthen the sound by the air passing to and fro over 

 them. By looking more particularly one will observe that the 

 anterior cutaneous plate at the side of the body also vibrates when 

 the sound is produced. I suppose that the action of the ligament, 

 which connects it with the circular plate of the muscular spring, 

 besides transferring the sound vibrations of the swim-bladder to the 

 water, consists in preventing a too violent recoil of the spring when 

 the muscle is relaxed." 



After describing the air bladder, and the arrangement of its 

 paired extrinsic muscles in Platystoma orbignyanum, and Pseud- 

 aroides clarias, Dr. Sorensen continues (p. 93) : " When the swim- 

 bladder is laid open in the living animal, it is very easy to perceive 

 that the contractions of the previously mentioned muscles [ex- 

 trinsic muscles] occur at the same time as the production of a 

 strong, deep, murmuring sound, whilst the wall of the swim-bladder 

 is put into strong vibratory motion. The majority of the specimens 

 I have examined of Pseudaroides had at the most a total length of 

 2535 cm. The walls of the swim-bladder were, therefore, not so 

 thick, but I was able to distinguish the internal transverse septa as 

 darker transverse Hues ; I could therefore see very distinctly that 

 when the sounds were produced, the septa were in a state of rapid 

 vibration forwards and backwards. This is sufficient to prove that 

 they play a very important part in tending to increase the sounds by 

 the fact that the air vibrates over their free edges, from one chamber 

 to the other. If one makes a small hole in the swim-bladder of 

 Platystoma, the strength of the sounds will not be very much dimi- 

 nished. If an even smaller incision is made the sound becomes fainter 

 and fainter, and at length dies away, even though the muscles are 

 functional." 



" So far as I have been able to see, only one muscular contraction 

 takes place, as in Doras, for every time sound is produced. This 

 always lasts a certain period, is fainter at the end, but ceases sud- 

 denly. About the nature of the sound, the same can be said as I have 

 stated about Doras. The sound a Platystoma produces can be heard 

 at a distance of more than 20 feet, when the animal is on the land." 



