82 SENESCENCE AND REJUVENESCENCE 



in cyanide and the death of the last part of the body in the animals 

 of each lot. Thus the differences in susceptibility of two or more lots 

 of worms are evident at a glance, for the farther to the right the 

 curve lies, the less the susceptibility, and vice versa. In Fig. 3, for 

 example, the susceptibility of the young worms, as indicated by 

 the curve ab, is very much greater than that of the old worms, as 

 indicated by the curve cd. 



The susceptibility curves in the following chapters are all drawn 

 in the same way as those in Fig. 3 and from data similar to those in 

 Table I. In general this method is more convenient than the 

 indirect method described below, and the results are less likely to 

 be affected by complicating factors. 



THE INDIRECT METHOD 



By this method the susceptibility or physiological resistance to 

 the depressing agent is determined indirectly, through the ability 

 of the animals to become acclimated to a given concentration. In 

 general, but with certain exceptions, the ability of an animal to 

 acclimate to the cyanides or other depressing agents varies with the 

 rate of metabolism, that is, animals with the higher rate live longer 

 than those with a lower rate. In experiments by this method a 

 concentration of the agent used is determined which does not kill 

 the animals directly, but allows more or less acclimation. The 

 concentration to be used depends to some extent upon the condition 

 of the animals to be tested. For those with a high rate of metab- 

 olism higher concentrations are necessary than for those with a 

 low rate. With different temperatures also different concentrations 

 must be used. For Planaria dorotocephala at temperatures near 

 20 C., potassium cyanide, o . 00002-0 . 00004 m l- (o. 00013-0 . 00026 

 per cent) serves in most cases and i-i| per cent alcohol or 0.2- 

 0.3 per cent ether gives essentially the same results. The details 

 of technique and certain complicating and limiting factors have 

 been considered elsewhere (Child, 'n, '130, '140). 



The results of such experiments are best presented in graphic 

 form. Fig. 4 shows the different ability of old and young indi- 

 viduals of Planaria dorotocephala to acclimate to ij per cent alco- 

 hol. Each small interval represents 2 per cent of the total number 



