ASEXUAL CYCLE OF PLANARIA VELATA. 185 



Lot i after one and one half hours in KCN, while the worms of 

 Lot 2 are still intact and slowly moving about. In Lot 2 dis- 

 integration begins after three hours. All the worms of Lot I 

 are completely disintegrated after three and one half hours, 

 those of Lot 2 after seven and one half hours, i. e., the survival 

 time of Lot 2 is nearly double that of Lot i. In other words 

 the worms of Lot i have a much higher rate of metabolism than 

 those of Lot 2. 



That the difference in size of the worms is not responsible 

 for the difference in survival time Is evident for two reasons : first 

 in these flattened elongated animals the surface increases almost 

 as rapidly as the volume and second the time of beginning of 

 disintegration (Stage II.) is much later in Lot 2 than in Lot I. 

 The earliest stages of disintegration involve the external surface 

 of the body and the surface of the large w r orms including the 

 cilia remains alive for a much longer time than that of the small 

 worms. Moreover, if the difference in size determined the dif- 

 ference in survival time we should expect that this would be 

 much greater since the small worms are only a minute fraction 

 of the size of the larger. The difference in the rate of the meta- 

 bolic processes affected by the KCN is the only factor which 

 will account for the results (Child, '130). 



Unfortunately it has thus far been impossible to compare the 

 worms emerging from the cysts with young worms hatched from 

 eggs because I have never observed sexual reproduction in this 

 species, but the difference in rate of metabolism between the 

 small and large worms is similar to the difference known to exist 

 in other forms between young animals sexually produced and old. 



It is, however, not necessary to use the extremes of the life 

 cycle for comparison. Animals in various stages of growth may 

 be compared and in all cases those which are nearer the stage 

 when encystment occurs, i. e., those which are older as regards 

 growth and development, show the higher resistance. 



In Table II. the survival times of a series consisting of five 

 worms 5-6 mm. in length (Lot i) and five worms 11-12 mm. 

 -(Lot 2) are given. 



