AGE DIFFERENCES IN SUSCEPTIBILITY 99 



essentially the same conditions for a considerable period and where 

 the animals are not undergoing fission, the length of the worm is a 

 real criterion of its physiological condition, the rate of metabohsm 

 being lower in the longer than in the shorter worms. 



Results obtained by the direct method, such as those presented 

 above, can be confirmed by the indirect or acclimation method, 

 which was described on pp. 82-85. Except where the differences of 

 size are extreme, the animals which have the higher rate of metab- 

 olism and die earlier in the concentrations of the direct method 

 live longer than those with the lower rate in the low concentrations 

 used for the accKmation method. In other words, the animals 

 which are larger and therefore physiologically older become less 

 readily and less completely acclimated to the depressing reagent, 

 and so die earlier than the younger animals. Since the results 

 obtained by this method in the present case merely confirm the 

 results of the direct method, it is unnecessary to consider them in 

 detail. 



AGE DIFFERENCES IN SUSCEPTIBILITY IN PlanaHa dorotoccphala 



In a stock of Planaria dorotocephala collected from the natural 

 habitat of this species, animals are found ranging in length from four 

 or five millimeters up to twenty millimeters or more. Since there 

 is reason to believe that sexual reproduction does not occur, or at 

 most occurs very rarely in this species under natural conditions in 

 the localities which have come under my observation, it is certain 

 that at least most of the animals collected have arisen by fission 

 (see pp. 125, 384-86). But, ignoring for the present the question of 

 their origin, we should naturally regard the smaller worms in such 

 a stock as the younger and the larger as the older, and we find as a 

 matter of fact that the same differences in susceptibility exist be- 

 tween the larger and the smaller worms as in P. maculaia. This 

 difference is shown in Fig. 3 and in Fig. 13. Fig. 13 gives the 

 susceptibility curves of four lots of ten worms each from a stock 

 which had been in the laboratory only one day. Curve ab shows 

 the susceptibility of worms five millimeters in length, curve ac of 

 worms seven milKmeters, curve ad of worms ten to twelve milli- 

 meters, and curve ef of worms eighteen to twenty millimeters in 



