The Ahr-Bladder ii^ 



a highly explosive state, and yet they make the round trip 

 every day of their own volition — two changes of twenty 

 atmospheres every twenty-four hours. The conclusion is that 

 each of these two species can adjust the air-bladder to the 

 conditions, but that it must be done gradually. The lantern- 

 fish is quicker at it than the rock cod, but each has the ca- 

 pacity to accomplish it. The old theory is to some extent 

 borne out, but appears in a new light. The air-bladder, in- 

 stead of being something which helps the fish to change 

 levels, is something which the fish has to guard against, to 

 keep under control. 



How does the fish make these adjustments? If he is one 

 of the species which retain that tube leading from the gullet 

 to the air-bladder, through which his ancestors used to take 

 in air, it might be assumed that he uses this. It may be that 

 in some cases he does, although at the time that he needs air 

 most, to expand his bladder against increasing pressure as he 

 descends, there is no air to be had, for he is under water 5 

 and further, it has been found that in some species the tube is 

 so grown together, or so clogged with mucus, that there is 

 little possibility of anything passing through it in either 

 direction. Such fish, and the many species which lack the 

 tube entirely, must have other methods. Principally, they 

 depend on secretion of gas into, or absorption of gas out of, 

 the air-bladder by the blood. This may seem like a mysterious 

 and unnatural kind of operation, but it is a kind very com- 

 monly used in the life processes, both animal and vegetable. 

 The fish's air-bladder has walls rich with thin-skinned blood- 

 vessels, and these vessels have the ability to put forth gas 

 into the interior of the bladder, thus pumping it up against 

 external pressure, or to absorb gas from the interior, of the 

 bladder, thus preventing it from overexpanding when the 

 external pressure is released. But the process is not, in most 



