RESPIRATION 35 



9. Since respiration is an essential, vital function, Adaptive 

 we find many beautiful adaptations connected with 



it. The body is suited to what may be called normal 

 conditions, but under special circumstances it has the 

 power of adjusting itself to a certain extent to the en- 

 vironment. Thus Dr. E. C. Schneider and others 

 made experiments on Pike's Peak, Colorado (14,109 

 feet above sea level), and found that at this high 

 altitude the rate of blood flow was increased from 30 

 to 76 per cent, and the number of red blood corpuscles 

 in circulation was very appreciably increased. Ob- 

 viously such changes would facilitate the carriage 

 through the body of the diminished supply of oxygen, 

 and thus make up for the disadvantages of the rarefied 

 atmosphere. Such plastic adaptations, if we may so 

 call them, have of course their basis in the structure of 

 the organism, just as in the case of an automobile 

 constructed to run on "high" or "low." 



10. The evolution of the respiratory apparatus in Evolution of 

 diverse forms is very interesting. In the lowest 

 animals oxygen is merely absorbed through the surface 



of the body. These animals being aquatic or parasitic, 

 the oxygen obtained is that dissolved in the fluid, 

 usually water, in which they live. The amount re- 

 quired may be small, but differs in different forms ; 

 thus coral animals flourish only where the water is in 

 motion, near the surface, especially where there is surf. 

 The tumbling waters inclose many bubbles of air, and 

 some of the oxygen is dissolved. Many aquatic animals, 

 as mollusks (Fig. 66, page 249) and various insect larvae 

 (young of May flies, etc.) possess external gills, which 

 are branched processes of a delicate nature, rich in 

 blood vessels. These take up oxygen, but in some 

 cases serve the needs of the animals only under special 

 environments. Thus May fly "nymphs," as they are 



