PHYSIOLOGICAL SENESCENCE IN HYDROMEDUS/E. 5! 



exists act directly upon the chemical reactions or that they all 

 act in the same way. Much remains to be learned concerning 

 the nature of the action of external agents on living protoplasm, 

 but the fact is already clear that protoplasmic structure, aggre- 

 gate condition, permeability, surface tension, etc., are associated 

 with metabolic condition. Susceptibility, to a given agent serves 

 at best merely as an indicator of protoplasmic and metabolic 

 condition and tells us nothing concerning the way in which the 

 agent acts. As our knowledge of susceptibility to different 

 agents and conditions increases, we find that under certain con- 

 ditions different agents give us different results, and such dif- 

 ferences in susceptibility relations often serves to throw addi- 

 tional light on the physiological condition of the protoplasm or 

 the action of the agent, or both. The general relations stated 

 above hold for a wide range of agents and conditions, and it is 

 primarily with these general relations that we are concerned in 

 the present paper. 



AGE-DIFFERENCES IN BEHAVIOR. 



A very conspicuous difference between young and old indi- 

 viduals is the much higher rate of pulsation in the young animal 

 and its progressive decrease with advancing age. Table I. 

 gives characteristic data on this point. 



TAPI s I. 



i lameter of Body Pulsations in 20 Ser. 



Species. in Mm. at 14 C. 



jEquorea ctzrulescens 7 40 



20 32 



5 20 



Mitrocoma discoidea 18 28 



60 16 



Sarsia rosaria 5 40 



10 30 



In every species observed, ten in all, a similar difference in the 

 rate of pulsation exists, the rate in the sexually mature animal 

 being in most cases about half that of the animal at the beginning 

 or in very early stages of gonad development. By repeated 

 stimulation the rate may often be somewhat increased, particu- 

 larly in old animals, but the differences in animals under ordinary 

 conditions are of the order of magnitude indicated. This dif- 



