lyo SENESCENCE AND REJUVENESCENCE 



frequently without fragmentation. The encysted fragments from 

 liver-fed animals give rise to physiologically yoimg animals which 

 are able to repeat the life cycle, and asexual breeding may be con- 

 tinued with liver as food through at least many generations. 



Animals fed with earthworm have a rather different Hfe history. 

 They attain a larger size before fragmentation and, when kept at a 

 low temperature, they continue to grow until very much larger 

 than any individuals ever seen in nature, and finally die, apparently 

 of old age, usually without fragmentation and always without 

 sexual reproduction. At higher temperatures they cease to feed 

 at a certain stage, and some give rise to two or a few fragments 

 which are usually larger than under natural conditions. Some 

 animals encyst whole without fragmentation, and some do not 

 encyst at all. 



The further history of these different groups is of interest. The 

 encysted fragments give rise to physiologically young worms. The 

 animals which encyst without fragmentation remain in the cysts 

 until they have used up their reserves and more or less of their own 

 tissues, and then emerge as smaller, physiologically younger animals 

 also capable of repeating the Hfe cycle. But the history of the 

 animals which do not encyst shows the most interesting features. 

 The normal form of a full-grown, well-fed animal is shown in Fig 8 

 (p. 94). At the time these animals cease to feed, the pharynx 

 disintegrates and no new pharynx develops in its place. In the 

 course of a few days the posterior end of the body becomes inactive 

 and assumes a rounded form, as in Fig. 58, being dragged about 

 by the rest of the body as if it were a dead mass or a foreign sub- 

 stance. During the next few days this change in form and behav- 

 ior extends farther anteriorly, so that the rounded mass becomes 

 larger and the active portion of the body smaller (Fig. 59). At 

 this stage this process may cease in some individuals, but in others 

 it continues still farther, as in Fig. 60, until only a short anterior 

 portion with the head remains active. In this condition the small, 

 active anterior region is scarcely able to drag the large inert mass 

 about, although it makes violent attempts to do so. 



In some cases the rounded mass disintegrates at this stage and 

 is lost, and the anterior region slowly undergoes reconstitution to a 



