156 SENESCENCE AND REJUVENESCENCE 



In the course of observations on Planaria dorotocephala I have 

 found that the lower limit of reduction differs rather widely accord- 

 ing to the original size of the animal. Animals of twenty-five 

 millimeters in length before starvation begin to die when they are 

 reduced to a length of five or six millimeters, while animals which 

 are six or seven millimeters in length before starvation may undergo 

 reduction to a length of one or two millimeters before death. 

 As I have suggested elsewhere, death in these cases is probably not 

 due to lack of available material, for pieces isolated from starving 

 animals are capable of reconstitution to whole animals and may then 

 undergo reduction to a much smaller size before death. Death, 

 at least in the larger animals reduced by starvation, is probably due 

 to altered correlative conditions resulting from changes in the 

 axial gradient in rate of metabolism (Child, '14, p. 443). 



In consequence of their ability to undergo extreme reduction 

 before death occurs from starvation the planarians would consti- 

 tute valuable material for the study of physiological and particu- 

 larly of metabolic changes connected with inanition if it were not 

 for their small size. But now with the Tashiro biometer and with 

 the susceptibility method we are able to obtain some light on at 

 least certain features of the metabohsm in these starving animals. 

 Some of the data bearing upon this problem are presented in the 

 following section. 



CHANGES IN SUSCEPTIBILITY DURING STARVATION IN 



Planaria dorotocephala and P. velata 



Since the animals reduced by starvation resemble young animals 

 morphologically, the question whether they are young physio- 

 logically at once suggests itself. If the reduced animals are fed, 

 growth begins again, and the animals are not only indistinguishable 

 from young, growing animals in appearance and behavior, but are 

 able to go through the life history again from the stage at which 

 feeding began. Moreover, the reduced animals are very active 



during starvation. In tlie iiigher animals decrease in size of gland cells, muscle cells, 

 and nerve cells during starvation ha,3 been recorded by various authors, among 

 whom are Heumann, '50; Rindfleisch, '68; Morpurgo, '88, '89; Downerowitsch, '92; 

 Statkewitsch, '94; Lukjanow, '97; Morgulis, 'ir. 



