174 SENESCENCE AND REJUVENESCENCE 



this process ends with the death of the whole animal and no resorp- 

 tion or rejuvenescence occurs. In some animals, however, the 

 involution mass disintegrates and is lost at the stage of Fig. 60, 

 and the anterior portion develops a new pharynx and posterior end. 

 With the mussel diet a few very small fragments arise from some 

 individuals. 



The animals which undergo partial involution and disintegration 

 followed by reconstitution feed a few times on mussel, but cease 

 to grow at about half the size of the preceding generation, and most 

 of them undergo involution and die. Some encyst entire and others 

 produce one or two fragments and then encyst, but in all cases thus 

 far the encysted animals or pieces die in the cysts and no third 

 generation appears, i.e., that portion of the second generation which 

 arises from the non-encysting members of the first generation dies 

 without giving rise to a third generation. 



As regards the encysted fragments from the first generation, 

 about half die in the cysts, the others emerge as small worms; 

 these feed a few times on mussel, grow slowly to about half the size 

 of the first generation, and undergo involution or in a few cases 

 fragmentation, as in the preceding generation. Most of these 

 worms die at this time, either as the result of involution or in the 

 cysts, but a very few emerge from cysts as a third generation. 

 These scarcely react to food at all, show almost no growth, and soon 

 undergo involution and die with or without fragmentation, or 

 die in the cysts. In no case has a single animal of the fourth gener- 

 ation been obtained from stocks fed on mussel, and very few Uve 

 to the third generation. 



These stocks show every indication of a progressive senescence 

 in successive generations. It is of interest to note that a few of the 

 animals from encysted fragments reach the third generation, while 

 the animals developed from the pieces surviving partial involution 

 or encystment without fragmentation all die in the second genera- 

 tion. The encysted fragments are smaller than the others and 

 undergo more extensive reorganization, aiid consequently a some- 

 what greater degree of rejuvenescence in the process of reconstitu- 

 tion to whole animals. But the animals after emergence from cysts 

 or reconstitution following partial involution are not as young 



