AGE DIFFERENCES IN SUSCEPTIBILITY 



97 



in such simple forms as the planarian worms. But as will appear 

 more clearly in following chapters, time is not a correct measure 

 of physiological age in these lower forms. The animal which has 

 lived longer is not necessarily the older: the older animal is the one 

 which has undergone more growth and development, but the 

 amount of growth and development is dependent upon nutrition, 

 temperature, and other external conditions. It is possible to 



Stages • 



II 



III 



R 



Hours I 234567 



Fig. II. — Susceptibility of Planaria macidata to KCN o.ooi mol.: ah, worms 

 12 mm. in length; cd, worms 16-18 mm. in length. 



measure the physiological age of these animals in terms of time 

 only when the conditions of existence are controlled. 



Fig. 12 will serve to illustrate this point. In this figure the 

 curve ah shows the susceptibihty of ten worms nine millimeters 

 long from a stock raised in the laboratory from eggs and only about 

 ten weeks "old," while the curve cb is plotted from worms ten milli- 

 meters long, but which had lived at least a year. The temperature 

 was somewhat higher in this series than in those preceding, and the 

 survival times are therefore shorter than they would be for animals 

 of this age at the temperature of the other series. 



