150 ANNALS NEW YORK ACADEMY OF SCIENCES 



communicates with the exterior. Besides these kinds of sound production, 

 which are of no special interest in this discussion, there are tu'o others. 

 One is the drumming of the squeteague, croaker and other drumfishes 

 (Scisenidse) ; the other is the so-called grunt of the sea-robin (Prianotus) 

 and the common toadfish (Opsanus). With the difference in the kind of 

 sound made by the drumming and the grunting fishes there will be found 

 to be a distinct difference in the structure of the swim-bladder, which is 

 the organ chiefly involved in the production of sound by these species. 



Anatomy of the Swim-Bladder. 



The Drumfishes. 



Bearded drum {Pogonias cromis). — The swim-bladder of the drum is 

 characterized by its large size and the enormous number of its diverticula. 

 The bladder occupies, as is the case in nearly all of the sciaenoid fishes, 

 the entire length of the abdominal cavity. The diverticula are finger-like 

 processes which arise laterally from the bladder and open into its large 

 cavity. These tube-like appendages in the adult ramify through the con- 

 nective tissue, and in many cases adhere firmly to the aponeuroses of the 

 neighboring muscles. The air-bladder itself lies free in the abdominal 

 cavity, attached on the dorsal side to the body of the fourth vertebra and 

 covered on the ventral side by the peritoneum, which is continued from 

 the parietal walls. When examined carefully, the air-bladder is seen to be 

 made up of three layers: the outside is of a hyaline character and is com- 

 posed of extremely tough fibrous tissue; the middle layer, which is sepa- 

 rated from the outer layer only with great difficulty, is connective tissue 

 containing elastic fibres ; the inner layer is a very delicate connecting tissue, 

 lined with pavement epithelium. Jager (1903) has recently discovered 

 that this inner layer does not cover the entire bladder-lumen, but on the 

 dorsal surface there is an oval space in which the inner layer disappears, 

 with the exception of the pavement epithelium. This space he calls the 

 "oval," and maintains that it can be increased or diminished by the action 

 of small muscles. In the middle layer ramify all the blood-vessels, which 

 break into small branches and then enter the inner layer, where, in the 

 region of the "oval," they form an anastomosing capillary net-work almost 

 as complete as is found in the "red-body." This net-work is thus sepa- 

 rated from the lumen of the air-bladder only by the single layer of pavement 

 epithelium. The function of the "oval," according to Jager, is the ab- 

 .sorption of oxygen and the diminution of the amount of gas in the bladders 



