TOWER, PRODUCTION OF SOUND IN CERTAIN FISHES 153 



would be instructive. For the drumming muscles the name musculus 

 sonificus has been suggested and is used in the following discussion.^ 



In young squeteague two inches long, it is impossible to distinguish 

 macroscopically a differentiation of this muscle. But if a piece of the peri- 

 toneum with the underlying fascia is removed and examined under the 

 microscope there are seen striations typical of voluntary muscles. The 

 muscle fibres run in the direction of the short diameter of the fish, i. e., 

 circularly around the air-bladder. These young squeteague have been 

 heard to "drum," and the contractions of the m. sonificiis can be easily 

 felt when the fish is held firmly in the hand. In the young this muscle 

 has not acquired the deep red color that so characterizes it in the adult. 



&I> a. Cim 



of 



of 



air- 



air- 



msv. 



Lateral horns 

 bladder. 



Hinder point 

 bladder. 



The muscles which must 

 make the air-bladder 

 act as a sound pro- 

 ducing organ. 



Fig. 2. Swim-bladder of Micropogon unduiatus after Sorensen. 



Croaker (Micropogon undulatus). — Another scisenoid, known in Amer- 

 ican waters as the croaker, is of interest from an anatomical standpoint. 

 The difference between the bladder of this fish and that of the squeteague, 

 except for its being considerably smaller, is that the central head is not 

 present, and the two lateral horns are reduced to two very small tubes. 

 It is therefore an even more simple organ than that of the squeteague. 

 The two bilateral sonifici have the same arrangement in both animals, 

 and the description of the muscles of one applies equally well to those of 

 the other. Sorensen states that "the form of the air-bladder needs no 

 other description than that given in figure 9" (a copy of which is here 

 appended as fig. 2). 



1 Dr. Hugh M. Smith and Dr. Theodore Gill suggested several anatomical names, from which 

 musculus sonificus was selected as being the most appropriate. The author is greatly indebted 

 to these two well-known ichthyologists for their assistance. 



II' 



