AGE DIFFERENCES IN METABOLISM OF PLANARIA. 389 



attempted to make a correction for these inert portions of the 

 body. 



As Benedict and Talbot ('14) concluded after a study of the 

 metabolism of infants, metabolism cannot be accurately meas- 

 ured either by weight or by surface, but only by the amount of 

 active protoplasm. It is probably impossible to determine 

 this but certainly in the soft-bodied lower invertebrates, weight 

 is a more accurate index of the amount of active protoplasm 

 than is the case in higher forms, where skeleton is present. For 

 this reason, the metabolism of the lower forms deserves more 

 attention than it has hitherto received from physiologists. 



Among the early investigators of the effect of size differences 

 on metabolic rate were Jolyet and Regnard ('77), who determined 

 the rate of oxygen consumption per unit weight of a large number 

 of fish, and some of the higher invertebrates. They found that 

 among related groups small species consume more oxygen per 

 unit weight per unit time than large species; and small indi- 

 viduals more than large individuals of the same species. Vernon 

 ('95) confirmed this conclusion. He measured the rate of oxygen 

 consumption per unit weight of large and small individuals of 

 the same species, using as material a hydrozoan medusa, a 

 scyphozoan medusa, two ctenophores, two gasteropods, and two 

 pelagic tunicates. With a few exceptions the smaller individuals 

 were found to consume relatively more oxygen than the large 

 ones. Bounhiol ('02) working with twelve species of polychsete 

 annelids, belonging to several different families, found that the 

 carbon dioxide production was greater the smaller the indi- 

 vidual. This was true for individuals of the same and of different 

 species. On the other hand, Montuori ('13) was unable to come 

 to any definite conclusion from his measurements of the oxygen 

 consumption of a large number of species belonging to most of 

 the aquatic groups. In some cases the small individuals respired 

 relatively faster than large ones of the same species; in others 

 the reverse was found ; and in still others there was no relation 

 between size and rate of respiration. The great irregularity of 

 Montuori's results, as well as their disagreement with the work 

 of others, suggests that he failed to control adequately the con- 

 ditions under which the experiments were performed. Child 



