MUSEUM OF COMPARATIVE ZOOLOGY. 13 



of bringing the slit in the roof of the throat, through which the gas 

 must be forced, into a position more favorable for its escape. If the 

 opening through which the gas is obliged to pass is directed downward, 

 its expulsion will require greater effort than if the opening is directed 

 sidewise. An advantage depending on the same physical properties of 

 the gas may perhaps also explain the universal habit of coming to the 

 surface of the water to disengage the bubble. At least, the pressure of 

 the water to be overcome in forcing out a bubble when the fish is at a 

 considerable depth must be greater than when it is near the surface. 



The escape of gas, which may be several times repeated during the 

 process of swallowing a large fish, shows clearly enough that the bubbles 

 are not simply air taken in at the mouth to be immediately discharged 

 through the gill openings. The repeated emission of gas from the same 

 fish, without the possibility of any fresh air having been taken in through 

 the mouth, even led me to conjecture at one time that air was never 

 taken in through the mouth. At least, it was certain that the young 

 fishes often discharged gas without lifting any portion of the body out 

 of the water. 



The rate at which gas escapes from the gill openings is extremely 

 variable, depending on the temperature of the water, the recency of 

 feeding, etc. Perhaps the following observations give a fair idea of 

 the rate during a warm summer day. In the course of ten minutes 

 (August 6) eight fishes together emitted forty bubbles, or an average of 

 one in two minutes for each fish. On August 17, a single fish, G2 mm. 

 long, came to the surface six times in ten minutes, and caused bubbles 

 to escape from the gills. 



In the case of older fishes, snapping at the prey is frequently accom- 

 panied by an escape of gas, which is apparently involuntary, the sudden 

 motion of the head and the opening of the jaws being sufficient to cause 

 the escape of a few bubbles. 



The nature of the gas will be considered in the following section. 



III. The Respiratory Function of the Air-Bladder. 



Lepidosteus, as well as some other fishes, has the habit of coming to 

 the surface of the water and emitting through the gill slits or the oral 

 opening bubbles of gas. This habit has attracted the attention of all 

 who have had the opportunity of examining the fish while living. 



Poey ('55, p. 13G) observed that, when placed in a basin of water, 

 "every five or eight minutes he [Lepidosteus] would come to the sur- 



