AGE DIFFERENCES IN SUSCEPTIBILITY 



95 



ing. A comparison of the resistance to cyanide of unhatched 

 worms capable of movement with that of worms just hatched 

 shows, as in Fig. lo, that the younger worms have the higher rate 

 of metabolism, although in this case also the difference in age meas- 

 ured by time is no more than a few days. 



But it is only during these earlier stages of the life cycle that 

 the rate of metabolism changes appreciably during such short 

 intervals of time. 

 The rate of metab- 

 ohsm decreases most 

 rapidly during the 

 earher stages, and as 

 development ad- 

 vances the decrease 

 in rate for a given 

 time interval becomes 

 always less. In ani- 

 mals eight or nine 

 millimeters in length, 

 for example, the 

 differences in rate of 

 metaboHsm for an in- 

 terval of two or three 

 weeks, under ordinary 

 conditions of nutri- 

 tion and temperature, 

 and in many cases 

 for a much longer 

 interval, are no 



greater than the differences shown in Fig. lo for an interval of four 

 days immediately after hatching. In still older animals the 

 decrease in rate of metaboKsm under constant conditions is even 

 slower. 



In Fig. 1 1 the susceptibilities of two lots of large old worms are 

 compared. The curve ab is from ten worms twelve millimeters in 

 length, and cd from ten worms sixteen to eighteen millimeters in 

 length. These worms were collected from their natural habitat 



Hours i\ 2\ 2\ 4i si 



Fig. 9. — Susceptibility of Planaria maculata to 

 KCN o.ooi mol.: ab, recently hatched worms; cd, 

 fuU-grown, sexually mature worms. 



