THE PROBLEM AND METHODS OF INVESTIGATION 83 



of worms in each lot compared, and each horizontal interval repre- 

 sents one day. Each point of the curve represents the percentages 

 of worms intact at a given time during the experiment. Each 

 curve is plotted from fifty worms and from examinations two days 

 apart. The curve ab shows the survival time of old, large indi- 

 viduals, the curve ac, that of fifty younger individuals of medium 

 size. 



It will be noted that the relation between survival time and rate 

 of metabolism is the opposite of that observed by the direct method. 



FIG. 4. Planaria dorotocephala: death curves of young and old animals in i . 5 per 

 cent alcohol; ab, curve of fifty old worms; ac, curve of fifty young worms. 



Here the younger animals with the higher rate live much longer 

 than the older with the lower rate. It is also evident that the rela- 

 tion between surface and volume in animals of different size plays 

 no part in the result, for the smaller animals live longer than the 

 larger. The results obtained with cyanide and other depressing 

 agents, and even with low temperatures, are essentially the same. 

 The difference in the ability of the animals to become acclimated 

 to low concentrations of depressing agents is apparent, not merely 

 in the length of life, but in the motor activity. The primary effect 

 of the depressing agent is greater upon the young than upon the 



