RESISTANCE OF FISHES TO LACK OF OXYGEN. 449 



suggests a fundamental similarity in the manner in which these 

 two toxic conditions interfere with the metabolism of organisms. 

 The actual meaning of the similarity is still to be discovered. 



The preceding results are of especial interest in their connection 

 with previous work on the metabolism of the lower and the 

 higher animals. Child ('16) has shown that flat worms (Plan- 

 aria) that are morphologically old, if starved, can be made to 

 retrace the metabolic steps taken toward old age and to again 

 attain the morphological appearance and high rate of metabolism 

 that are concomitant in nature with young worms; furthermore 

 these worms are not apparently but really young and cannot 

 be distinguished by appearance, physiological activity, or be- 

 havior from "naturally" young worms. 1 In this regressive proc- 

 ess the planarian becomes smaller, the renewed youth being a 

 result of the tearing down and throwing off of those morpho- 

 logical and physiological structures that slow up cell activity. 

 Rejuvenation is then possible, in the planarian, because of the 

 absence of stable structures such as are present in the higher 

 forms. 



Animals with a fixed supporting tissue may perhaps become 

 somewhat rejuvenated by clearing the body cells of obstructions 

 to metabolism but they cannot appreciably diminish the bulk 

 of supporting tissue which they possess. In the mammals, 

 starvation results in emaciation and there is no extensive re- 

 organization such as is found in the flat worms. When a mammal 

 is starved we get a decrease instead of an increase in the rate of 

 metabolism, if we measure this rate by the carbon dioxide output. 

 This depression in the rate of the oxidative processes persists 

 throughout the entire starvation period, there being little evi- 

 dence at the present time that the metabolism of a starving 

 mammal shows any tendency to increase above the normal rate, 

 even though the starvation period be continued till death results. 



In mammals and flat worms then, we have represented, the 

 two extremes of starvation effects so far as rate of oxidations 

 is concerned. In man, starvation effects a depression in rate 



1 It is necessary before a starved planarian will show the same capacity for 

 acclimation to low concentrations of killing agents such as KCN, that it be fed at 

 least once (Child, '16, p. 164). 



' 



