42 HOMOIOTHERMISM 



differs characteristically and widely (Helff, 1928). 

 Planarians, insects, and mammals after feeding show 

 an increase in respiration which takes place before 

 the food is being digested and appears to be dne to in- 

 creased activity of entoderm cells (Amberson, Mayerson, 

 and Scott, 1924). When animals are desiccated carbon 

 dioxide production increases rapidly when about half the 

 vital limit is exceeded (Caldwell, 1925). In many inverte- 

 brates oxygen tension is related more or less directly to 

 oxygen consumption, but this is not true of a vertebrate 

 like Fundulus, which reaches a critical point and dies 

 when the oxygen supply decreases. The metabolism of 

 a fish cannot fall below a certain minimum limit (Amber- 

 son, Mayerson, and Scott, 1924). Some vertebrates, how- 

 ever, like turtles, which have a large storage capacity in 

 their lungs, can get along for several hours at rather 

 high temperatures without carrying on any external 

 respiration (Lumsden, 1924). 



Many factors influence respiratory processes directly 

 or indirectly. Numerous invertebrates are able to reduce 

 their respiration or cease to breathe altogether for long 

 periods of time when oxygen is lacking in the environ- 

 ment (Pearse, 1926). More highly organized animals, 

 in which more authority has been delegated to nervous 

 control, cannot do this to such an extent and homoio- 

 therms of course quickly suffocate if a considerable sup- 

 ply of oxygen is not continually available. Gesell (1926) 

 has recently suggested that ** changes in the hydrogen- 

 ion concentration of the respiratory center rather than 

 of the blood constitute the prime factor in respiratory 

 control." He believes that oxygen supply controls the 

 amount of lactic acid present and that the effects of this 

 substance and carbon dioxide exert a combined influence 

 indirectly when present in the blood and directly when 



