PHYSIOLOGICAL SENESCENCE IN HYDROMEDUS^. 63 



the membrane is not independent of the metabolic condition of 

 the protoplasm. Peimeability to various agents is itself within 

 certain limits an indicator of age or physiological condition of 

 the cell. 



The progressive change in behavior, the decrease in pulsation - 

 rate and "spontaneity" indicate very clearly a process of phys- 

 iological senescence and the data on susceptibility only confirm 

 and extend the facts of observation. 



SUMMARY. 



Four species of hydromedusae show evidence of a process of 

 physiological sensecence in the decrease in pulsation-rate and 

 apparent spontaneity and in the decrease in susceptibility to 

 KNC, HC1, KOH and "vital" dyes in various concentrations 

 with advancing development. The criteria of susceptibility 

 are change in rate or cessation of pulsation, loss of contractility, 

 shrinkage of the jelly and disintegration of cellular tissues, and 

 all these criteria agree. The facts justify the conclusion that a 

 decrease in the rate of metabolism, or more specifically of oxida- 

 tions, is a characteristic feature of this process of senescence. 



HULL ZOOLOGICAL LABORATORY, 

 UNIVERSITY OF CHICAGO. 



REFERENCES. 

 Child, C. M. 



'14 Starvation, Rejuvenescence and Acclimation in Planaria dorotocephala. 



Arch. f. Entwickelungsmech., XXXVIII. 

 '15 Senescence and Rejuvenescence. Chicago. 

 'i6a Experimental Control and Modification of Larval Development in the 



Sea Urchin in Relation to the Axial Gradients. Jour. Morphol., XXVIII. 

 'i6b Further Observations on Axial Susceptibility Gradients in Certain Algae. 



BIOL. BULL., XXXI. 

 Mayer, A. G. 



'06 Rhythmical Pulsation in Scyphomedusae. Carnegie Inst. Publ. No. 47. 



