50 C. M. CHILD. 



expected, if age differences exist, to be physiologically older. As 

 has been shown for Planaria, and other simple animals (Child, 

 '15, Chapter IV. VII.) physiological age is not necessarily a 

 function of the length of time which the individual has lived, for 

 nutrition, agamic reproduction and various other factors play a 

 role in determining the physiological age at any given time. 

 Size is therefore a far more adequate criterion of age in such forms 

 than time. Moreover, since the development of the gonads in 

 medusae progresses from a relatively early stage to sexual ma- 

 turity the condition of the gonads also serves as a measure of the 

 stage in its life history which the animal has reached. 



The chief method employed was that of determining the sus- 

 ceptibility of younger and older animals to various agents. The 

 general relation between susceptibility and age has been dis- 

 cussed elsewhere (Child, '14, '15) and need only be briefly re- 

 stated. In general, in concentrations or intensities sufficiently 

 high to kill without permitting acclimation, the susceptibility 

 decreases with advancing age. In low concentrations, which* 

 permit some degree of acclimation, the susceptibility increases 

 with advancing age because the young individual possesses a 

 greater capacity for acclimation then the older, therefore becomes 

 more rapidly and more completely acclimated and so dies later 

 than the older, or lives indefinitely, while the older dies sooner or 

 later. 



It has been shown that a relation exists between susceptibility 

 and metabolic activity, more particularly the oxidative or energy- 

 freeing reactions. In general, susceptibility to the higher con- 

 centrations or intensities varies directly, and susceptibility to 

 the lower concentrations varies inversely as the rate of these reac- 

 tions, because acclimation occurs more rapidly and more com- 

 pletely where metabolic activity is greater. This relation be- 

 tween susceptibility and metabolic condition has been shown to 

 hold good not only for different individuals, e. g., young and old 

 (Child, '15) but for different regions of the body of a single 

 individual (Child, 'i6a). 



It does not follow, however, from the existence of a relation 

 between susceptibility and rate of metabolism or of certain 

 metabolic reactions, that all agents for which such a relation 



