

182 C. M. CHILD. 



containing vessel it may leave behind it a series of such pieces. 

 The pieces vary considerably in size, some being as large as 1.5 

 mm. in diameter, some only about 0.5 mm. The process con- 

 tinues until half or two thirds or sometimes even more 

 of the worm is separated into pieces and then the 

 anterior region including the head may encyst without 

 further division or in some cases dies. 



Under natural conditions the encysted pieces remain 

 quiescent during the summer and the following winter 

 and in early spring emerge from the cysts as minute, 

 very active worms which at once begin to feed and 

 grow and repeat the cycle. As I have determined by 

 experiment, the encysted pieces are not capable of with- 

 standing desiccation and it is probable that this fact is 

 connected with the occurrence of the worms in ditches 

 and pools partly filled with dead leaves. In such locali- 

 ties even though the water disappears, the bottom under 

 the thick layer of leaves is always more or less wet and 

 the encysted pieces are not subjected to drying. 



During the last thirteen years I have collected these 

 worms almost every year and have never found a single 

 individual with mature sexual organs or even any indi- 

 cation of sexual reproduction. Every year the active 

 period ends with decrease in activity, cessation of feed- 

 1 ing, loss of pigment, fragmentation and encystment of 



the fragments. 



In this species then, under the conditions where it. occurs in 

 this locality, development and growth result in a process of 

 senescence, the individual breaks up into fragments which under- 

 go regulation within the cysts to small whole animals, and these 

 are to all appearances physiologically as well as morphologically 

 young and are capable of repeating the life cycle. In short, 

 senescence is followed in these animals not by death but by asex- 

 ual reproduction and rejuvenescence. 



During a number of years I have kept a stock of these worms 

 in the laboratory, have bred them through several asexual 

 generations and have subjected them to various experimental 

 conditions. The results of this asexual breeding and the experi- 



