322 BULLETIN OF THE BUREAU OF FISHERIES. 
Packard (1905, 1907, 1908) kept top minnows, Fundulus heteroclitus, in oxygen-free 
water and found that they were able to live about three hours. He believes that these 
fishes must get oxygen from other sources than the atmosphere. His experiments support 
Mathews’ (1905) theory of respiration, which supposes that the oxygen of the atmosphere 
acts as a depolarizer and combines with nascent hydrogen produced during metabolism. 
If oxygen can be replaced by some other substance which will neutralize hydrogen, its 
presence is not necessary. Packard found that he could prolong the life of Fundulus in 
water which contained no oxygen by injecting carbohydrates into the body cavity. 
Wells (1913) determined that abundant oxygen and carbon dioxide was less injurious to 
fishes than a very small amount of both gases or than much carbon dioxide and little 
oxygen. He says (p. 345): “Oxygen in large amounts (10 ¢. c. per liter) antagonizes 
the detrimental effects of high carbon dioxide (50 c. c. per liter).” He also found that 
the most active fishes succumbed to a large amount of carbon dioxide before more slug- 
gish individuals, and that oxygen deficiency was more quickly fatal when the water was 
alkaline than when it had an acid reaction. The observations just reviewed, then, 
indicate (1) that fish are able to live for some time in water without oxygen; (2) that 
lack of oxygen is generally more injurious than excess of carbon dioxide; and (3) 
that gas conditions unfavorable for respiration are more quickly fatal in water with an 
alkaline reaction. 
The general resistance which fishes show to suffocation is in many species assisted 
materially by the use of the swim bladder as a storage reservoir for oxygen. ‘This organ 
apparently serves various functions and in different fishes may be used as a lung, an organ 
for making sounds, a hydrostatic organ, and as a respiratory reservoir. Woodland 
(r911) proved the hydrostatic function by weighing fish after subjecting them to differ- 
ent pressures. The use of the bladder as a storage reservoir for oxygen has been the 
subject of a number of investigations. Tower (1902), for example, studied a number of 
miatine fishes and found there were three gases present—oxygen, carbon dioxide, and 
nitrogen; that the amount of carbon dioxide might increase a little (0.25 per cent) 
during suffocation, but that it was usually 0.06 to 0.1 per cent of the total gases; 
and that the deeper the water from which fishes were taken, the higher the proportion of 
oxygen (in some fishes captured at considerable depths the gas in the bladder was 
practically all oxygen). Bridge (1891) showed that the secretion of gases into the swim 
bladder was under the control of the nervous system; and he found that there was 
usually an increase in the amount of carbon dioxide when the fish died of suffocation. 
The Cambridge Natural History states that in general the amount of oxygen in the 
bladder is less in fresh-water fishes than in those from the ocean. In fishes like the 
perch, in which the swim bladder has no duct connecting it with the outside, its functions 
are confined to storing reserve oxygen and regulating the specific gravity of the body 
(hydrostatic function). Finally, the density of the water in which fishes live affects 
respiratory activities. As pressure grows greater on account of increase in depth, the 
ability of the water to absorb gases is also increased. Furthermore, the comparative 
“hardness” or ‘‘softness’’ of water not only affects the density, but has a marked 
influence on the pressure of the gases present in solution. Sumner (1906) asserts that 
the membranes of fresh-water fishes are highly adapted to resist changes in density. 
There is an “irreducible minimum”’ of salts in the blood which is not released even when 
