PHYSIOLOGICAL SENESCENCE IN HYDROMEDUS/E. 57 



the lower limit of concentration for^temporary inhibition only 

 the young member of a pair is inhibited; with somewhat higher 

 concentration both are inhibited for a short time but the young 

 animal usually resumes pulsation before the old if the differences 

 in age are not too extreme. As the concentration increases, the 

 length of the inhibition period increases more rapidly in the young 

 than in the old, until about the upper limit permanent inhibition 

 occurs at once in the young animal while some slight pulsation 

 may reappear sooner or later in the old. In short the age dif- 

 ferences in susceptibility appear here as in other features of the 

 action of external agents. 



Experiments on recovery after temporary exposure to an agent 

 showed that the relation between recovery and age is the same 

 as in other forms examined. The reappearance of the pulsation 

 response to stimulation is the criterion of recovery used. Where 

 the concentration is not too high, the exposure too long or the 

 difference in age between the animals too great, the young will 

 recover before the old on return to water, but with higher con- 

 centrations longer exposures and greater differences in age the 

 old animal recovers earlier and usually more completely than the 

 young. These relations between recovery, age, concentration 

 and period of exposure are of course merely a special case of the 

 age-susceptibility relation. To higher concentrations or longer 

 times of exposure the young animal is more susceptible than the 

 old, but to low concentrations or short times of exposure it is able 

 to adjust itself more rapidly or to recover more rapidly afterward 

 than the old (Child '15, 7 i6a). The degree of difference between 

 young and old animals also differs widely with different agents, 

 because acclimation to some agents occurs much more rapidly 

 than to others. The observations on recovery and its limits are 

 as yet only fragmentary and further work is necessary before a 

 complete statement can be made. 



Some of the data of an experiment with HC1 772/500 will serve 

 as an example of recovery. Animals 15 mm. and 60 mm. are 

 placed in HC1 m/$oo and at intervals a pair, one of each size, is 

 returned to water. In the young animals all pulsation ceases 

 at once in HC1, in the old it ceases after two minutes. On return 

 to water after 5-10 minutes in HC1 pulsation usually reappears 



