FUNCTIONAL ACTIVITIES 



89 



the oxidation or burning of substances in the protoplasm, and 

 for this oxygen is necessary, hence the need of respiration. Ni- 

 trogenous waste (urea) and carbon dioxide are products of the 

 continuingbreakdown of protoplasmic substances by oxidation and 

 by other chemical processes; they act as poisons and must be dis- 

 posed of, if protoplasmic activities are to continue unhampered. 





Fig. 53— TWO SISTER CELLS OF DILEPTUS 

 GTGAS 



B, a normal individual properly nourished; A, an 



individual, sister cell of B, which had been starved 



for two weeks. Same magnification 



From photomicrographs by the author 

 Magnification, 65 



A 



B 



This disposal is effected by the processes of excretion. The 

 continued breakdown of protoplasmic substances leads to a con- 

 tinuous loss of living substance, but the loss is constantly made 

 good through the activities of the function of nutrition. Thus, 

 repair and growth, and finally the vital function of reproduction, 

 follow nutrition. 



Let us see how this works out in the case of a starving Para- 

 mecium, which normally feeds on the bacteria of infusions. Ob- 

 servations of this nature were first carefully made in 1902 by 

 Professor Hans Thure Sigurd Wallengren, a well-known Swed- 

 ish naturalist. He kept normal individuals in tubes of pure 

 water and watched them daily for about eleven days. At the 

 end of this time he found that most of them had died and that 

 the few survivors were much vacuolated and abnormal in appear- 

 ance. After the first day of the experiment they appeared clear 

 and transparent, indicating the loss of their nutritional reserves. 

 After five days small vacuoles (minute cavities filled with a 

 watery fluid) appeared, and these increased in size and number 

 until little more than a skeleton of the organism was left. 



