114 F - G. HALL. 



SUMMARY. 



1. The normal gaseous content of the swimbladder of fresh- 

 water fishes near the surface is approximately that of the atmos- 

 phere. The composition varies with the species, pressure, 

 temperature, amounts and kinds of dissolved gases, and with 

 the seasons of the year. 



2. When fishes are placed in water containing little or no 

 oxygen the oxygen in the swimbladder diminishes; indicating 

 that the swimbladder may act as a reservoir on which the blood 

 may draw for oxygen in times of need. A perch is enabled to 

 go into water of low oxygen content without asphixiation. 



3. The effect of increased pressure in the surrounding water 

 is to increase both the percentage of oxygen and carbon dioxide 

 in the swimbladder. 



4. If carbon dioxide is increased in the medium in which perch 

 are living, the volume of the fishes is changed and the fishes auto- 

 matically rise in the water. This response would be of adaptive 

 value, causing the fish to move out of deeper water containing 

 larger amounts of carbon dioxide into the safer zones above. 



5. The primary function of the swimbladder of most of the 

 fresh-water fishes is hydrostatic. 



6. Perch apparently possess no voluntary muscular control 

 over the size of the swimbladder. 



7. Under conditions where high oxygen percentages were 

 found in the swimbladder, a higher tension of the gases existed 

 than in the blood. This indicates an active secretion. 



8. Under conditions where fishes are not changing their depth 

 rapidly, the gases in the swimbladder are probably kept constant 

 by simple diffusion of gases from the blood. 



9. A "rete mirabile" partially surrounds the walls of the swim- 

 bladder and furnishes a rich supply of blood. It is apparently 

 the means by which the gases are transferred from the blood to 

 the swimbladder. 



10. The mechanism by which gas is secreted into the swim- 

 bladder can apparently be explained on a chemical and physical 

 basis. The writer's experiments show that the hydrogen ion 

 concentration of the swimbladder gland is increased by external 

 stimulation. This indicates the secretion of a substance by 



