PHYSIOLOGICAL SENESCENCE IN HYDROMEDUS/E. 53 



course a considerable range of variation in different individuals, 

 but are nevertheless striking features of the behavior of these 

 animals when attention is directed to them and they remind one 

 irresistibly of age differences in behavior in much higher animals. 

 The apparent restlessness and spontaneity, the more rapid pul- 

 sation rate, and the greater irritability of the young animal as 

 compared with the old all suggest that in these, as in other 

 organisms, so for as known, the life history of the individual is 

 from an early stage a process of physiological senescence which 

 expresses itself dynamically as a decrease in rate or intensity of 

 the fundamental energy-liberating metabolic reactions. 



SUSCEPTIBILITY. 



In determining the susceptibility of the animals various other 

 criteria besides death and disintegration may be used as a check, 

 viz., cessation of rhythmic pulsation, the disappearance of mus- 

 cular contractility and shrinkage of jelly in some agents. As 

 regards the relation between susceptibility and age all of these 

 criteria give the same results. It is usually difficult or impossible 

 to determine the exact time when an animal ceases to respond to 

 stimulation by rhythmic pulsation or by slow muscular contrac- 

 tions and the early stages of shrinkage and of disintegration are 

 not less difficult to determine. The times given in the following 

 tables represent approximations only. Frequently pulsation or 

 contractility is present at one observation and absent at the next, 

 and the time of its disappearance can only be estimated. Never- 

 theless there is no difficulty in distinguishing the differences in 

 susceptibility between young and old animals. The experiments 

 were mostly performed with single pairs, one young, one old, but 

 in some cases two or three of each were used. 



ctzrulescens . 



This, being one of the most abundant species, was used to a 

 large extent. The smallest and youngest individuals found were 

 8-10 mm. in diameter, without gonads, the largest, and oldest, 

 sexually mature animals 60-75 mm. in diameter. The chief 

 results are tabulated below. In these tables the sign following 

 a given time indicates that this is estimated from two observa- 



