AGAMIC REPRODUCTION AND REJUVENESCENCE 133 



Since the pieces are usually very small, the reorganization is ex- 

 tensive and the degree of rejuvenescence is very much greater than 

 in the larger pieces separated in agamic reproduction in P. doroto- 

 cephala and P. maculata. In cases where large instead of small 

 fragments are formed the animals which develop from them are of 

 course longer than those from the small fragments, the reconsti- 

 tutional changes are less extensive, and the degree of rejuvenescence 

 is less than in the small fragments. 



Apparently the degree of rejuvenescence is essentially the same 

 in successive generations, for this method of reproduction is ade- 

 quate for the maintenance of the species without visible decrease 

 in vigor or advance in senescence, at least for a considerable number 

 of generations. In the laboratory a stock of these worms has been 

 bred asexually over three years and has passed through fifteen 

 generations without any apparent progressive change in the 

 physiological condition of the animals in successive generations. 

 In each generation the rate of metabolism decreases and the process 

 of senescence ends in fragmentation and encystment, and young 

 animals emerge from the cysts and repeat the life cycle. 



This case is of particular interest because the process of senes- 

 cence, as it occurs under the usual conditions of existence, does not 

 end in death but leads directly to reproduction and rejuvenescence. 

 The occurrence of fragmentation in these animals is very clearly 

 associated with the decrease in rate of metabolism which is the 

 characteristic dynamic feature of senescence (Child, '136). As 

 the animal grows old its decreasing rate of metabolism makes im- 

 possible the maintenance of physiological individuality. Physio- 

 logical isolation of parts (see chap, ix) occurs and is followed by 

 physical isolation, and the isolated parts of the old individual 

 undergo reconstitution into new, young individuals. Senescence 

 itself is the physiological factor inducing reproduction and re- 

 juvenescence. 



AGAMIC REPRODUCTION AND REJUVENESCENCE IN StenOStOMUm 



AND CERTAIN ANNELIDS 



In certain flatworms, among which is the genus Stenostomum, 

 the morphological development of the new zooids reaches an 



